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Getting decent error reports in Bash when you're using 'set -e'

July 23, 2025

Suppose that you have a shell script that's not necessarily complex
but is at least long. For reliability, you use 'set -e' so that the
script will immediately stop on any unexpected errors from commands,
and sometimes this happens. Since this isn't supposed to happen, it
would be nice to print some useful information about what went wrong,
such as where it happened, what the failing command's exit status
was, and what the command was. The good news is that if you're
willing to make your script specifically a Bash script, you can do
this quite easily.

The Bash trick you need is:

    trap 'echo "Exit status $? at line $LINENO from: $BASH_COMMAND"' ERR

This uses three Bash features: the special '$LINENO' and
'$BASH_COMMAND' environment variables (which have the command
executed just before the trap and the line number), and the special
'ERR' Bash 'trap' condition that causes your 'trap' statement to be
invoked right when 'set -e' is causing your script to fail and exit.

Using 'ERR' instead of 'EXIT' (or '0' if you're a traditionalist like
me) is necessary in order to get the correct line number in Bash. If
you switch this to 'trap ... EXIT', the line number that Bash will
report is the line that the 'trap' was defined on, not the line that
the failing command is on (although the command being executed
remains the same). This makes a certain amount of sense from the
right angle; the shell is currently on that line as it's exiting.

As far as I know, no other version of the Bourne shell can do all of
this. The OpenBSD version of /bin/sh has a '$LINENO' variable and
'trap ... 0' preserves its value (instead of resetting it to the line
of the 'trap'), but it has no access to the current command. The
FreeBSD version of /bin/sh resets '$LINENO' to the line of your 'trap
... 0', so the best you can do is report the exit status. Dash, the
Ubuntu 24.04 default /bin/sh, doesn't have '$LINENO', effectively
putting you in the same situation as FreeBSD.

(On Fedora, /bin/sh is Bash, and the Fedora version of Bash supports
all of 'trap .. ERR', $LINENO, and $BASH_COMMAND even when invoked as
'#!/bin/sh' by your script. You probably shouldn't count on this; if
you want Bash, use '#!/bin/bash'.)

(One comment.)
Written on 23 July 2025.

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