Linux Blimp

echo is one of the most commonly and widely used built-in command for Linux bash and C shells, that typically used in scripting language and batch files to display a line of text/string on standard output or a file.

The syntax for echo is:

echo [option(s)] [string(s)]

1. Input a line of text and display on standard output

$ echo Tecmint is a community of Linux Nerds 

Outputs the following text:

Tecmint is a community of Linux Nerds 

2. Declare a variable and echo its value. For example, Declare a variable of x and assign its value=10.

$ x=10

echo its value:

$ echo The value of variable x = $x 

The value of variable x = 10 

Note: The ‘-e‘ option in Linux acts as interpretation of escaped characters that are backslashed.

3. Using option ‘\b‘ – backspace with backslash interpretor ‘-e‘ which removes all the spaces in between.

$ echo -e "Tecmint \bis \ba \bcommunity \bof \bLinux \bNerds" 

TecmintisacommunityofLinuxNerds 

4. Using option ‘\n‘ – New line with backspace interpretor ‘-e‘ treats new line from where it is used.

$ echo -e "Tecmint \nis \na \ncommunity \nof \nLinux \nNerds" 

Tecmint 
is 
a 
community 
of 
Linux 
Nerds 

5. Using option ‘\t‘ – horizontal tab with backspace interpretor ‘-e‘ to have horizontal tab spaces.

$ echo -e "Tecmint \tis \ta \tcommunity \tof \tLinux \tNerds" 

Tecmint 	is 	a 	community 	of 	Linux 	Nerds 

6. How about using option new Line ‘\n‘ and horizontal tab ‘\t‘ simultaneously.

$ echo -e "\n\tTecmint \n\tis \n\ta \n\tcommunity \n\tof \n\tLinux \n\tNerds" 

	Tecmint 
	is 
	a 
	community 
	of 
	Linux 
	Nerds 

7. Using option ‘\v‘ – vertical tab with backspace interpretor ‘-e‘ to have vertical tab spaces.

$ echo -e "\vTecmint \vis \va \vcommunity \vof \vLinux \vNerds" 

Tecmint 
        is 
           a 
             community 
                       of 
                          Linux 
                                Nerds 

8. How about using option new Line ‘\n‘ and vertical tab ‘\v‘ simultaneously.

$ echo -e "\n\vTecmint \n\vis \n\va \n\vcommunity \n\vof \n\vLinux \n\vNerds" 


Tecmint 

is 

a 

community 

of 

Linux 

Nerds 

Note: We can double the vertical tab, horizontal tab and new line spacing using the option two times or as many times as required.

9. Using option ‘\r‘ – carriage return with backspace interpretor ‘-e‘ to have specified carriage return in output.

$ echo -e "Tecmint \ris a community of Linux Nerds" 

is a community of Linux Nerds 

10. Using option ‘\c‘ – suppress trailing new line with backspace interpretor ‘-e‘ to continue without emitting new line.

$ echo -e "Tecmint is a community \cof Linux Nerds" 

Tecmint is a community avi@tecmint:~$ 

11. Omit echoing trailing new line using option ‘-n‘.

$ echo -n "Tecmint is a community of Linux Nerds" 
Tecmint is a community of Linux Nerdsavi@tecmint:~/Documents$ 

12. Using option ‘\a‘ – alert return with backspace interpretor ‘-e‘ to have sound alert.

$ echo -e "Tecmint is a community of \aLinux Nerds" 
Tecmint is a community of Linux Nerds

Note: Make sure to check Volume key, before firing.

13. Print all the files/folder using echo command (ls command alternative).

$ echo * 

103.odt 103.pdf 104.odt 104.pdf 105.odt 105.pdf 106.odt 106.pdf 107.odt 107.pdf 108a.odt 108.odt 108.pdf 109.odt 109.pdf 110b.odt 110.odt 110.pdf 111.odt 111.pdf 112.odt 112.pdf 113.odt linux-headers-3.16.0-customkernel_1_amd64.deb linux-image-3.16.0-customkernel_1_amd64.deb network.jpeg 

14. Print files of a specific kind. For example, let’s assume you want to print all ‘.jpeg‘ files, use the following command.

$ echo *.jpeg 

network.jpeg 

15. The echo can be used with redirect operator to output to a file and not standard output.

$ echo "Test Page" > testpage 

## Check Content
avi@tecmint:~$ cat testpage 
Test Page 

echo Options

Options	Description
-n	do not print the trailing newline.
-e	enable interpretation of backslash escapes.
\b	backspace
\\	backslash
\n	new line
\r	carriage return
\t	horizontal tab
\v	vertical tab

That’s all for now and don’t forget to provide us with your valuable feedback in the comments below.

Source

The Nmap aka Network Mapper is an open source and a very versatile tool for Linux system/network administrators. Nmap is used for exploring networks, perform security scans, network audit and finding open ports on remote machine. It scans for Live hosts, Operating systems, packet filters and open ports running on remote hosts.

I’ll be covering most of NMAP usage in two different parts and this is the first part of nmap serious. Here in this setup, I have used two servers without firewall to test the working of the Nmap command.

1. 192.168.0.100 – server1.tecmint.com
2. 192.168.0.101 – server2.tecmint.com

Nmap command usage

# nmap [Scan Type(s)] [Options] {target specification}

How to Install NMAP in Linux

Most of the today’s Linux distributions like Red Hat, CentOS, Fedoro, Debian and Ubuntu have included Nmap in their default package management repositories called Yum and APT. The both tools are used to install and manage software packages and updates. To install Nmap on distribution specific use the following command.

# yum install nmap		[on Red Hat based systems]

$ sudo apt-get install nmap	[on Debian based systems]

Once you’ve install latest nmap application, you can follow the example instructions provided in this article.

1. Scan a System with Hostname and IP Address

The Nmap tool offers various methods to scan a system. In this example, I am performing a scan using hostname as server2.tecmint.com to find out all open ports, services and MAC address on the system.

Scan using Hostname

[root@server1 ~]# nmap server2.tecmint.com

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 15:42 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.415 seconds
You have new mail in /var/spool/mail/root

Scan using IP Address

[root@server1 ~]# nmap 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-18 11:04 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
958/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.465 seconds
You have new mail in /var/spool/mail/root

2. Scan using “-v” option

You can see that the below command with “-v” option is giving more detailed information about the remote machine.

[root@server1 ~]# nmap -v server2.tecmint.com

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 15:43 EST
Initiating ARP Ping Scan against 192.168.0.101 [1 port] at 15:43
The ARP Ping Scan took 0.01s to scan 1 total hosts.
Initiating SYN Stealth Scan against server2.tecmint.com (192.168.0.101) [1680 ports] at 15:43
Discovered open port 22/tcp on 192.168.0.101
Discovered open port 80/tcp on 192.168.0.101
Discovered open port 8888/tcp on 192.168.0.101
Discovered open port 111/tcp on 192.168.0.101
Discovered open port 3306/tcp on 192.168.0.101
Discovered open port 957/tcp on 192.168.0.101
The SYN Stealth Scan took 0.30s to scan 1680 total ports.
Host server2.tecmint.com (192.168.0.101) appears to be up ... good.
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.485 seconds
               Raw packets sent: 1681 (73.962KB) | Rcvd: 1681 (77.322KB)

Scan Multiple Hosts

You can scan multiple hosts by simply writing their IP addresses or hostnames with Nmap.

[root@server1 ~]# nmap 192.168.0.101 192.168.0.102 192.168.0.103

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 16:06 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)
Nmap finished: 3 IP addresses (1 host up) scanned in 0.580 seconds

4. Scan a whole Subnet

You can scan a whole subnet or IP range with Nmap by providing * wildcard with it.

[root@server1 ~]# nmap 192.168.0.*

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 16:11 EST
Interesting ports on server1.tecmint.com (192.168.0.100):
Not shown: 1677 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
111/tcp open  rpcbind
851/tcp open  unknown

Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 256 IP addresses (2 hosts up) scanned in 5.550 seconds
You have new mail in /var/spool/mail/root

On above output you can see that nmap scanned a whole subnet and gave the information about those hosts which are Up in the Network.

5. Scan Multiple Servers using last octet of IP address

You can perform scans on multiple IP address by simple specifying last octet of IP address. For example, here I performing a scan on IP addresses 192.168.0.101, 192.168.0.102 and 192.168.0.103.

[root@server1 ~]# nmap 192.168.0.101,102,103

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 16:09 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 3 IP addresses (1 host up) scanned in 0.552 seconds
You have new mail in /var/spool/mail/root

6. Scan list of Hosts from a File

If you have more hosts to scan and all host details are written in a file , you can directly ask nmap to read that file and perform scans. Let’s see how to do that.

