#!/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 $ret
How can I use a command-string as I have in the 2nd part? Think the pipe is the problem. Tried to escape it but it did not help. Get some snytax error of ps.
You need eval:
ret=$(eval "$cmd")
ret=eval $cmd runs $cmd with ret temporarily set to the string eval, and because the eval is data there instead of a command, the invocation of $cmd is subject to the bugs in BashFAQ #50. Second: if you don't quote the "$cmd", then its contents get word-split and glob-expanded before being glued back into a single string and passed to eval. The details of exactly how that corrupts your command depends on the details of the command and the files in your current working directory, but it's basically never something good.
Commented
Aug 31, 2023 at 15:30
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!
