The bash man page is close to 40K words. It’s not
\nquite War and Peace, but it could hold its
\nown in a rack of cheap novels.
\nGiven the size of bash’s documentation, missing a useful feature
\nis easy to do when looking through the man page.
\nFor that reason, as well as to look for new features,
\nrevisiting the man page occasionally can be a useful
\nthing to do.<\/p>\n
The sub-section of interest today is Parameter Expansion\u2014that is, $var in its many forms.
\nDon’t be confused by the name though, it’s really about parameter
\nand variable expansion.<\/p>\n
I’m not going to cover all the different forms of parameter Gone are the days of using tr ‘[[:lower:]]’ ‘[[:upper:]]’ $ a=hello And for going to lowercase:<\/p>\n $ a=HELLO You also can specify a character after the operator and change $ a=hello Need a list of all the variables whose names match a certain prefix? Do this:<\/p>\n $ mya=1 Bash even can give you a taste of the good-old days of programming $ var=somevalue What’s happening here is that the value of var_name gives you As a bit of an aside, because it’s not really about “parameter $ var=no The variable “ref” is a reference to the variable “var”. $ cat nref.sh<\/b> aval=old_value Running that, you get:<\/p>\n $ bash nref.sh<\/b> Since indirection is automatic with nameref variables, There are also a number of expansions of the form $, $ declare -a array=(1 2) Check the man page for more information about these “@” expansions.<\/p>\n And to wrap it up, one other subtle thing that can be easy to unset var var= And now if you leave out the colon:<\/p>\n unset var var= So leaving out the colon changes the test from “unset or null” to If something doesn’t seem to work, check your bash version:<\/p>\n $ echo $BASH_VERSION
\nexpansion here, just some that I think may not be as
\nwidely known as others.
\nIf you’re completely new to parameter expansion,
\ncheck out my ancient post<\/a> or one of the
\nmany articles elsewhere on the internet.<\/p>\nCase Conversion<\/h3>\n
\nto convert strings to uppercase:<\/p>\n
\n$ echo $ # First character only
\nHello
\n$ echo $ # All characters
\nHELLO<\/p>\n
\n$ echo $ # First character only
\nhELLO
\n$ echo $ # All characters
\nhello<\/p>\n
\nthe case only of characters that match:<\/p>\n
\n$ echo $ # First character if it is an ‘l’
\nhello
\n$ echo $ # All characters that are ‘l’s
\nheLLo<\/p>\nNames Starting with Some Prefix<\/h3>\n
\n$ myb=2
\n$ yourc=3
\n$ echo ${!my*}
\nmya myb<\/p>\nIndirection<\/h3>\n
\nC and Assembler and using indirect addressing\u2014well sort of:<\/p>\n
\n$ var_name=var
\n$ echo ${!var_name}
\nsomevalue<\/p>\n
\nthe name of the actual variable to be expanded.
\nThat variable then is expanded and becomes the result of the
\nexpansion.
\nIn this case, “var_name” has the value “var”,
\nso the variable “var” is expanded to yield the ultimate
\nvalue of “somevalue”.<\/p>\nShort Detour into Namerefs<\/h3>\n
\nexpansion”, let’s take a quick look at namerefs in bash.
\nA nameref variable is a variable that references another variable:<\/p>\n
\n$ declare -n ref=var # -n == nameref
\n$ ref=yes
\n$ echo $ref
\nyes<\/p>\n
\nWhen you assign to “ref”, you actually change the value of “var”.
\nThis can be particularly handy in getting values out of a
\nfunction by passing the name of a variable to the function:<\/p>\n
\nfunction func()
\n{
\nlocal -n up_value=$1 # -n == nameref
\nup_value=new_value
\necho “Changing ‘${!up_value}’ in $”
\n}<\/p>\n
\necho
\necho “Before function call, aval is $aval”
\nfunc aval # pass var *name* to func
\necho “After function call, aval is $aval”<\/p>\n
\nBefore function call, aval is old_value
\nChanging ‘aval’ in func
\nAfter function call, aval is new_value<\/p>\n
\nyou don’t use the exclamation point expansion to get the value
\nof the referenced variable; normal $var expansion works.
\nIn the case of namerefs, the exclamation point expansion
\nyields a different result: the name of the referenced variable.
\nSo, this slight detour dealt with parameter expansion after all.<\/p>\nTransformation<\/h3>\n
\nwhere the “?” is one of the letters “Q”, “E”, “P”, “A” or “a”
\nthat can transform the value or get you information about
\nthe variable itself.
\nFor example:<\/p>\n
\n$ echo Attributes: $
\nAttributes: a # i.e. array was declared with -a<\/p>\nUnset or Null<\/h3>\n
\noverlook when reading the parameter expansion section relates
\nto the colon (:) in many of the expansions.
\nFor example, the :- form of expansion allows a default value
\nto be specified if a variable is unset or null:<\/p>\n
\n$ echo var: $
\nvar: default<\/p>\n
\n$ echo var: $
\nvar: default<\/p>\n
\n$ echo var: $
\nvar: default<\/p>\n
\n$ echo var: $
\nvar:<\/p>\n
\njust a test for “unset”.
\nThis applies to the :-, :=, :?, and :+ forms of
\nparameter expansion as well.<\/p>\nYour Mileage May Vary<\/h3>\n
\n4.4.23(1)-release<\/p>\n