Create a text file called “nmaptest.txt” and define all the IP addresses or hostname of the server that you want to do a scan.

[root@server1 ~]# cat > nmaptest.txt

localhost
server2.tecmint.com
192.168.0.101

Next, run the following command with “iL” option with nmap command to scan all listed IP address in the file.

[root@server1 ~]# nmap -iL nmaptest.txt

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-18 10:58 EST
Interesting ports on localhost.localdomain (127.0.0.1):
Not shown: 1675 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
25/tcp  open  smtp
111/tcp open  rpcbind
631/tcp open  ipp
857/tcp open  unknown

Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
958/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
958/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 3 IP addresses (3 hosts up) scanned in 2.047 seconds

7. Scan an IP Address Range

You can specify an IP range while performing scan with Nmap.

[root@server1 ~]# nmap 192.168.0.101-110

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 16:09 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 10 IP addresses (1 host up) scanned in 0.542 seconds

8. Scan Network Excluding Remote Hosts

You can exclude some hosts while performing a full network scan or when you are scanning with wildcards with “–exclude” option.

[root@server1 ~]# nmap 192.168.0.* --exclude 192.168.0.100

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 16:16 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 255 IP addresses (1 host up) scanned in 5.313 seconds
You have new mail in /var/spool/mail/root

9. Scan OS information and Traceroute

With Nmap, you can detect which OS and version is running on the remote host. To enable OS & version detection, script scanning and traceroute, we can use “-A” option with NMAP.

[root@server1 ~]# nmap -A 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 16:25 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE VERSION
22/tcp   open  ssh     OpenSSH 4.3 (protocol 2.0)
80/tcp   open  http    Apache httpd 2.2.3 ((CentOS))
111/tcp  open  rpcbind  2 (rpc #100000)
957/tcp  open  status   1 (rpc #100024)
3306/tcp open  mysql   MySQL (unauthorized)
8888/tcp open  http    lighttpd 1.4.32
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)
No exact OS matches for host (If you know what OS is running on it, see http://www.insecure.org/cgi-bin/nmap-submit.cgi).
TCP/IP fingerprint:
SInfo(V=4.11%P=i686-redhat-linux-gnu%D=11/11%Tm=52814B66%O=22%C=1%M=080027)
TSeq(Class=TR%IPID=Z%TS=1000HZ)
T1(Resp=Y%DF=Y%W=16A0%ACK=S++%Flags=AS%Ops=MNNTNW)
T2(Resp=N)
T3(Resp=Y%DF=Y%W=16A0%ACK=S++%Flags=AS%Ops=MNNTNW)
T4(Resp=Y%DF=Y%W=0%ACK=O%Flags=R%Ops=)
T5(Resp=Y%DF=Y%W=0%ACK=S++%Flags=AR%Ops=)
T6(Resp=Y%DF=Y%W=0%ACK=O%Flags=R%Ops=)
T7(Resp=Y%DF=Y%W=0%ACK=S++%Flags=AR%Ops=)
PU(Resp=Y%DF=N%TOS=C0%IPLEN=164%RIPTL=148%RID=E%RIPCK=E%UCK=E%ULEN=134%DAT=E)

Uptime 0.169 days (since Mon Nov 11 12:22:15 2013)

Nmap finished: 1 IP address (1 host up) scanned in 22.271 seconds
You have new mail in /var/spool/mail/root

In above Output, you can see that nmap is came up with TCP/IP fingerprint of the OS running on remote hosts and being more specific about the port and services running on the remote hosts.

10. Enable OS Detection with Nmap

Use the option “-O” and “-osscan-guess” also helps to discover OS information.

[root@server1 ~]# nmap -O server2.tecmint.com

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 17:40 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)
No exact OS matches for host (If you know what OS is running on it, see http://www.insecure.org/cgi-bin/nmap-submit.cgi).
TCP/IP fingerprint:
SInfo(V=4.11%P=i686-redhat-linux-gnu%D=11/11%Tm=52815CF4%O=22%C=1%M=080027)
TSeq(Class=TR%IPID=Z%TS=1000HZ)
T1(Resp=Y%DF=Y%W=16A0%ACK=S++%Flags=AS%Ops=MNNTNW)
T2(Resp=N)
T3(Resp=Y%DF=Y%W=16A0%ACK=S++%Flags=AS%Ops=MNNTNW)
T4(Resp=Y%DF=Y%W=0%ACK=O%Flags=Option -O and -osscan-guess also helps to discover OS
R%Ops=)
T5(Resp=Y%DF=Y%W=0%ACK=S++%Flags=AR%Ops=)
T6(Resp=Y%DF=Y%W=0%ACK=O%Flags=R%Ops=)
T7(Resp=Y%DF=Y%W=0%ACK=S++%Flags=AR%Ops=)
PU(Resp=Y%DF=N%TOS=C0%IPLEN=164%RIPTL=148%RID=E%RIPCK=E%UCK=E%ULEN=134%DAT=E)

Uptime 0.221 days (since Mon Nov 11 12:22:16 2013)

Nmap finished: 1 IP address (1 host up) scanned in 11.064 seconds
You have new mail in /var/spool/mail/root

11. Scan a Host to Detect Firewall

The below command will perform a scan on a remote host to detect if any packet filters or Firewall is used by host.

[root@server1 ~]# nmap -sA 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 16:27 EST
All 1680 scanned ports on server2.tecmint.com (192.168.0.101) are UNfiltered
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.382 seconds
You have new mail in /var/spool/mail/root

12. Scan a Host to check its protected by Firewall

To scan a host if it is protected by any packet filtering software or Firewalls.

[root@server1 ~]# nmap -PN 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 16:30 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.399 seconds

13. Find out Live hosts in a Network

With the help of “-sP” option we can simply check which hosts are live and up in Network, with this option nmap skips port detection and other things.

[root@server1 ~]# nmap -sP 192.168.0.*

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-18 11:01 EST
Host server1.tecmint.com (192.168.0.100) appears to be up.
Host server2.tecmint.com (192.168.0.101) appears to be up.
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)
Nmap finished: 256 IP addresses (2 hosts up) scanned in 5.109 seconds

14. Perform a Fast Scan

You can perform a fast scan with “-F” option to scans for the ports listed in the nmap-services files and leaves all other ports.

[root@server1 ~]# nmap -F 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 16:47 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1234 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.322 seconds

15. Find Nmap version

You can find out Nmap version you are running on your machine with “-V” option.

[root@server1 ~]# nmap -V

Nmap version 4.11 ( http://www.insecure.org/nmap/ )
You have new mail in /var/spool/mail/root

16. Scan Ports Consecutively

Use the “-r” flag to don’t randomize.

[root@server1 ~]# nmap -r 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 16:52 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.363 seconds

17. Print Host interfaces and Routes

You can find out host interface and route information with nmap by using “–iflist” option.

[root@server1 ~]# nmap --iflist

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 17:07 EST
************************INTERFACES************************
DEV  (SHORT) IP/MASK          TYPE     UP MAC
lo   (lo)    127.0.0.1/8      loopback up
eth0 (eth0)  192.168.0.100/24 ethernet up 08:00:27:11:C7:89

**************************ROUTES**************************
DST/MASK      DEV  GATEWAY
192.168.0.0/0 eth0
169.254.0.0/0 eth0

In above output, you can see that map is listing interfaces attached to your system and their respective routes.

18. Scan for specific Port

There are various options to discover ports on remote machine with Nmap. You can specify the port you want nmap to scan with “-p” option, by default nmap scans only TCP ports.

[root@server1 ~]# nmap -p 80 server2.tecmint.com

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 17:12 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
PORT   STATE SERVICE
80/tcp open  http
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) sca

19. Scan a TCP Port

You can also specify specific port types and numbers with nmap to scan.

[root@server1 ~]# nmap -p T:8888,80 server2.tecmint.com

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 17:15 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
PORT     STATE SERVICE
80/tcp   open  http
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.157 seconds

20. Scan a UDP Port

[root@server1 ~]# nmap -sU 53 server2.tecmint.com

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 17:15 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
PORT     STATE SERVICE
53/udp   open  http
8888/udp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.157 seconds

21. Scan Multiple Ports

You can also scan multiple ports using option “-p“.

[root@server1 ~]# nmap -p 80,443 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-18 10:56 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
PORT    STATE  SERVICE
80/tcp  open   http
443/tcp closed https
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.190 seconds

22. Scan Ports by Network Range

You can scan ports with ranges using expressions.

