Why does the following script work as expected (prints hello)
#!/bin/bash
foo=$(bash -c 'echo hello')
echo $foo
while this script:
#!/bin/bash
cmd="bash -c 'echo hello'"
foo=$($cmd)
echo $foo
gives the following error:
hello': -c: line 0: unexpected EOF while looking for matching `''
hello': -c: line 1: syntax error: unexpected end of file
In,
cmd="bash -c 'echo hello'"
$cmd
You're not running the bash -c 'echo hello' command, you're running the $cmd simple command.
That unquoted $cmd means invoking the split+glob operator. Here, with the default value of $IFS, the content of $cmd is split into bash, -c, 'echo and hello'. So, you're running bash with those 4 arguments, it's as if you had typed:
bash -c "'echo" "hello'"
And that 'echo code has a missing closing quote (the hello' argument goes into the $0 of that inline script).
If you want to evaluate the content of $cmd as shell code, it's
eval "$cmd"
So:
cmd="bash -c 'echo hello'"
foo=$(eval "$cmd")
echo "$foo"
Though you could also use your split+glob operator differently:
cmd='bash,-c,echo hello'
IFS=, # split on comma
set -f # disable glob
foo=$($cmd)
echo "$foo"
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