How to encapsulate shell execution into a function

Question

I have a script that needs to execute many commands, and I want to be able to encapsulate the execution as a function.

The sample functions are as follows

#!/bin/bash

cmd="redis-cli info | grep cluster_enabled"
cmd1="cmd1"
cmd2="cmd2"

exec(){
    local cmd=$1
    EXEC_RET=$($cmd 2>&1)
    EXEC_STATUS=$?
    echo "$EXEC_RET"
    echo "$EXEC_STATUS"
}

main(){
    exec "$cmd"
}

main "$@"

However, the following error is reported after execution

$ bash test.sh
ERR syntax error
0

But if I don't extract the execution command, it won't report an error

exec(){
    EXEC_RET=$(redis-cli info | grep cluster_enabled 2>&1)
    EXEC_STATUS=$?
    echo "$EXEC_RET"
    echo "$EXEC_STATUS"
}
main(){
    exec 
}

main "$@"

# stdout
$ bash temp.sh 
cluster_enabled:1
0

What causes this error and how to solve it? I really appreciate any help with this.

Don't store commands in variables. Variables are for data, not for executable code. See BashFAQ #50: "I'm trying to put a command in a variable, but the complex cases always fail!" — Gordon Davisson
– Gordon Davisson, Commented Jan 10, 2023 at 9:02
Don't use exec as a function name - that name is already a shell built-in — Chris Davies
– Chris Davies, Commented Jan 10, 2023 at 10:17

Stéphane Chazelas · Accepted Answer · 2023-01-10 13:03:14Z

redis-cli info | grep cluster_enabled is shell syntax to execute /path/to/redis-cli and /path/to/grep commands, the first one with redis-cli and info as two separate arguments and second one with grep and cluster_enabled as two separate arguments, concurrently and with a pipe connecting the stdout of the first with the stdin of the second.

$cmd, as you use in $(cmd 2>&1) expands the $cmd variable, does IFS-splitting on it (note that it has nothing to do with what the shell does when it parses a command line other than space and tab are in the default value of $IFS also happen to be token delimiters in the shell syntax), followed by filename generation (so called split+glob), and executes the command derived from the first resulting word with all the words as arguments. So it's neither evaluating the code in $cmd, nor running the command whose name is stored in $cmd, for which you'd need "$cmd".

So, it looks like you want to evaluate the argument passed to your function as code in the shell syntax as opposed to executing a command.

Also, exec is a special builtin of the shell already, and with several shells, including bash when called as sh or more generally is in POSIX mode, can't be redefined as a function.

So

run(){
    local shell_code="$1"
    RUN_STDOUT_AND_ERR=$(eval " $shell_code" 2>&1)
    RUN_STATUS="$?"
    printf '%s\n' "$RUN_STDOUT_AND_ERR" "$RUN_STATUS"
    return "$RUN_STATUS"
}

run "$cmd"

exec can be redefined (and used) as a function in bash version 4.4.12(3)-release. When you later unset its definition the word reverts to being the expected built-in — Chris Davies
– Chris Davies, Commented Jan 10, 2023 at 10:19
@roaima, not in POSIX mode. Sorry I had omitted to mention that. — Stéphane Chazelas
– Stéphane Chazelas, Commented Jan 10, 2023 at 13:05

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