[root@server1 ~]#  nmap -p 80-160 192.168.0.101

23. Find Host Services version Numbers

We can find out service’s versions which are running on remote hosts with “-sV” option.

[root@server1 ~]# nmap -sV 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 17:48 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE VERSION
22/tcp   open  ssh     OpenSSH 4.3 (protocol 2.0)
80/tcp   open  http    Apache httpd 2.2.3 ((CentOS))
111/tcp  open  rpcbind  2 (rpc #100000)
957/tcp  open  status   1 (rpc #100024)
3306/tcp open  mysql   MySQL (unauthorized)
8888/tcp open  http    lighttpd 1.4.32
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 12.624 seconds

24. Scan remote hosts using TCP ACK (PA) and TCP Syn (PS)

Sometimes packet filtering firewalls blocks standard ICMP ping requests, in that case, we can use TCP ACK and TCP Syn methods to scan remote hosts.

[root@server1 ~]# nmap -PS 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 17:51 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.360 seconds
You have new mail in /var/spool/mail/root

25. Scan Remote host for specific ports with TCP ACK

[root@server1 ~]# nmap -PA -p 22,80 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 18:02 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
PORT   STATE SERVICE
22/tcp open  ssh
80/tcp open  http
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.166 seconds
You have new mail in /var/spool/mail/root

26. Scan Remote host for specific ports with TCP Syn

[root@server1 ~]# nmap -PS -p 22,80 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 18:08 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
PORT   STATE SERVICE
22/tcp open  ssh
80/tcp open  http
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.165 seconds
You have new mail in /var/spool/mail/root

27. Perform a stealthy Scan

[root@server1 ~]# nmap -sS 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 18:10 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.383 seconds
You have new mail in /var/spool/mail/root

28. Check most commonly used Ports with TCP Syn

[root@server1 ~]# nmap -sT 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 18:12 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
111/tcp  open  rpcbind
957/tcp  open  unknown
3306/tcp open  mysql
8888/tcp open  sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 0.406 seconds
You have new mail in /var/spool/mail/root

29. Perform a tcp null scan to fool a firewall

[root@server1 ~]# nmap -sN 192.168.0.101

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-11-11 19:01 EST
Interesting ports on server2.tecmint.com (192.168.0.101):
Not shown: 1674 closed ports
PORT     STATE         SERVICE
22/tcp   open|filtered ssh
80/tcp   open|filtered http
111/tcp  open|filtered rpcbind
957/tcp  open|filtered unknown
3306/tcp open|filtered mysql
8888/tcp open|filtered sun-answerbook
MAC Address: 08:00:27:D9:8E:D7 (Cadmus Computer Systems)

Nmap finished: 1 IP address (1 host up) scanned in 1.584 seconds
You have new mail in /var/spool/mail/root

That’s it with NMAP for now,

Source

In this article we are going to review some of the useful and frequently used Linux or Unix commands for Linux System Administrators that are used in their daily life. This is not a complete but it’s a compact list of commands to refer when needed. Let us start one by one how we can use those commands with examples.

1. Uptime Command

In Linux uptime command shows since how long your system is running and the number of users are currently logged in and also displays load average for 1,5 and 15 minutes intervals.

# uptime

08:16:26 up 22 min,  1 user,  load average: 0.00, 0.03, 0.22

Check Uptime Version

Uptime command don’t have other options other than uptime and version. It gives information only in hours:mins if it less than 1 day.

[tecmint@tecmint ~]$ uptime -V
procps version 3.2.8

2. W Command

It will displays users currently logged in and their process along-with shows load averages. also shows the login name, tty name, remote host, login time, idle time, JCPU, PCPU, command and processes.

# w

08:27:44 up 34 min,  1 user,  load average: 0.00, 0.00, 0.08
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT
tecmint  pts/0    192.168.50.1     07:59    0.00s  0.29s  0.09s w

Available options

1. -h : displays no header entries.
2. -s : without JCPU and PCPU.
3. -f : Removes from field.
4. -V : (upper letter) – Shows versions.

3. Users Command

Users command displays currently logged in users. This command don’t have other parameters other than help and version.

# users

tecmint

4. Who Command

who command simply return user name, date, time and host information. who command is similar to wcommand. Unlike w command who doesn’t print what users are doing. Lets illustrate and see the different between who and w commands.

# who

tecmint  pts/0        2012-09-18 07:59 (192.168.50.1)

# w

08:43:58 up 50 min,  1 user,  load average: 0.64, 0.18, 0.06
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT
tecmint  pts/0    192.168.50.1     07:59    0.00s  0.43s  0.10s w

Who command Options

1. -b : Displays last system reboot date and time.
2. -r : Shows current runlet.
3. -a, –all : Displays all information in cumulatively.

5. Whoami Command

whoami command print the name of current user. You can also use “who am i” command to display the current user. If you are logged in as a root using sudo command “whoami” command return root as current user. Use “who am i” command if you want to know the exact user logged in.

# whoami

tecmint

6. ls Command

ls command display list of files in human readable format.

# ls -l

total 114
dr-xr-xr-x.   2 root root  4096 Sep 18 08:46 bin
dr-xr-xr-x.   5 root root  1024 Sep  8 15:49 boot

Sort file as per last modified time.

# ls -ltr

total 40
-rw-r--r--. 1 root root  6546 Sep 17 18:42 install.log.syslog
-rw-r--r--. 1 root root 22435 Sep 17 18:45 install.log
-rw-------. 1 root root  1003 Sep 17 18:45 anaconda-ks.cfg

For more examples of ls command, please check out our article on 15 Basic ‘ls’ Command Examples in Linux.

7. Crontab Command

List schedule jobs for current user with crontab command and -l option.

# crontab -l

00 10 * * * /bin/ls >/ls.txt

Edit your crontab with -e option. In the below example will open schedule jobs in VI editor. Make a necessary changes and quit pressing :wq keys which saves the setting automatically.

# crontab -e

For more examples of Linux Cron Command, please read our earlier article on 11 Cron Scheduling Task Examples in Linux.

8. Less Command

less command allows quickly view file. You can page up and down. Press ‘q‘ to quit from less window.

# less install.log

Installing setup-2.8.14-10.el6.noarch
warning: setup-2.8.14-10.el6.noarch: Header V3 RSA/SHA256 Signature, key ID c105b9de: NOKEY
Installing filesystem-2.4.30-2.1.el6.i686
Installing ca-certificates-2010.63-3.el6.noarch
Installing xml-common-0.6.3-32.el6.noarch
Installing tzdata-2010l-1.el6.noarch
Installing iso-codes-3.16-2.el6.noarch

9. More Command

more command allows quickly view file and shows details in percentage. You can page up and down. Press ‘q‘ to quit out from more window.

# more install.log

Installing setup-2.8.14-10.el6.noarch
warning: setup-2.8.14-10.el6.noarch: Header V3 RSA/SHA256 Signature, key ID c105b9de: NOKEY
Installing filesystem-2.4.30-2.1.el6.i686
Installing ca-certificates-2010.63-3.el6.noarch
Installing xml-common-0.6.3-32.el6.noarch
Installing tzdata-2010l-1.el6.noarch
Installing iso-codes-3.16-2.el6.noarch
--More--(10%)

10. CP Command

Copy file from source to destination preserving same mode.

# cp -p fileA fileB

You will be prompted before overwrite to file.

# cp -i fileA fileB

11. MV Command

Rename fileA to fileB. -i options prompt before overwrite. Ask for confirmation if exist already.

# mv -i fileA fileB

12. Cat Command

cat command used to view multiple file at the same time.

# cat fileA fileB

You combine more and less command with cat command to view file contain if that doesn’t fit in single screen / page.

# cat install.log | less

# cat install.log | more

For more examples of Linux cat command read our article on 13 Basic Cat Command Examples in Linux.

13. Cd command (change directory)

with cd command (change directory) it will goes to fileA directory.

# cd /fileA

14. pwd command (print working directory)

pwd command return with present working directory.

# pwd

/root

15. Sort command

Sorting lines of text files in ascending order. with -r options will sort in descending order.

#sort fileA.txt

#sort -r fileA.txt

16. VI Command

Vi is a most popular text editor available most of the UNIX-like OS. Below examples open file in read only with -Roption. Press ‘:q‘ to quit from vi window.

# vi -R /etc/shadows

17. SSH Command (Secure Shell)

SSH command is used to login into remote host. For example the below ssh command will connect to remote host (192.168.50.2) using user as narad.

# ssh narad@192.168.50.2

To check the version of ssh use option -V (uppercase) shows version of ssh.

