Bash: execute content of variable including pipe [duplicate]

Question

#!/bin/bash

# 1st part
ret=$(ps aux | grep -v grep)    # thats OK 
echo $ret

# 2nd part
cmd="ps aux | grep -v grep"     # a problem with the pipe | 
ret=$($cmd)         
echo $ret

How can I use a command-string as I have in the 2nd part? Think the pipe is the problem. Tried to escape it but it did not help. Get some snytax error of ps.

Thanks!

Answer 1

You need eval:

ret=$(eval "$cmd")

Answer 2

Using eval is not recommended here. It can lead to unexpected results, especially when variables can be read from untrusted sources (See BashFAQ/048 - Eval command and security issues.

You can solve this in a simple way by defining and calling a function as below

ps_cmd() {
    ps aux | grep -v grep
}

and use it in the script as

output="$(ps_cmd)"
echo "$output"

Also a good read would be to see why storing commands in a variable is not a good idea and has a lot of potential pitfalls - BashFAQ/050 - I'm trying to put a command in a variable, but the complex cases always fail!