Passing Command to Function in Bash - bash

Question

I am creating a setup bash script so I can quickly setup my servers.
Inside the run function I want to pass the command, which the run function will then verify whether is successful.
function run () {
if output=$( $1 );
then printf 'OK. («%s»)\n' "$output";
else printf 'Failed! («%s»)\n' "$output";
fi
}
printf 'Setting up «uni» as system group...'
run " if [ ! $( getent group uni ) ];
then sudo addgroup --system uni;
else echo 'Group exists.';
fi "
However this results in a error: setup.sh: line 5: if: command not found
When I do this it works fine, but I want to eliminate repetitive code as I ha ve many commands:
if output=$(
if [ ! $( getent group uni ) ]; then sudo addgroup --system uni; else echo 'Group exists.'; fi
);
then printf 'OK. («%s»)\n' "$output";
else printf 'Failed! («%s»)\n' "$output";
fi
What am I doing wrong?

Answer 1

The safe way to pass code to functions is... to encapsulate that code in another function.
run() {
local output
if output=$("$#"); then
printf 'OK. («%s»)\n' "$output";
else
printf 'Failed! («%s»)\n' "$output";
fi
}
printf 'Setting up «uni» as system group...'
step1() {
if [ ! "$(getent group uni)" ]; then
sudo addgroup --system uni;
else
echo 'Group exists.';
fi
}
run step1
If your code didn't involve flow control operators (and otherwise fit into the definition of a "simple command"), you wouldn't even need to do that; with the above run definition (using "$#" instead of $1),
run sudo addgroup --system uni
...would work correctly as-is.
Using either eval or sh -c exposes you to serious security problems; see BashFAQ #48 for a high-level overview, and see BashFAQ #50 for a discussion of why code shouldn't be passed around as text (and preferred ways to avoid the need to do so in the first place!)

Answer 2

You have chosen to pass a shell command string to your function (aka system(3) semantics). This means that you have to use eval to evaluate it:
function run () {
if output=$( eval "$1" );
then printf 'OK. («%s»)\n' "$output";
else printf 'Failed! («%s»)\n' "$output";
fi
}
Note that the parameter with your command will be expanded as normal, so if you want the $(getent) to be evaluated in the run function instead of before it, you will need to escape it:
run " if [ ! \$( getent group uni ) ];
then sudo addgroup --system uni;
else echo 'Group exists.';
fi "

Answer 3

If you want to pass a bash piece of code as a function args to be evaluated in a subshell and test the success, you can do it like that:
#!/bin/bash
run () {
# Here I'm not sure what you want to test, so I assume that you want to test if the piece of bash pass as an arg is failing or not
# Do not forget to quote the value in order to take all the string
# You can also try with "eval" instead of "bash -c"
if output=$( bash -c "${1}" ); then
echo "OK. («${output}»)"
else
echo "Failed! («${output}»)"
fi
}
# Do not forget to escape the $ here to be evaluated after in the `run` function
# Do not forget to exit with a code different of 0 to indicate there is a failure for the test in the "run" function
run "if [[ ! \$( getent group uni ) ]]; then sudo addgroup --system uni else echo 'Group exists.'; exit 1; fi"
Warning: I posted this piece of code to make your script works the way you seems to want (have a function which will eval your code from a string and interpret the result) but it's not safe at all for this reasons as Charles Duffy pointed out in the comment section.
It's probably safer to make something like:
#!/bin/bash
run () {
if output=$("${#}"); then
echo "OK. («${output}»)"
else
echo "Failed! («${output}»)"
fi
}
one_of_your_functions_you_want_to_eval() {
if [[ ! $( getent group uni ) ]]; then
sudo addgroup --system uni
else
echo 'Group exists.'
# do not forget to exit with something different than 0 here
exit 1
fi
}
run one_of_your_functions_you_want_to_eval
Note: in order to declare a function, either you declare it like that (POSIX compliant syntax):
your_function() {
}
Either using a bash only syntax (I think it's better to avoid "bashism" when it not bring real values to your scripts):
function your_function {
}
But no need to mix both syntaxes ;)