Question

What is the correct way to call some command stored in variable?

Are there any differences between 1 and 2?

#!/bin/sh
cmd="ls -la $APPROOTDIR | grep exception"

#1
$cmd

#2
eval "$cmd"

Answer 1

Unix shells operate a series of transformations on each line of input before executing them. For most shells it looks something like this (taken from the Bash man page):

• initial word splitting
• brace expansion
• tilde expansion
• parameter, variable and arithmetic expansion
• command substitution
• secondary word splitting
• path expansion (aka globbing)
• quote removal

Using $cmd directly gets it replaced by your command during the parameter expansion phase, and it then undergoes all following transformations.

Using eval "$cmd" does nothing until the quote removal phase, where $cmd is returned as is, and passed as a parameter to eval, whose function is to run the whole chain again before executing.

So basically, they’re the same in most cases and differ when your command makes use of the transformation steps up to parameter expansion. For example, using brace expansion:

$ cmd="echo foo{bar,baz}"

$ $cmd
foo{bar,baz}

$ eval "$cmd"
foobar foobaz