# ssh -V

OpenSSH_5.3p1, OpenSSL 1.0.0-fips 29 Mar 2010

18. Ftp or sftp Command

ftp or sftp command is used to connect to remote ftp host. ftp is (file transfer protocol) and sftp is (secure file transfer protocol). For example the below commands will connect to ftp host (192.168.50.2).

# ftp 192.168.50.2

# sftp 192.168.50.2

Putting multiple files in remote host with mput similarly we can do mget to download multiple files from remote host.

# ftp > mput *.txt

# ftp > mget *.txt

19. Service Command

Service command call script located at /etc/init.d/ directory and execute the script. There are two ways to start the any service. For example we start the service called httpd with service command.

# service httpd start
OR
# /etc/init.d/httpd start

20. Free command

Free command shows free, total and swap memory information in bytes.

# free
             total       used       free     shared    buffers     cached
Mem:       1030800     735944     294856          0      51648     547696
-/+ buffers/cache:     136600     894200
Swap:      2064376          0    2064376

Free with -t options shows total memory used and available to use in bytes.

# free -t
             total       used       free     shared    buffers     cached
Mem:       1030800     736096     294704          0      51720     547704
-/+ buffers/cache:     136672     894128
Swap:      2064376          0    2064376
Total:     3095176     736096    2359080

21. Top Command

top command displays processor activity of your system and also displays tasks managed by kernel in real-time. It’ll show processor and memory are being used. Use top command with ‘u‘ option this will display specific User process details as shown below. Press ‘O‘ (uppercase letter) to sort as per desired by you. Press ‘q‘ to quit from top screen.

# top -u tecmint

top - 11:13:11 up  3:19,  2 users,  load average: 0.00, 0.00, 0.00
Tasks: 116 total,   1 running, 115 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.0%us,  0.3%sy,  0.0%ni, 99.7%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   1030800k total,   736188k used,   294612k free,    51760k buffers
Swap:  2064376k total,        0k used,  2064376k free,   547704k cached

PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
1889 tecmint   20   0 11468 1648  920 S  0.0  0.2   0:00.59 sshd
1890 tecmint   20   0  5124 1668 1416 S  0.0  0.2   0:00.44 bash
6698 tecmint   20   0 11600 1668  924 S  0.0  0.2   0:01.19 sshd
6699 tecmint   20   0  5124 1596 1352 S  0.0  0.2   0:00.11 bash

For more about top command we’ve already compiled a list of 12 TOP Command Examples in Linux.

22. Tar Command

tar command is used to compress files and folders in Linux. For example the below command will create a archive for /home directory with file name as archive-name.tar.

# tar -cvf archive-name.tar /home

To extract tar archive file use the option as follows.

# tar -xvf archive-name.tar

To understand more about tar command we’ve created a complete how-to guide on tar command at 18 Tar Command Examples in Linux.

23. Grep Command

grep search for a given string in a file. Only tecmint user displays from /etc/passwd file. we can use -i option for ignoring case sensitive.

# grep tecmint /etc/passwd

tecmint:x:500:500::/home/tecmint:/bin/bash

24. Find Command

Find command used to search files, strings and directories. The below example of find command search tecmint word in ‘/‘ partition and return the output.

# find / -name tecmint

/var/spool/mail/tecmint
/home/tecmint
/root/home/tecmint

For complete guide on Linux find command examples fount at 35 Practical Examples of Linux Find Command.

25. lsof Command

lsof mean List of all open files. Below lsof command list of all opened files by user tecmint.

# lsof -u tecmint

COMMAND  PID    USER   FD   TYPE     DEVICE SIZE/OFF   NODE NAME
sshd    1889 tecmint  cwd    DIR      253,0     4096      2 /
sshd    1889 tecmint  txt    REG      253,0   532336 298069 /usr/sbin/sshd
sshd    1889 tecmint  DEL    REG      253,0          412940 /lib/libcom_err.so.2.1
sshd    1889 tecmint  DEL    REG      253,0          393156 /lib/ld-2.12.so
sshd    1889 tecmint  DEL    REG      253,0          298643 /usr/lib/libcrypto.so.1.0.0
sshd    1889 tecmint  DEL    REG      253,0          393173 /lib/libnsl-2.12.so
sshd    1889 tecmint  DEL    REG      253,0          412937 /lib/libkrb5support.so.0.1
sshd    1889 tecmint  DEL    REG      253,0          412961 /lib/libplc4.so

For more lsof command examples visit 10 lsof Command Examples in Linux.

26. last command

With last command we can watch user’s activity in the system. This command can execute normal user also. It will display complete user’s info like terminal, time, date, system reboot or boot and kernel version. Useful command to troubleshoot.

# last

tecmint  pts/1        192.168.50.1     Tue Sep 18 08:50   still logged in
tecmint  pts/0        192.168.50.1     Tue Sep 18 07:59   still logged in
reboot   system boot  2.6.32-279.el6.i Tue Sep 18 07:54 - 11:38  (03:43)
root     pts/1        192.168.50.1     Sun Sep 16 10:40 - down   (03:53)
root     pts/0        :0.0             Sun Sep 16 10:36 - 13:09  (02:32)
root     tty1         :0               Sun Sep 16 10:07 - down   (04:26)
reboot   system boot  2.6.32-279.el6.i Sun Sep 16 09:57 - 14:33  (04:35)
narad    pts/2        192.168.50.1     Thu Sep 13 08:07 - down   (01:15)

You can use last with username to know for specific user’s activity as shown below.

# last tecmint

tecmint  pts/1        192.168.50.1     Tue Sep 18 08:50   still logged in
tecmint  pts/0        192.168.50.1     Tue Sep 18 07:59   still logged in
tecmint  pts/1        192.168.50.1     Thu Sep 13 08:07 - down   (01:15)
tecmint  pts/4        192.168.50.1     Wed Sep 12 10:12 - 12:29  (02:17)

27. ps command

ps command displays about processes running in the system. Below example show init process only.

# ps -ef | grep init

root         1     0  0 07:53 ?        00:00:04 /sbin/init
root      7508  6825  0 11:48 pts/1    00:00:00 grep init

28. kill command

Use kill command to terminate process. First find process id with ps command as shown below and kill process with kill -9 command.

# ps -ef | grep init
root         1     0  0 07:53 ?        00:00:04 /sbin/init
root      7508  6825  0 11:48 pts/1    00:00:00 grep init

# kill- 9 7508

29. rm command

rm command used to remove or delete a file without prompting for confirmation.

# rm filename

Using -i option to get confirmation before removing it. Using options ‘-r‘ and ‘-f‘ will remove the file forcefully without confirmation.

# rm -i test.txt

rm: remove regular file `test.txt'?

30. mkdir command example.

mkdir command is used to create directories under Linux.

# mkdir directoryname

This is a handy day to day useable basic commands in Linux / Unix-like operating system. Kindly share through our comment box if we missed out.

Source

Adapting to using the command line or terminal can be very hard for beginners who want to learn Linux. Because the terminal gives more control over a Linux system than GUIs programs, one has to get a used to running commands on the terminal. Therefore to memorize different commands in Linux, you should use the terminal on a daily basis to understand how commands are used with different options and arguments.

Please go through our previous parts of this Linux Tricks series.

1. 5 Interesting Command Line Tips and Tricks in Linux – Part 1
2. 10 Useful Commandline Tricks for Newbies – Part 2

In this article, we are going to look at some tips and tricks of using 10 commands to work with files and time on the terminal.

File Types in Linux

In Linux, everything is considered as a file, your devices, directories and regular files are all considered as files.

There are different types of files in a Linux system:

1. Regular files which may include commands, documents, music files, movies, images, archives and so on.
2. Device files: which are used by the system to access your hardware components.

There are two types of device files block files that represent storage devices such as harddisks, they read data in blocks and character files read data in a character by character manner.

1. Hardlinks and softlinks: they are used to access files from any where on a Linux filesystem.
2. Named pipes and sockets: allow different processes to communicate with each other.

