Bash – Best practice to use $? in bash

basherror handlingshell

When I read this answer about $? another question comes to mind.

Is there any best practice for how to use $? in bash?

Let's have a example:

We have a linear script and I we would like to know that all the command was executed ok.
Do you think it is ok to call a small function (let's call it "did_it_work"),
to check the error code and break if it's not.

#!/bin/bash 

function did_it_work {
    code=$1
    if [ "$code" -ne "0" ]
    then
        echo "Error failure: code $code "
        exit 1
    fi
}

dir=some/path

mkdir -p $dir
did_it_work $? 

cd $dir
did_it_work $? 

run_some_command
did_it_work $?

This approach of course means that I have to manually solve the problem if there is any and rerun the script.

Do you think this is a good idea or is there some other best practice to do this?

/Thanks

Best Answer

One common way is:

die() {
    IFS=' ' # make sure "$*" is joined with spaces

    # output the arguments if any on stderr:
    [ "$#" -eq 0 ] || printf '%s\n' "$*" 1>&2
    exit 1
}

then you use it like this:

mkdir -p some/path || die "mkdir failed with status $?"

Or if you want it to include the exit status, you could change it to:

die() {
    last_exit_status=$?
    IFS=' '
    printf '%s\n' "FATAL ERROR: $* (status $last_exit_status)" 1>&2
    exit 1
}

and then using it is a bit easier:

mkdir -p some/path || die "mkdir failed"

When it fails, mkdir will likely already have issued an error message, so that second one may be seen as redundant, and you could just do:

mkdir -p some/path || exit   # with the same (failing) exit status as mkdir's
mkdir -p some/path || exit 1 # with exit status 1 always

(or use the first variant of die above without argument)

Just in case you haven't seen command1 || command2 before, it runs command1, and if command1 fails, it runs command2.

So you can read it like "make the directory or die".

Your example would look like:

mkdir -p some/path || die "mkdir failed"
cd some/path || die "cd failed"
some_command || die "some_command failed"

Or you can align the dies further on the right so that the main code is more obvious.

mkdir -p some/path         || die "mkdir failed"
cd some/path               || die "cd failed"
some_command               || die "some_command failed"

Or on the following line when the command lines are long:

mkdir -p some/path ||
  die "mkdir failed"

cd some/path ||
  die "cd failed"

some_command ||
  die "some_command failed"

Also, if you are going to use the name some/path multiple times, store it in a variable so you don't have to keep typing it, and can easily change it if you need to. And when passing variable arguments to commands, make sure to use the -- option delimiter so that the argument is not taken as an option if it starts with -.

dir=some/path
mkdir -p -- "$dir"         || die "Cannot make $dir"
cd -P -- "$dir"            || die "Cannot cd to $dir"
some_command               || die "Cannot run some_command"
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