Bash Bash Parameter Expansion Parameter indirection
Example
Bash indirection permits to get the value of a variable whose name is contained in another variable. Variables example:
$ red="the color red"
$ green="the color green"
$ color=red
$ echo "${!color}"
the color red
$ color=green
$ echo "${!color}"
the color green
Some more examples that demonstrate the indirect expansion usage:
$ foo=10
$ x=foo
$ echo ${x} #Classic variable print
foo
$ foo=10
$ x=foo
$ echo ${!x} #Indirect expansion
10
One more example:
$ argtester () { for (( i=1; i<="$#"; i++ )); do echo "${i}";done; }; argtester -ab -cd -ef
1 #i expanded to 1
2 #i expanded to 2
3 #i expanded to 3
$ argtester () { for (( i=1; i<="$#"; i++ )); do echo "${!i}";done; }; argtester -ab -cd -ef
-ab # i=1 --> expanded to $1 ---> expanded to first argument sent to function
-cd # i=2 --> expanded to $2 ---> expanded to second argument sent to function
-ef # i=3 --> expanded to $3 ---> expanded to third argument sent to function
