I'm trying to create some error reporting using a Trap to call a function on all errors:

Trap "_func" ERR

Is it possible to get what line the ERR signal was sent from? The shell is bash.

If I do that, I can read and report what command was used and log/perform some actions.

Or maybe I'm going at this all wrong?

I tested with the following:


echo hello | grep "asdf"

And $LINENO is returning 2. Not working.

  • 1
    You can look at the bash debugger script bashdb. It seems that the first argument to trap can contain variables that are evaluated in the desired context. So trap 'echo $LINENO' ERR' should work. May 29 '12 at 18:53
  • hmm just tried this with a bad echo | grep command and it returns the line of the Trap statement. But I'll take a look at bashdb May 29 '12 at 18:56
  • I'm so sorry... I didn't specify in my original question that I need a native solution. I edited the question. May 29 '12 at 19:05
  • Sorry, I borked the example line: trap 'echo $LINENO' ERR. The first argument to trap is the entire echo $LINENO hardquoted. This is in bash. May 29 '12 at 19:43
  • 5
    @Mechaflash It would have to be trap 'echo $LINENO' ERR, with single quotes, not double quotes. With the command you wrote, $LINENO is expanded when line 2 is parsed, so the trap is echo 2 (or rather ECHO 2, which would output bash: ECHO: command not found). May 29 '12 at 23:56

As pointed out in comments, your quoting is wrong. You need single quotes to prevent $LINENO from being expanded when the trap line is first parsed.

This works:

#! /bin/bash

err_report() {
    echo "Error on line $1"

trap 'err_report $LINENO' ERR

echo hello | grep foo  # This is line number 9

Running it:

 $ ./test.sh
 Error on line 9
  • thanks for the example with a function call. I didn't know that double quotes expanded the variable in this case. May 30 '12 at 14:23
  • echo hello | grep foo doesn't seem to throw error for me. Am I misunderstanding something?
    – geotheory
    Dec 2 '15 at 22:15
  • @geotheory On my system grep has an exit status of 0 if there was a match, 1 if there was no match and >1 for an error. You can check the behavior on your system with echo hello | grep foo; echo $?
    – Patrick
    Dec 7 '15 at 23:17
  • No you're right it is an error :)
    – geotheory
    Dec 8 '15 at 9:56
  • Don't you need to use -e on the invocation line, to cause error on command failure? That is: #!/bin/bash -e ?
    – Tim Bird
    Mar 31 '17 at 22:39

You can also use the bash builtin 'caller':


err_report() {
  echo "errexit on line $(caller)" >&2

trap err_report ERR

echo hello | grep foo

it prints filename too:

$ ./test.sh
errexit on line 9 ./test.sh

I really like the answer given by @Mat above. Building on this, I wrote a little helper which gives a bit more context for the error:

We can inspect the script for the line which caused the failure:

err() {
    echo "Error occurred:"
    awk 'NR>L-4 && NR<L+4 { printf "%-5d%3s%s\n",NR,(NR==L?">>>":""),$0 }' L=$1 $0
trap 'err $LINENO' ERR

Here it is in a small test script:


set -e

err() {
    echo "Error occurred:"
    awk 'NR>L-4 && NR<L+4 { printf "%-5d%3s%s\n",NR,(NR==L?">>>":""),$0 }' L=$1 $0
trap 'err $LINENO' ERR

echo one
echo two
echo three
echo four
echo five
echo six
echo seven
echo eight

When we run it we get:

$ /tmp/test.sh
Error occurred:
12      echo two
13      echo three
14      echo four
15   >>>false
16      echo five
17      echo six
18      echo seven
  • 3
    This would be even better using $(caller)'s data to give the context even if the failure is not in the current script but one of its imports. Very nice though!
    – tricasse
    Mar 7 '19 at 18:42
  • I'm confused about "set -e", I think this means exit on error. But you're handling the error. I cannot understand the purpose. Jun 17 at 13:13
  • 1
    @TarekEldeeb - This isn't like catching an error in other languages, by the time that the error trap is called, the script is in the process of stopping. There's no option here to return to the command which caused the errexit to be invoked.
    – unpythonic
    Jun 18 at 17:04

Is it possible to get what line the ERR signal was sent from?

Yes, LINENO and BASH_LINENO variables are supper useful for getting the line of failure and the lines that lead up to it.

Or maybe I'm going at this all wrong?

Nope, just missing -q option with grep...

echo hello | grep -q "asdf"

... With the -q option grep will return 0 for true and 1 for false. And in Bash it's trap not Trap...

trap "_func" ERR

... I need a native solution...

Here's a trapper that ya might find useful for debugging things that have a bit more cyclomatic complexity...


## Outputs Front-Mater formatted failures for functions not returning 0
## Use the following line after sourcing this file to set failure trap
##    trap 'failure "LINENO" "BASH_LINENO" "${BASH_COMMAND}" "${?}"' ERR
    local -n _lineno="${1:-LINENO}"
    local -n _bash_lineno="${2:-BASH_LINENO}"
    local _last_command="${3:-${BASH_COMMAND}}"
    local _code="${4:-0}"

    ## Workaround for read EOF combo tripping traps
    if ! ((_code)); then
        return "${_code}"

    local _last_command_height="$(wc -l <<<"${_last_command}")"

    local -a _output_array=()
        "lines_history: [${_lineno} ${_bash_lineno[*]}]"
        "function_trace: [${FUNCNAME[*]}]"
        "exit_code: ${_code}"

    if [[ "${#BASH_SOURCE[@]}" -gt '1' ]]; then
        for _item in "${BASH_SOURCE[@]}"; do
            _output_array+=("  - ${_item}")
        _output_array+=("source_trace: [${BASH_SOURCE[*]}]")

    if [[ "${_last_command_height}" -gt '1' ]]; then
            'last_command: ->'
        _output_array+=("last_command: ${_last_command}")

    printf '%s\n' "${_output_array[@]}" >&2
    exit ${_code}

... and an example usage script for exposing the subtle differences in how to set the above trap for function tracing too...