1. Determining the type of a file using ‘file’ command

You can determine the type of a file by using the file command as follows. The screenshot below shows different examples of using the file command to determine the types of different files.

tecmint@tecmint ~/Linux-Tricks $ dir
BACKUP				      master.zip
crossroads-stable.tar.gz	      num.txt
EDWARD-MAYA-2011-2012-NEW-REMIX.mp3   reggea.xspf
Linux-Security-Optimization-Book.gif  tmp-link

tecmint@tecmint ~/Linux-Tricks $ file BACKUP/
BACKUP/: directory 

tecmint@tecmint ~/Linux-Tricks $ file master.zip 
master.zip: Zip archive data, at least v1.0 to extract

tecmint@tecmint ~/Linux-Tricks $ file crossroads-stable.tar.gz
crossroads-stable.tar.gz: gzip compressed data, from Unix, last modified: Tue Apr  5 15:15:20 2011

tecmint@tecmint ~/Linux-Tricks $ file Linux-Security-Optimization-Book.gif 
Linux-Security-Optimization-Book.gif: GIF image data, version 89a, 200 x 259

tecmint@tecmint ~/Linux-Tricks $ file EDWARD-MAYA-2011-2012-NEW-REMIX.mp3 
EDWARD-MAYA-2011-2012-NEW-REMIX.mp3: Audio file with ID3 version 2.3.0, contains: MPEG ADTS, layer III, v1, 192 kbps, 44.1 kHz, JntStereo

tecmint@tecmint ~/Linux-Tricks $ file /dev/sda1
/dev/sda1: block special 

tecmint@tecmint ~/Linux-Tricks $ file /dev/tty1
/dev/tty1: character special 

2. Determining the file type using ‘ls’ and ‘dir’ commands

Another way of determining the type of a file is by performing a long listing using the ls and dir commands.

Using ls -l to determine the type of a file.

When you view the file permissions, the first character shows the file type and the other charcters show the file permissions.

tecmint@tecmint ~/Linux-Tricks $ ls -l
total 6908
drwxr-xr-x 2 tecmint tecmint    4096 Sep  9 11:46 BACKUP
-rw-r--r-- 1 tecmint tecmint 1075620 Sep  9 11:47 crossroads-stable.tar.gz
-rwxr----- 1 tecmint tecmint 5916085 Sep  9 11:49 EDWARD-MAYA-2011-2012-NEW-REMIX.mp3
-rw-r--r-- 1 tecmint tecmint   42122 Sep  9 11:49 Linux-Security-Optimization-Book.gif
-rw-r--r-- 1 tecmint tecmint   17627 Sep  9 11:46 master.zip
-rw-r--r-- 1 tecmint tecmint       5 Sep  9 11:48 num.txt
-rw-r--r-- 1 tecmint tecmint       0 Sep  9 11:46 reggea.xspf
-rw-r--r-- 1 tecmint tecmint       5 Sep  9 11:47 tmp-link

Using ls -l to determine block and character files.

tecmint@tecmint ~/Linux-Tricks $ ls -l /dev/sda1
brw-rw---- 1 root disk 8, 1 Sep  9 10:53 /dev/sda1

tecmint@tecmint ~/Linux-Tricks $ ls -l /dev/tty1
crw-rw---- 1 root tty 4, 1 Sep  9 10:54 /dev/tty1

Using dir -l to determine the type of a file.

tecmint@tecmint ~/Linux-Tricks $ dir -l
total 6908
drwxr-xr-x 2 tecmint tecmint    4096 Sep  9 11:46 BACKUP
-rw-r--r-- 1 tecmint tecmint 1075620 Sep  9 11:47 crossroads-stable.tar.gz
-rwxr----- 1 tecmint tecmint 5916085 Sep  9 11:49 EDWARD-MAYA-2011-2012-NEW-REMIX.mp3
-rw-r--r-- 1 tecmint tecmint   42122 Sep  9 11:49 Linux-Security-Optimization-Book.gif
-rw-r--r-- 1 tecmint tecmint   17627 Sep  9 11:46 master.zip
-rw-r--r-- 1 tecmint tecmint       5 Sep  9 11:48 num.txt
-rw-r--r-- 1 tecmint tecmint       0 Sep  9 11:46 reggea.xspf
-rw-r--r-- 1 tecmint tecmint       5 Sep  9 11:47 tmp-link

3. Counting number of files of a specific type

Next we shall look at tips on counting number of files of a specific type in a given directory using the ls, grep and wc commands. Communication between the commands is achieved through named piping.

1. grep – command to search according to a given pattern or regular expression.
2. wc – command to count lines, words and characters.

Counting number of regular files

In Linux, regular files are represented by the – symbol.

tecmint@tecmint ~/Linux-Tricks $ ls -l | grep ^- | wc -l
7

Counting number of directories

In Linux, directories are represented by the d symbol.

tecmint@tecmint ~/Linux-Tricks $ ls -l | grep ^d | wc -l
1

Counting number of symbolic and hard links

In Linux, symblic and hard links are represented by the l symbol.

tecmint@tecmint ~/Linux-Tricks $ ls -l | grep ^l | wc -l
0

Counting number of block and character files

In Linux, block and character files are represented by the b and c symbols respectively.

tecmint@tecmint ~/Linux-Tricks $ ls -l /dev | grep ^b | wc -l
37
tecmint@tecmint ~/Linux-Tricks $ ls -l /dev | grep ^c | wc -l
159

4. Finding files on a Linux system

Next we shall look at some commands one can use to find files on a Linux system, these include the locate, find, whatis and which commands.

Using the locate command to find files

In the output below, I am trying to locate the Samba server configuration for my system.

tecmint@tecmint ~/Linux-Tricks $ locate samba.conf
/usr/lib/tmpfiles.d/samba.conf
/var/lib/dpkg/info/samba.conffiles

Using the find command to find files

To learn how to use the find command in Linux, you can read our following article that shows more than 30+practical examples and usage of find command in Linux.

1. 35 Examples of ‘find’ Command in Linux

Using the whatis command to locate commands

The whatis command is mostly used to locate commands and it is special because it gives information about a command, it also finds configurations files and manual entries for a command.

tecmint@tecmint ~/Linux-Tricks $ whatis bash
bash (1)             - GNU Bourne-Again SHell

tecmint@tecmint ~/Linux-Tricks $ whatis find
find (1)             - search for files in a directory hierarchy

tecmint@tecmint ~/Linux-Tricks $ whatis ls
ls (1)               - list directory contents

Using which command to locate commands

The which command is used to locate commands on the filesystem.

tecmint@tecmint ~/Linux-Tricks $ which mkdir
/bin/mkdir

tecmint@tecmint ~/Linux-Tricks $ which bash
/bin/bash

tecmint@tecmint ~/Linux-Tricks $ which find
/usr/bin/find

tecmint@tecmint ~/Linux-Tricks $ $ which ls
/bin/ls

5. Working with time on your Linux system

When working in a networked environment, it is a good practice to keep the correct time on your Linux system. There are certain services on Linux systems that require correct time to work efficiently on a network.

We shall look at commands you can use to manage time on your machine. In Linux, time is managed in two ways: system time and hardware time.

The system time is managed by a system clock and the hardware time is managed by a hardware clock.

To view your system time, date and timezone, use the date command as follows.

tecmint@tecmint ~/Linux-Tricks $ date
Wed Sep  9 12:25:40 IST 2015

Set your system time using date -s or date –set=”STRING” as follows.

tecmint@tecmint ~/Linux-Tricks $ sudo date -s "12:27:00"
Wed Sep  9 12:27:00 IST 2015

tecmint@tecmint ~/Linux-Tricks $ sudo date --set="12:27:00"
Wed Sep  9 12:27:00 IST 2015

You can also set time and date as follows.

tecmint@tecmint ~/Linux-Tricks $ sudo date 090912302015
Wed Sep  9 12:30:00 IST 2015

Viewing current date from a calendar using cal command.

tecmint@tecmint ~/Linux-Tricks $ cal
   September 2015     
Su Mo Tu We Th Fr Sa  
       1  2  3  4  5  
 6  7  8  9 10 11 12  
13 14 15 16 17 18 19  
20 21 22 23 24 25 26  
27 28 29 30      

View hardware clock time using the hwclock command.

tecmint@tecmint ~/Linux-Tricks $ sudo hwclock
Wednesday 09 September 2015 06:02:58 PM IST  -0.200081 seconds

To set the hardware clock time, use hwclock –set –date=”STRING” as follows.

tecmint@tecmint ~/Linux-Tricks $ sudo hwclock --set --date="09/09/2015 12:33:00"

tecmint@tecmint ~/Linux-Tricks $ sudo hwclock
Wednesday 09 September 2015 12:33:11 PM IST  -0.891163 seconds

The system time is set by the hardware clock during booting and when the system is shutting down, the hardware time is reset to the system time.

Therefore when you view system time and hardware time, they are the same unless when you change the system time. Your hardware time may be incorrect when the CMOS battery is weak.

You can also set your system time using time from the hardware clock as follows.

$ sudo hwclock --hctosys

It is also possible to set hardware clock time using the system clock time as follows.

