[SOLVED] Bash "set -e" confusion

drtebi · 2017-02-22 21:54:03

Hello,

I hope someone can help me with this apparently simple problem.

It's probably best explained by just showing you the code:

#!/usr/bin/env bash


#set -e   # "interferes" with for loop?

REQUIRED=( 'rsync' 'nonexistantprogram' 'tar' )

checkRequired() {
    echo -n ">>> Checking required programs: "
    local NOT_FOUND=()
    for X in "${REQUIRED[@]}"; do
        TMP=$(which "${X}" 2>&1)
        if [[ $? != 0 ]]; then
            NOT_FOUND+=("${X}")
        fi
    done
    if [[ ${#NOT_FOUND[@]} != 0 ]]; then
        echo 'error'
        echo "the following programs are required: ${NOT_FOUND[@]}"
        exit 1
    fi
    echo 'ok'
}

checkRequired

The function is supposed to check each program in $REQUIRED for existence with "which". If it doesn't exist, it's thrown into the list $NOT_FOUND. Then, if $NOT_FOUND is not 0 in length, the program exits with a message.

The problem is, that I would like to use 'set -e' in the script, so that when an error occurs in another place in the script, the script won't keep running havoc... but if I include the 'set -e' command on the top of the script, then the loop will cause the program to exit because 'which' returns a non-zero exit status.

Is there a way to avoid that? E.g. to have 'set -e' but still be able to check for a non-zero exit code of a command?

Last edited by drtebi (2017-02-22 22:15:17)

Trilby · 2017-02-22 22:00:11

Just put a `set +e` before the loop, and `set -e` again after the loop.

Or, alternatively, just relace the for loop with a simplification:

    for X in "${REQUIRED[@]}"; do
        which "${X}" > /dev/null 2>&1 || NOT_FOUND+=("${X}")
    done

drtebi · 2017-02-22 22:08:09

That was quick! Thank you, it works.

Just wondering though, as far as I understand, if the error occurs in an 'if' statement or in the condition of a loop, than 'set -e' would not stop the script.
Is it possible to put the 'which ...' command into an 'if' statement?

ayekat · 2017-02-22 22:08:42

You do not seem to be using the value of $TMP, so how about simply

    for X in "${REQUIRED[@]}"; do
        if ! which "${X}" >/dev/null 2>&1; then
            ...

--edit--
ninja'd twice - but well... yes, Trilby's suggestion uses less lines

Last edited by ayekat (2017-02-22 22:10:08)

drtebi · 2017-02-22 22:14:38

Great, problem solved, marking thread as SOLVED.

Thank you!

drtebi · 2017-02-22 22:20:32

But just another question so I really understand what's going on...

If I do:

which "${X}" > /dev/null 2>&1 || NOT_FOUND+=("${X}")

Doesn't the 'which' command return an error, which in turn should cause 'set -e' to stop the script? Or is it the redirection to /dev/null that causes 'set -e' to ignore that?

Trilby · 2017-02-22 22:21:35

drtebi wrote:

Just wondering though, as far as I understand, if the error occurs in an 'if' statement or in the condition of a loop...

It's not really the conditional that makes it work, any time the final exit status of a pipeline is zero set -e will allow it to continue. That's why the `which ... || ...` works because if which fails, as long as the array can be appended to, the pipeline exits with a zero. This is covered in `man set`.

drtebi · 2017-02-22 22:27:44

OK, I understand now. I read 'man set', but found it a bit difficult to understand...

Interesting is also the 'set -o pipefail' option, which, if I am not mistaken, would cause the script to stop if the 'which' command fails.

I think I got it now, thanks again.

bulletmark · 2017-02-22 22:46:28

OP, you don't really need to make that an array. I would just do:

NOT_FOUND=""
REQUIRED="rsync nonexistantprogram tar"
for X in $REQUIRED; do
    type $X &>/dev/null || NOT_FOUND+=" $X"
done

I use type rather than which (e.g. see this and this). You specified bash so may as well use compact and neater looking &>.

Last edited by bulletmark (2017-02-22 22:47:28)

drtebi · 2017-02-22 23:58:29

Thanks. I am learning more and more neat tricks

Using mostly Python, it is a bit awkward using arrays in Bash. This 'for' loop would also do something different in Python... just need to make that switch in my head

Arch Linux

#1 2017-02-22 21:54:03

[SOLVED] Bash "set -e" confusion

#2 2017-02-22 22:00:11

Re: [SOLVED] Bash "set -e" confusion

#3 2017-02-22 22:08:09

Re: [SOLVED] Bash "set -e" confusion

#4 2017-02-22 22:08:42

Re: [SOLVED] Bash "set -e" confusion

#5 2017-02-22 22:14:38

Re: [SOLVED] Bash "set -e" confusion

#6 2017-02-22 22:20:32

Re: [SOLVED] Bash "set -e" confusion

#7 2017-02-22 22:21:35

Re: [SOLVED] Bash "set -e" confusion

#8 2017-02-22 22:27:44

Re: [SOLVED] Bash "set -e" confusion

#9 2017-02-22 22:46:28

Re: [SOLVED] Bash "set -e" confusion

#10 2017-02-22 23:58:29

Re: [SOLVED] Bash "set -e" confusion

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