Dynamic variable names in Bash

Question

I am confused about a bash script.

I have the following code:

function grep_search() {
    magic_way_to_define_magic_variable_$1=`ls | tail -1`
    e         


        

Answer 1

Use declare

There is no need on using prefixes like on other answers, neither arrays. Use just declare, double quotes, and parameter expansion.

I often use the following trick to parse argument lists contanining one to n arguments formatted as key=value otherkey=othervalue etc=etc, Like:

# brace expansion just to exemplify
for variable in {one=foo,two=bar,ninja=tip}
do
  declare "${variable%=*}=${variable#*=}"
done
echo $one $two $ninja 
# foo bar tip

But expanding the argv list like

for v in "$@"; do declare "${v%=*}=${v#*=}"; done

---

Extra tips

# parse argv's leading key=value parameters
for v in "$@"; do
  case "$v" in ?*=?*) declare "${v%=*}=${v#*=}";; *) break;; esac
done
# consume argv's leading key=value parameters
while (( $# )); do
  case "$v" in ?*=?*) declare "${v%=*}=${v#*=}";; *) break;; esac
  shift
done

Answer 2

For indexed arrays, you can reference them like so:

foo=(a b c)
bar=(d e f)

for arr_var in 'foo' 'bar'; do
    declare -a 'arr=("${'"$arr_var"'[@]}")'
    # do something with $arr
    echo "\$$arr_var contains:"
    for char in "${arr[@]}"; do
        echo "$char"
    done
done

Associative arrays can be referenced similarly but need the -A switch on declare instead of -a.

Answer 3

I want to be able to create a variable name containing the first argument of the command

script.sh file:

#!/usr/bin/env bash
function grep_search() {
  eval $1=$(ls | tail -1)
}

Test:

$ source script.sh
$ grep_search open_box
$ echo $open_box
script.sh

---

As per help eval:

Execute arguments as a shell command.

---

You may also use Bash ${!var} indirect expansion, as already mentioned, however it doesn't support retrieving of array indices.

---

For further read or examples, check BashFAQ/006 about Indirection.

We are not aware of any trick that can duplicate that functionality in POSIX or Bourne shells without eval, which can be difficult to do securely. So, consider this a use at your own risk hack.

However, you should re-consider using indirection as per the following notes.

Normally, in bash scripting, you won't need indirect references at all. Generally, people look at this for a solution when they don't understand or know about Bash Arrays or haven't fully considered other Bash features such as functions.

Putting variable names or any other bash syntax inside parameters is frequently done incorrectly and in inappropriate situations to solve problems that have better solutions. It violates the separation between code and data, and as such puts you on a slippery slope toward bugs and security issues. Indirection can make your code less transparent and harder to follow.

Answer 4

This should work:

function grep_search() {
    declare magic_variable_$1="$(ls | tail -1)"
    echo "$(tmpvar=magic_variable_$1 && echo ${!tmpvar})"
}
grep_search var  # calling grep_search with argument "var"

Answer 5

This will work too

my_country_code="green"
x="country"

eval z='$'my_"$x"_code
echo $z                 ## o/p: green

In your case

eval final_val='$'magic_way_to_define_magic_variable_"$1"
echo $final_val

Answer 6

An extra method that doesn't rely on which shell/bash version you have is by using envsubst. For example:

newvar=$(echo '$magic_variable_'"${dynamic_part}" | envsubst)