$ sudo hwclock --systohc

To view how long your Linux system has been running, use the uptime command.

tecmint@tecmint ~/Linux-Tricks $ uptime
12:36:27 up  1:43,  2 users,  load average: 1.39, 1.34, 1.45

tecmint@tecmint ~/Linux-Tricks $ uptime -p
up 1 hour, 43 minutes

tecmint@tecmint ~/Linux-Tricks $ uptime -s
2015-09-09 10:52:47

Summary

Understanding file types is Linux is a good practice for begginers, and also managing time is critical especially on servers to manage services reliably and efficiently. Hope you find this guide helpful. If you have any additional information, do not forget to post a comment.

Source

Are you making most out of the Linux? There are lots of helpful features which appears to be Tips and Tricks for many of Linux Users. Sometimes Tips and Tricks become the need. It helps you get productive with the same set of commands yet with enhanced functionality.

Here we are starting a new series, where we will be writing some tips and tricks and will try to yield as more as we can in small time.

1. To audit the commands we’d run in past, we use history command. Here is a sample output of history command.

# history

Obvious from output, the history command do not output the time stamp with the log of last executed commands. Any solution for this? Yeah! Run the below command.

# HISTTIMEFORMAT="%d/%m/%y %T "
# history

If you want to permanently append this change, add the below line to ~/.bashrc.

export HISTTIMEFORMAT="%d/%m/%y %T "

and then, from terminal run,

# source ~/.bashrc

Explanation of commands and switches.

1. history – GNU History Library
2. HISTIMEFORMAT – Environmental Variable
3. %d – Day
4. %m – Month
5. %y – Year
6. %T – Time Stamp
7. source – in short send the contents of file to shell
8. .bashrc – is a shell script that BASH runs whenever it is started interactively.

2. The next gem in the list is – how to check disk write speed? Well one liner dd command script serves the purpose.

# dd if=/dev/zero of=/tmp/output.img bs=8k count=256k conv=fdatasync; rm -rf /tmp/output.img

Explanation of commands and switches.

1. dd – Convert and Copy a file
2. if=/dev/zero – Read the file and not stdin
3. of=/tmp/output.img – Write to file and not stdout
4. bs – Read and Write maximum upto M bytes, at one time
5. count – Copy N input block
6. conv – Convert the file as per comma separated symbol list.
7. rm – Removes files and folder
8. -rf – (-r) removes directories and contents recursively and (-f) Force the removal without prompt.

3. How will you check the top six files that are eating out your space? A simple one liner script made from du command, which is primarily used as file space usages.

# du -hsx * | sort -rh | head -6

Explanation of commands and switches.

1. du – Estimate file space usages
2. -hsx – (-h) Human Readable Format, (-s) Summaries Output, (-x) One File Format, skip directories on other file format.
3. sort – Sort text file lines
4. -rh – (-r) Reverse the result of comparison, (-h) for compare human readable format.
5. head – output first n lines of file.

4. The next step involves statistics in terminal of a file of every kind. We can output the statistics related to a file with the help of stat (output file/fileSystem status) command.

# stat filename_ext  (viz., stat abc.pdf)

5. The next and last but not the least, this one line script is for those, who are newbies. If you are an experienced user you probably don’t need it, unless you want some fun out of it. Well newbies are Linux-command-line phobic and the below one liner will generate random man pages. The benefit is as a newbie you always get something to learn and never get bored.

# man $(ls /bin | shuf | head -1)

Explanation of commands and switches.

1. man – Linux Man pages
2. ls – Linux Listing Commands
3. /bin – System Binary file Location
4. shuf – Generate Random Permutation
5. head – Output first n line of file.

That’s all for now. If you know any such tips and tricks you may share with us and we will post the same in your words on our reputed Tecmint.com website.

If you want to share any tips and tricks that you cannot make into article you may share it at tecmint[dot]com[at]gmail[dot]com and we will include it in our article. Don’t forget to provide us with your valuable feedback in the comments below. Keep connected. Like and share us and help us get spread.

Don’t Miss:

1. 10 Useful Commandline Tricks for Newbies – Part 2
2. 5 Useful Commands to Manage Linux File Types and System Time – Part 3

Source

Bootiso is a powerful Bash script to easily and securely create a bootable USB device from one ISO file. It helps you create a bootable USB from an ISO with a single command from the terminal. It is a well tailored script that carefully organized and validated using shellcheck.

It has to be run with root authority, and if external programs it requires are not available on your system, it will ask you to install them and exits. Bootiso checks that the selected ISO has the correct mime-type, otherwise it exits. To prevent system damages, it ensures that the selected device is connected only via USB.

Read Also: 3 Ways to Extract and Copy Files from ISO Image in Linux

Before formating and partitioning your USB device, it prompts you to accept execution of the actions to prevent any data loss. Importantly, it manages any failure from an internal command appropriately exits. In addition, it performs a cleanup of any temporary files on exit by employing the trap utility.

Install Bootiso Script in Linux

The easy way to install bootiso from sources is to clone the git repository and set execute permission as shown.

$ git clone https://github.com/jsamr/bootiso.git
$ cd bootiso/
$ chmod +x bootiso

Next, move the script to a bin path (for example ~/bin/ or /usr/local/bin/) to run it like any other Linux commands on your system.

$ mv bootiso ~/bin/

Once installed, the syntax for running bootiso is to provide the ISO as first argument.

$ bootiso myfile.iso

To create a bootable USB device from ISO file, first you need to list all available USB drives attached to your system using the -l flag as shown.

$ bootiso -l

Listing USB drives available in your system:
NAME    HOTPLUG   SIZE STATE   TYPE
sdb           1   14.9G running disk

Next, to make the device (/dev/sdb) as a bootable device, simply provide the ISO as the first argument. Note that if there is only one USB device attached to the system (as in the case above), the script will automatically select it, otherwise, it will ask you to select from an auto-generated list of all attached USB drives.

$ sudo bootiso ~/Templates/eXternOS.iso 

You may also use the -a flag to enable autoselecting USB drives in conjunction with -y (disables prompting user before formating USB drive) option as shown.

$ sudo bootiso -a -y ~/Templates/eXternOS.iso

If you have multiple USB devices connected to the system, you can use the -d flag to explicitly specify the USB device you want to make bootable from the command line as shown.

$ sudo bootiso -d /dev/sdb ~/Templates/eXternOS.iso  

By default, bootiso uses mount + rsync to employ dd command instead, add the --dd flag as shown.

$ sudo bootiso --dd -d ~/Templates/eXternOS.iso      

In addition, for non-hybrid ISOs, you can install a bootloader with syslinux with the -b option, as follows. This option however does not support the dd command.

$ sudo bootiso -b /ptah/to/non-hybrid/file.iso
OR
$ sudo bootiso -bd /usb/device /ptah/to/non-hybrid/file.iso

For more information on other bootiso capabilities and options, see the help message.

$ bootiso -h  

Bootiso Github repository: https://github.com/jsamr/bootiso

That’s It! Bootiso is a powerful Bash script to easily and securely create a bootable USB device from one ISO file, with a single command on the terminal. Use the comment form below to share your thoughts about it or ask questions.

Source

The Linux system boots so fast that most of the output scrolls by too quickly to read the text (showing services being started) sent to the console. Therefore observing boot issues/errors becomes a little of a challenge for us.

In this article, we will briefly explain the different stages in a Linux system boot process, then learn how to establish and get to the bottom of boot issues: in terms files to look into or commands to view system boot messages.

Summary Of Linux Boot Process

In summary, once we press the Power On button, the BIOS (Basic Input Output System) a program integrated in a motherboard performs a POST (Power on Self Test) – where hardware such as disks, RAM (Random Access Memory), keyboard, etc are scanned. In case of an error (missing/malfunctioning hardware), it is reported on the screen.

During POST, the BIOS also looks for the boot device, the disk to stat up from (usually the first hard disk, however we may configure it to be a DVD, USB, Network Card etc instead).

Then the system will connect to the disk and search for the Master Boot Record (512 bytes in size) which stores the boot loader (446 bytes in size) and the rest of the space stores info about disk partitions (four maximum) and the MBR itself.

Suggested Read: 4 Best Linux Boot Loaders

The boot loader will identify and point to, as well as load the Kernel and the initrd file (initialization ram disk – provides the kernel access to the mounted root filesystem and modules/drivers stored in /lib directory), which are generally stored in the /boot directory of the filesystem.

After the kernel is loaded, it executes init (or systemd on newer Linux distros), the first process with PID 1, which in turn starts all other processes on the system. It is also the last process to be executed on system shutdown.

How to Find Out Linux Boot Issues or Error Messages

As we mentioned before, the Linux boot processes happens rapidly that we can’t even clearly read most of the output sent to the console.