#!/usr/bin/env bash

set -E -o functrace

## Optional, but recommended to find true directory this script resides in
while [[ -h "${__SOURCE__}" ]]; do
    __SOURCE__="$(find "${__SOURCE__}" -type l -ls | sed -n 's@^.* -> \(.*\)@\1@p')"
__DIR__="$(cd -P "$(dirname "${__SOURCE__}")" && pwd)"

## Source module code within this script
source "${__DIR__}/modules/trap-failure/failure.sh"

trap 'failure "LINENO" "BASH_LINENO" "${BASH_COMMAND}" "${?}"' ERR

something_functional() {
    _req_arg_one="${1:?something_functional needs two arguments, missing the first already}"
    printf 'something_functional: %s %s %s' "${_req_arg_one}" "${_opt_arg_one}" "${_opt_arg_two}"
    ## Generate an error by calling nothing

## Ignoring errors prevents trap from being triggered
something_functional || echo "Ignored something_functional returning $?"
if [[ "$(something_functional 'Spam!?')" == '0' ]]; then
    printf 'Nothing somehow was something?!\n' >&2 && exit 1

## And generating an error state will cause the trap to _trace_ it
something_functional '' 'spam' 'Jam'

The above where tested on Bash version 4+, so leave a comment if something for versions prior to four are needed, or Open an Issue if it fails to trap failures on systems with a minimum version of four.

The main takeaways are...

set -E -o functrace
  • -E causes errors within functions to bubble up

  • -o functrace causes allows for more verbosity when something within a function fails

trap 'failure "LINENO" "BASH_LINENO" "${BASH_COMMAND}" "${?}"' ERR
  • Single quotes are used around function call and double quotes are around individual arguments

  • References to LINENO and BASH_LINENO are passed instead of the current values, though this might be shortened in later versions of linked to trap, such that the final failure line makes it into output

  • Values of BASH_COMMAND and exit status ($?) are passed, first to get the command that returned an error, and second for ensuring that the trap does not trigger on non-error statuses

And while others may disagree I find it's easier to build an output array and use printf for printing each array element on it's own line...

printf '%s\n' "${_output_array[@]}" >&2

... also the >&2 bit at the end causes errors to go where they should (standard error), and allows for capturing just errors...

## ... to a file...
some_trapped_script.sh 2>some_trapped_errros.log

## ... or by ignoring standard out...
some_trapped_script.sh 1>/dev/null

As shown by these and other examples on Stack Overflow, there be lots of ways to build a debugging aid using built in utilities.


Inspired by other answer, here's a simpler contextual error handler:

trap '>&2 echo Command failed: $(tail -n+$LINENO $0 | head -n1)' ERR

You can also use awk instead of tail & head if needed.

  • 1
    there's a reason the other answer provides context by way of 3 lines above and 3 lines below the offending line - what if the error emanates from a continuation line?
    – iruvar
    Mar 23 '19 at 3:13
  • @iruvar this is understood, but I don't need any of that extra context; one line of context is as simple as it gets, and as sufficient as I need
    – sanmai
    Mar 23 '19 at 4:42
  • Ok my friend,+1
    – iruvar
    Mar 23 '19 at 6:24

Here's another version, inspired by @sanmai and @unpythonic. It shows script lines around the error, with line numbers, and the exit status - using tail & head as that seems simpler than the awk solution.

Showing this as two lines here for readability - you can join these lines into one if you prefer (preserving the ;):

trap 'echo >&2 "Error - exited with status $? at line $LINENO:"; 
         pr -tn $0 | tail -n+$((LINENO - 3)) | head -n7' ERR

This works quite well with set -euo pipefail (unofficial strict mode) - any undefined variable error gives a line number without firing the ERR pseudo-signal, but the other cases do show context.

Example output:

myscript.sh: line 27: blah: command not found
Error - exited with status 127 at line 27:
   24   # Do something
   25   lines=$(wc -l /etc/passwd)
   26   # More stuff
   27   blah
   29   # Check time
   30   time=$(date)
  • Loved the mention about set -euo pipefail!!! But... is there any way to trap the undefined var case?
    – MestreLion
    Aug 15 '20 at 4:29
  • 1
    I don't know of a way to trap the undefined var error, which seems to be detected without firing this trap. However, the built-in error message is quite clear and has a line number: foo.sh: line 7: x: unbound variable.
    – RichVel
    Aug 15 '20 at 6:09
  • Yeah, just noticed that. That builtin message is enough for me. By the way, you might add -E to your "unofficial strict mode" so the trap also catches errors inside functions. My final strict mode became set -Eeuo pipefail
    – MestreLion
    Aug 15 '20 at 7:06

The trap is very useful for finding undefined variables and array elements. There are a couple of "gotchas":

  1. Incremented variables

    ((i++)) post-increments. If i is zero, the return code is 1, triggering an apparent error. See Why does a=0; let a++ return exit code 1?

    Change to pre-increment ((++i)) and the problem goes away

  2. Testing for unset array elements

    if [[ -z ${array[$element]} ]]

    will report 'unbound variable'. The +_ syntax does not trigger this error

    if ! [[ ${array[$element]+_} ]]

    This form is not exactly easy to remember 8-{

Apologies for posting a comment as an answer: the "gotchas" can easily put you off using a trap so it seemed worth noting the ways to avoid them.

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