Using Bash Shell Parameter Expansions

Knowing shell parameter extensions is quite handy to manipulate parameters in Bash. You can remove and replace characters, select substrings, check if a variable is present, find variables and convert strings to upper and lower case.

A simple example is to emulate the behavior of the coreutils commands dirname and basename. After setting a variable FILE=/tmp/a.txt the output of dirname $FILE and echo ${FILE%/*} is /tmp. We can also create a parameter expansion that emulates basename $FILE that returns a.txt: echo ${FILE##*/}.

As you can see from these two examples parameter expansions are powerful. It’s also obvious that the parameter expansions are quite cryptic. They do not use a syntax that is used elsewhere and are hard to remember. The advantage of parameter expansions is that you can use them directly for ad hoc scripting in the shell.

The remainder of this blog demonstrates all parameter expansions with examples. The Bash manual contains a complete description of parameter expansions.

Examples

Lines starting with $ contain the command line input and lines starting with > contain the output of commands. Each section contains a shell session - all variables have the scope of this section.

Get the value of a variable

$ A=value
$ echo $A
> value
$ echo ${A}
> value

Assign variable if null

$ echo $F
>
$ echo ${F:=A}
> A
$ echo ${F:=B}
> A
$ echo $F
> A

Substitute variable if null

$ echo $F
>
$ echo ${F:-A}
> A
$ echo $F
>
$ G=B
$ echo ${G:-A}
> B

Exit shell if variable is null

$ bash -c "echo ${A:?died}"
> bash: A: died
$ echo $?  #return code
> 1
$ A=ok
$ bash -c "echo ${A:?died}"
> ok
$ echo $?  #return code
> 0

Substitute variable if not null

$ echo ${F:+A}
>
$ F=G
$ echo ${F:+A}
> A

Substrings from index

$ A=abcdefg
$ echo ${A:1}
> bcdefg
$ echo ${A:1:3} #with optional length parameter
> bcd

Length of a string

$ A=abc
$ echo ${#A}
> 3

Delete from left

$ A=abcdefabc

$ echo ${A#a}
> bcdefabc

$ echo ${A#ab}
> cdefabc
$ echo ${A#b} #no match
> abcdefabc
$ echo ${A#?b}
> cdefabc
$ echo ${A#?c} #no match
> abcdefabc
$ echo ${A#*c}
> defabc

$ echo ${A#*b} #shorted match
> cdefabc
$ echo ${A##*b} #longest match
> c

Delete from right

$ A=abcdefabc

$ echo ${A%c}
> abcdefab

$ echo ${A%b*} #shortest match
> abcdefa
$ echo ${A%%b*} #longest match
> a

Replace variable value

$A=abcdefabc

$ echo ${A/a/x}
> xbcdefabc
$ echo ${A/b/x}
> axcdefabc
$ echo ${A/#a/x} #from left
> xbcdefabc
$ echo ${A/#b/x}
> abcdefabc
$ echo ${A/%c/x} #from right
> abcdefabx
$ echo ${A/%b/x}
> abcdefabc
$ echo ${A/b*b/x}
> axc
$ echo ${A//b/x} #replace all
> axcdefaxc
$ echo ${A//b}
> acdefac

Case conversion

$ A=abc
$ echo ${A^} #upper case first
> Abc
$ echo ${A^^} #upper case all
> ABC
$ B=DEF
$ echo ${B,} #lower case first
> dEF
$ echo ${B,,} #lower case all
> def

Find variable name with prefix

$ echo ${!US*}
> USER

Get the value of a parameter defined by the value of another parameter

$ A=B
$ B=b_value
$ echo ${!A} #Indirect expansion
> b_value

You should always keep in mind that the shell parameter expansions can be used with nested parameter expansion, tilde expansions, command substitutions and arithmetic expansions.

Used together with parameter expansions

$ A=abc
$ B=a
$ echo ${A#$B}
> bc
$ echo ${A#$B?}
> c

Used together with tilde expansion

$ A=./a
$ echo ${A/#./~}
> /home/user/a

Used together with command substitution

$ A=./a
$ cd /usr
$ echo ${A/#./$(pwd)}
> /usr/a

Used together with arithmetic expansion

$ echo ${A/#./$((1 + 10))}
> 11/a

After getting so far in the examples I suggest you have also a look at the other shell expansions supported by Bash.