So taking note of boot issues/errors calls for a system administrator to look into certain important files in conjunction with particular commands. And these include:

/var/log/boot.log – Logs System Boot Messages

This is probably the first file you want to look into, to view all that unfolded during the system boot.

Rather than trying so hard to follow the output on the screen during boot, we can view this file after the boot process has completed to assist us in determining and resolving boot issues/errors.

We use the cat command for that purpose as follows (below is a sample of this file):

# cat /var/log/boot.log

[  OK  ] Started Load/Save RF Kill Switch Status.
[ TIME ] Timed out waiting for device dev-disk-by\x2duuid-53e41ce9\x2ddc18\x2d458c\x2dbc08\x2d584c208ed615.device.
[DEPEND] Dependency failed for /dev/disk/by-uuid/53e41ce9-dc18-458c-bc08-584c208ed615.
[DEPEND] Dependency failed for Swap.
[  OK  ] Reached target System Initialization.
[  OK  ] Listening on UUID daemon activation socket.
[  OK  ] Started Daily Cleanup of Temporary Directories.
[  OK  ] Listening on CUPS Scheduler.
[  OK  ] Started Daily apt activities.
[  OK  ] Reached target Timers.
[  OK  ] Listening on Avahi mDNS/DNS-SD Stack Activation Socket.
[  OK  ] Started ACPI Events Check.
[  OK  ] Started Trigger resolvconf update for networkd DNS.
[  OK  ] Started CUPS Scheduler.
[  OK  ] Reached target Paths.
[  OK  ] Listening on D-Bus System Message Bus Socket.
[  OK  ] Listening on ACPID Listen Socket.
         Starting Console System Startup Logging...
[  OK  ] Listening on Cockpit Web Service Socket.
[  OK  ] Reached target Sockets.
[  OK  ] Reached target Basic System.
         Starting LSB: Set the CPU Frequency Scaling governor to "ondemand"...
[  OK  ] Started ACPI event daemon.
[  OK  ] Started mintsystem.service.
         Starting Detect the available GPUs and deal with any system changes...
         Starting LSB: daemon to balance interrupts for SMP systems...
         Starting Bluetooth service...
[  OK  ] Started ClamAV virus database updater.
         Starting LSB: Starts syslogd...
[  OK  ] Started Regular background program processing daemon.
         Starting Modem Manager...
         Starting Accounts Service...
......

From the output above, we can see that there is a boot issues indicated by the lines below.

[DEPEND] Dependency failed for /dev/disk/by-uuid/53e41ce9-dc18-458c-bc08-584c208ed615.
[DEPEND] Dependency failed for Swap.

Issue: Problem with the swap partition; the system either failed to read the swap file/device/partition or it’s not present.

Let’s check if the system is using swap space with free command.

# free

total        used        free      shared  buff/cache   available
Mem:        3742792     2421060      433696      287376      888036      967000
Swap:             0           0           0

Alternatively, we may run the swapon command to view the system swap space usage summary (we’ll not get any output).

# swapon -s

We can resolve this issue, by creating a swap space in Linux.

Note: The content of this file is cleared at system shutdown: new data is stored in it on a new boot.

/var/log/messages – General System Logs

This file stores general system messages, including the messages that are logged during system boot.

To view it, type:

# cat /var/log/messages

Jun  4 13:04:44 tecmint syslogd (GNU inetutils 1.9.4): restart
Jun  4 13:19:55 tecmint -- MARK --
Jun  4 13:39:55 tecmint -- MARK --
Jun  4 13:59:55 tecmint -- MARK --
Jun  4 14:19:55 tecmint -- MARK --
Jun  4 14:20:17 tecmint vmunix: [ 4945.388740] i915 0000:00:02.0: BAR 6: [??? 0x00000000 flags 0x2] has bogus alignment
Jun  4 14:20:17 tecmint vmunix: [ 4945.388837] i915 0000:00:02.0: BAR 6: [??? 0x00000000 flags 0x2] has bogus alignment
Jun  4 14:20:17 tecmint vmunix: [ 4945.388903] i915 0000:00:02.0: BAR 6: [??? 0x00000000 flags 0x2] has bogus alignment
Jun  4 14:20:17 tecmint vmunix: [ 4945.388930] i915 0000:00:02.0: BAR 6: [??? 0x00000000 flags 0x2] has bogus alignment
Jun  4 14:20:17 tecmint vmunix: [ 4945.389334] i915 0000:00:02.0: BAR 6: [??? 0x00000000 flags 0x2] has bogus alignment
Jun  4 14:20:17 tecmint vmunix: [ 4945.389402] pcieport 0000:00:1c.0: BAR 15: assigned [mem 0xdfa00000-0xdfbfffff 64bit pref]
.....

Because this file may be relatively lengthy, we can view it in pages use the more command (which even shows a percentage) like this.

# more /var/log/messages

The contents of /var/log/messages unlike the previous file is not cleared shutdown because it does not only contain boot messages but messages concerning other system activities as well. So older files are compressed and kept on the system for later inspection as shown below.

# ls -l message*

-rw-r--r-- 1 root root 1206127 Jun 10 14:20 messages
-rw-r--r-- 1 root root 1419494 Jun  4 13:00 messages.1
-rw-r--r-- 1 root root  153011 May 28 09:30 messages.2.gz

dmesg – Shows Kernel Messages

The dmesg command can show operations once the boot process has completed, such as command line options passed to the kernel; hardware components detected, events when a new USB device is added, or errors like NIC (Network Interface Card) failure and the drivers report no link activity detected on the network and so much more.

# dmesg

[    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Initializing cgroup subsys cpuacct
[    0.000000] Linux version 4.4.0-21-generic (buildd@lgw01-21) (gcc version 5.3.1 20160413 (Ubuntu 5.3.1-14ubuntu2) ) #37-Ubuntu SMP Mon Apr 18 18:33:37 UTC 2016 (Ubuntu 4.4.0-21.37-generic 4.4.6)
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-4.4.0-21-generic root=UUID=57b36d48-1938-43c2-bf85-e97bc9f423ea ro quiet splash
[    0.000000] KERNEL supported cpus:
[    0.000000]   Intel GenuineIntel
[    0.000000]   AMD AuthenticAMD
[    0.000000]   Centaur CentaurHauls
[    0.000000] x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256
[    0.000000] x86/fpu: Supporting XSAVE feature 0x01: 'x87 floating point registers'
[    0.000000] x86/fpu: Supporting XSAVE feature 0x02: 'SSE registers'
[    0.000000] x86/fpu: Supporting XSAVE feature 0x04: 'AVX registers'
[    0.000000] x86/fpu: Enabled xstate features 0x7, context size is 832 bytes, using 'standard' format.
[    0.000000] x86/fpu: Using 'eager' FPU context switches.
[    0.000000] e820: BIOS-provided physical RAM map:
[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000008ffff] usable
[    0.000000] BIOS-e820: [mem 0x0000000000090000-0x00000000000bffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000001fffffff] usable
[    0.000000] BIOS-e820: [mem 0x0000000020000000-0x00000000201fffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000020200000-0x0000000040003fff] usable
[    0.000000] BIOS-e820: [mem 0x0000000040004000-0x0000000040004fff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000040005000-0x0000000080b2ffff] usable
[    0.000000] BIOS-e820: [mem 0x0000000080b30000-0x0000000080d31fff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000080d32000-0x00000000ce3eefff] usable
[    0.000000] BIOS-e820: [mem 0x00000000ce3ef000-0x00000000ce5eefff] type 20
[    0.000000] BIOS-e820: [mem 0x00000000ce5ef000-0x00000000daeeefff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000daeef000-0x00000000daf9efff] ACPI NVS
....

journalctl – Query Contents of Systemd Journal

This is useful in most current Linux distros using systemd: the newest system and service manager. Under it, system logging is done via the journal component – a centralized location for all messages logged by different components.

These messages include kernel and boot messages; messages from syslog or various services. We can use it to review boot messages and establish boot issues by reading through the output and identifying lines of interest (errors pointed out by red lines depending on terminal text color settings).

