How do I get the effect and usefulness of “set -e” inside a shell function?

Question

set -e (or a script starting with #!/bin/sh -e) is extremely useful to automatically bomb out if there is a problem. It saves me having to error ch

Answer 1

You may directly use a subshell as your function definition and set it to exit immediately with set -e. This would limit the scope of set -e to the function subshell only and would later avoid switching between set +e and set -e.

In addition, you can use a variable assignment in the if test and then echo the result in an additional else statement.

# use subshell for function definition
f() (
   set -exo pipefail
   echo a
   false
   echo Should NOT get HERE
   exit 0
)

# next line also works for non-subshell function given by agsamek above
#if ret="$( set -e && f )" ; then 
if ret="$( f )" ; then
   true
else
   echo "$ret"
fi

# prints
# ++ echo a
# ++ false
# a

Answer 2

You will need to call your function in a sub shell (inside brackets ()) to achieve this.

I think you want to write your script like this:

#!/bin/sh -e

my_function() {
    echo "the following command could fail:"
    false
    echo "this is after the command that fails"
}

(my_function)

if [ $? -ne 0 ] ; then
    echo "dealing with the problem"
fi

echo "run this all the time regardless of the success of my_function"

Then the output is (as desired):

the following command could fail:
dealing with the problem
run this all the time regardless of the success of my_function

Answer 3

From documentation of set -e:

When this option is on, if a simple command fails for any of the reasons listed in Consequences of Shell Errors or returns an exit status value > 0, and is not part of the compound list following a while, until, or if keyword, and is not a part of an AND or OR list, and is not a pipeline preceded by the ! reserved word, then the shell shall immediately exit.

In your case, false is a part of a pipeline preceded by ! and a part of if. So the solution is to rewrite your code so that it isn't.

In other words, there's nothing special about functions here. Try:

set -e
! { false; echo hi; }

Answer 4

Note/Edit: As a commenter pointed out, this answer uses bash, and not sh like the OP used in his question. I missed that detail when I originaly posted an answer. I will leave this answer up anyway since it might be interested to some passerby.

Y'aaaaaaaaaaaaaaaaaaallll ready for this?

Here's a way to do it with leveraging the DEBUG trap, which runs before each command, and sort of makes errors like the whole exception/try/catch idioms from other languages. Take a look. I've made your example one more 'call' deep.

#!/bin/bash

# Get rid of that disgusting set -e.  We don't need it anymore!
# functrace allows RETURN and DEBUG traps to be inherited by each
# subshell and function.  Plus, it doesn't suffer from that horrible
# erasure problem that -e and -E suffer from when the command 
# is used in a conditional expression.
set -o functrace

# A trap to bubble up the error unless our magic command is encountered 
# ('catch=$?' in this case) at which point it stops.  Also don't try to
# bubble the error if were not in a function.
trap '{ 
    code=$?
    if [[ $code != 0 ]] && [[ $BASH_COMMAND != '\''catch=$?'\'' ]]; then
        # If were in a function, return, else exit.
        [[ $FUNCNAME ]] && return $code || exit $code
    fi
}' DEBUG

my_function() {
    my_function2
}

my_function2() {
    echo "the following command could fail:"
    false
    echo "this is after the command that fails"
}

# the || isn't necessary, but the 'catch=$?' is.
my_function || catch=$?
echo "Dealing with the problem with errcode=$catch (⌐■_■)"

echo "run this all the time regardless of the success of my_function"

and the output:

the following command could fail:
Dealing with the problem with errcode=1 (⌐■_■)
run this all the time regardless of the success of my_function

I haven't tested this in the wild, but off the top of my head, there are a bunch of pros:

1. It's actually not that slow. I've ran the script in a tight loop with and without the functrace option, and times are very close to each other under 10 000 iterations.

2. You could expand on this DEBUG trap to print a stack trace without doing that whole looping over $FUNCNAME and $BASH_LINENO nonsense. You kinda get it for free (besides actually doing an echo line).

3. Don't have to worry about that shopt -s inherit_errexit gotcha.