How to detect that RETURN trap was called from source?

Question

I am writing a library and would want to detect where from a RETURN trap was called. Kindly consider the following example:

#!/bin/bash

on_return() {
  echo "on_return: ${BASH_SOURCE[1]}:${BASH_LINENO[1]}:${FUNCNAME[1]}() returned" >&2
  declare -p BASH_SOURCE BASH_LINENO FUNCNAME
  echo
}
set -o functrace
trap 'on_return' RETURN

f() {
  return 101
}

f

. <(echo '
  echo "FROM INSIDE SOURCE:"
  declare -p BASH_SOURCE BASH_LINENO FUNCNAME
  echo
  f
  return 102
')

On Bash5.3 on archlinux this prints the following, with my comment:

on_return: ./test.sh:15:f() returned   # correct!
declare -a BASH_SOURCE=([0]="./test.sh" [1]="./test.sh" [2]="./test.sh")
declare -a BASH_LINENO=([0]="12" [1]="15" [2]="0")
declare -a FUNCNAME=([0]="on_return" [1]="f" [2]="main")

FROM INSIDE SOURCE:
declare -a BASH_SOURCE=([0]="/dev/fd/63" [1]="./test.sh")
declare -a BASH_LINENO=([0]="17" [1]="0")
declare -a FUNCNAME    # FUNCNAME is only set inside functions

on_return: ./test.sh:5:f() returned   # correct!
declare -a BASH_SOURCE=([0]="./test.sh" [1]="./test.sh" [2]="/dev/fd/63" [3]="./test.sh")
declare -a BASH_LINENO=([0]="12" [1]="5" [2]="17" [3]="0")
declare -a FUNCNAME=([0]="on_return" [1]="f" [2]="source" [3]="main")
 #                                           ^^^^^^^^^^^^ - here it is

on_return: ./test.sh:0:main() returned   # wrong, source returned
declare -a BASH_SOURCE=([0]="./test.sh" [1]="./test.sh")
declare -a BASH_LINENO=([0]="17" [1]="0")
declare -a FUNCNAME=([0]="on_return" [1]="main")  # no source in RETURN trap from source?

Because source is not present in FUNCNAME when sourcing a script, is there a way to know if RETURN trap is called from a source script return or from a function return?

LMC · Accepted Answer · 2025-10-06 00:06:19Z

3

The BASH_COMMAND variable could be used

Expands to the command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap. If BASH_COMMAND is unset, it loses its special properties, even if it is subsequently reset.

Showing BASH_COMMAND with declare -p the source command with its argument is shown by the last trap

#!/bin/bash

on_return() {
  echo "on_return: ${BASH_SOURCE[1]}:${BASH_LINENO[1]}:${FUNCNAME[1]}() returned" >&2
  is_source="$(declare -p BASH_COMMAND)"
  if [ "${is_source:25:2}" == '. ' ] || [ "${is_source:25:7}" == 'source ' ];then
    echo "source '${is_source:25:2}' return was trapped" >&2
  else
    echo "Command at trap: '${is_source:25}'" >&2
  fi
  echo
}
set -o functrace
trap 'on_return' RETURN

f() {
  return 101
}

f

. <(echo '
  echo "FROM INSIDE SOURCE:"
  declare -p BASH_COMMAND
  echo
  f
  return 102
')

on_return: ./ret.sh:20:f() returned
Command at trap: 'return 101"'

FROM INSIDE SOURCE:
declare -- BASH_COMMAND="declare -p BASH_COMMAND"

on_return: ./ret.sh:5:f() returned
Command at trap: 'return 101"'

on_return: ./ret.sh:0:main() returned
source '. ' return was trapped

edited Oct 6 at 0:06

answered Oct 5 at 22:43

LMC

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3 Comments

KamilCuk Oct 6 at 8:41

nice! "${is_source:25:2}" == '. ' just [[ "$BASH_COMMAND" = ". "* ]]

KamilCuk Oct 6 at 18:28

btw, fun fact, this does not work on bash 4.0, 4.1 and 4.2 . Probably a bug.

LMC Oct 6 at 18:34

I wrote the answer on GNU bash, version 4.4.23

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