# journalctl

Jun 13 16:35:32 tecmint mtp-probe[963]: checking bus 2, device 5: "/sys/devices/pci0000:00/0000:00:1d.0/u
Jun 13 16:35:32 tecmint mtp-probe[963]: bus: 2, device: 5 was not an MTP device
Jun 13 16:35:54 tecmint systemd[1]: dev-disk-by\x2duuid-53e41ce9\x2ddc18\x2d458c\x2dbc08\x2d584c208ed615.
Jun 13 16:35:54 tecmint systemd[1]: Timed out waiting for device dev-disk-by\x2duuid-53e41ce9\x2ddc18\x2d
Jun 13 16:35:54 tecmint systemd[1]: Dependency failed for /dev/disk/by-uuid/53e41ce9-dc18-458c-bc08-584c2
Jun 13 16:35:54 tecmint systemd[1]: Dependency failed for Swap.
Jun 13 16:35:54 tecmint systemd[1]: swap.target: Job swap.target/start failed with result 'dependency'.
Jun 13 16:35:54 tecmint systemd[1]: dev-disk-by\x2duuid-53e41ce9\x2ddc18\x2d458c\x2dbc08\x2d584c208ed615.
Jun 13 16:35:54 tecmint systemd[1]: dev-disk-by\x2duuid-53e41ce9\x2ddc18\x2d458c\x2dbc08\x2d584c208ed615.
Jun 13 16:35:54 tecmint systemd[1]: Reached target System Initialization.
Jun 13 16:35:54 tecmint systemd[1]: Started ACPI Events Check.
Jun 13 16:35:54 tecmint systemd[1]: Listening on CUPS Scheduler.
Jun 13 16:35:54 tecmint systemd[1]: Starting Console System Startup Logging...
Jun 13 16:35:54 tecmint systemd[1]: Started Daily Cleanup of Temporary Directories.

The above is a sample of the command output showing an error we already identified by viewing /var/log/boot.log: the swap partition error. To view more output lines, simply press [Enter] button.

Also check out these useful guides about Linux system boot and service management in relation to common system and service managers:

1. Process Management in RHEL 7: Boot, Shutdown, and Everything in Between
2. Managing System Startup Process and Services (SysVinit, Systemd and Upstart)
3. The Story Behind ‘init’ and ‘systemd’: Why ‘init’ Needed to be Replaced with ‘systemd’

In this article, we explained in short the different stages in the Linux system boot process as well as learned how to establish and possibly resolve boot issues. To add any thought to this guide, use the comment form below.

Source

ZIP is a very popular compression and file packaging utility for Unix-like operating systems as well as Windows. While perusing through the zip man page, I discovered some useful options for protecting zip archives.

Read Also: How to Password Protect a Vim File in Linux

In this post, I will show you how to create a password protected zip file on the terminal in Linux. This will help you learn a practical way of encrypting and decrypting contents of zip archive files.

First install zip utility in your Linux distribution using the package manger as shown.

$ sudo yum install zip    [On CentOS/RHEL]
$ sudo dnf install zip    [On Fedora 22+]
$ sudo apt install zip    [On Debian/Ubuntu]

How to Create Password Protected ZIP in Linux

Once installed, you can use the zip command with -p flag to create a password protected zip archive called ccat-command.zip from the directory of files called ccat-1.1.0 as follows.

$ zip -p pass123 ccat-command.zip ccat-1.1.0/

However, the above method is absolutely insecure, because here the password is provided as clear-text on the command line. Secondly, it will also be stored in the history file (e.g ~.bash_history for bash), meaning another user with access to your account (more especially root user) will easily see the password.

Therefore, try to always use the -e flag, it shows a prompt allowing you to enter a hidden password as shown.

$ zip -e ccat-command.zip ccat-1.1.0/

How to Unzip Password Protected ZIP in Linux

To unzip and decrypt the content of the archive file called ccat-command.zip, use the unzip program and provide the password you entered above.

$ unzip ccat-command.zip

That’s It! In this post, I described how to create a password protected zip file on the terminal in Linux. If you have any queries, or other useful related tip/tricks to share, use the comment form below ping us.

Source

In this article, i will explain how to solve “failed to mount /etc/fstab” boot error in Linux. The file in question contains descriptive information concerning the filesystems the system can mount automatically at boot time.

This information is static and is read by other programs on the system such as mount, umount, dump and fsck. It has six important filesystem mount specification fields: the first field describes the block special device or remote filesystem to be mounted, the second field defines the mount point for the filesystem and the third specifies the filesystem type.

The fourth field defines the mount options associated with the filesystem, and the fifth field is read by dump tool. The last field is used by fsck tool to establish the order of filesystem-checks.

Suggested Read: How to Determine and Fix Boot Issues in Linux

After editing the /etc/fstab to create an automount and rebooting my system; it booted into emergency mode showing the error message below.

I logged in as root from the interface above, and typed the following command to look through the systemd journal; then I saw the errors shown in the screen shot (indicated using red).

As you can see, the main error (failure of etc-fstab.mount unit) lead to several other errors (systemd unit dependency issues) such as failure of local-fs.target, rhel-autorelabel-mark.service etc.

# journalctl -xb

Causes of “failed to mount /etc/fstab” Error in Linux

The error above may result from any of the issues below, in the /etc/fstab file:

• missing /etc/fstab file
• wrong specification of filesystem mount options,
• failing mount points or
• unrecognized characters in the file.

To solve it, you can use the original file if you created a backup, otherwise comment out any changes you made using the “#” character (and also ensure that all the uncommented lines are filesystem mount lines).

So I opened the /etc/fstab using vi/m text editor to check for any errors.

# vi /etc/fstab

I realized I had typed an “r” letter at the beginning of the file as shown in the screen shot above – this was recognized by the system as a special device which did not actually exist in the filesystem, thus resulting to the sequential errors shown above.

This took me several hours before noticing and fixing it. So I had to remove the letter, commented out the first line in the file, closed and saved it. After running a reboot, the system booted well again.

How to Avoid Such Issues in the Future

To avoid encountering such issues on your system, take note of the following:

Always create a backup of your config files before editing them. In case of any errors in your configs, you can revert to the default/working file.

For instance:

# cp /etc/fstab /etc/fstab.orig

Secondly, check config files for any errors before saving them, certain applications offer utilities to check syntax of config files before running the application. Use these utilities where possible.

However, if you happen to get any system errors messages:

First look through the systemd journal using the journalctl utility to determine what exactly caused them:

# journal -xb

If you can’t resolve the errors in one way or the other, run to any of the millions of Linux forums on the web and post the issue there.

Do check out some useful related articles.

1. A Basic Guide to Linux Boot Process
2. 4 Best Linux Boot Loaders
3. Manage Log Messages Under Systemd Using Journalctl [Comprehensive Guide]
4. Managing System Startup Process and Services (SysVinit, Systemd and Upstart)
5. Process Management in RHEL 7: Boot, Shutdown, and Everything in Between

That’s it for now. In this article, I explained how to solve the “failed to mount /etc/fstab” boot error in Linux. Once again, to avoid such issues (or if you encounter any boot issues), remember to follow the guidelines offered above. Lastly, you can add your thoughts to this guide via the feedback form below.

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In Unix-like operating systems such as Linux, “everything is a file” and a file is fundamentally a link to an inode (a data structure that stores everything about a file apart from its name and actual content).

A hard link is a file that points to the same underlying inode, as another file. In case you delete one file, it removes one link to the underlying inode. Whereas a symbolic link (also known as soft link) is a link to another filename in the filesystem.

Read Also: How to Perform File and Directory Management

Another important difference between the two types of links is that hard links can only work within the same filesystem while symbolic links can go across different filesystems.

How to Create Hard Links in Linux

To create a hard links in Linux, we will use ln utility. For example, the following command creates a hard link named tp to the file topprocs.sh.

$ ls -l
$ ln topprocs.sh tp
$ ls -l

Looking at the output above, using ls command, the new file is not indicated as a link, it is shown as a regular file. This implies that tp is just another regular executable file that points to the same underlying inode as topprocs.sh.

To make a hard link directly into a soft link, use the -P flag like this.

$ ln -P topprocs.sh tp

How to Create Symbolic Links in Linux

To create a symbolic links in Linux, we will use same ln utility with -s switch. For example, the following command creates a symbolic link named topps.sh to the file topprocs.sh.

$ ln -s ~/bin/topprocs.sh topps.sh
$ ls -l topps.sh

From the above output, you can see from the file permissions section that topps.sh is a link indicated by l:meaning it is a link to another filename.

If the symbolic link already exist, you may get an error, to force the operation (remove exiting symbolic link), use the -f option.

$ ln -s ~/bin/topprocs.sh topps.sh
$ ln -sf ~/bin/topprocs.sh topps.sh

To enable verbose mode, add the -v flag to prints the name of each linked file in the output.

$ ln -sfv ~/bin/topprocs.sh topps.sh
$ $ls -l topps.sh

That’s It! Do check out these following related articles.

1. fdupes – A Command Line Tool to Find and Delete Duplicate Files in Linux
2. 5 Useful Commands to Manage File Types and System Time in Linux

In this article, we’ve learned how to create hard and symbolic links in Linux. You can ask any question(s) or share your thoughts about this guide via the feedback form below.

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