In the bash shell, we can define a function f with
f(){ echo Hello; } and then redeclare/override it, without any error or warning messages, with
f(){ echo Bye; } I believe there is a way to protect functions from being overridden in this way.
You may declare a function foo as a read-only function using readonly -f foo or declare -g -r -f foo (readonly is equivalent to declare -g -r). It's the -f option to these built-in utilities that makes them act on foo as the name of a function, rather than on the variable foo.
$ foo () { echo Hello; } $ readonly -f foo $ foo () { echo Bye; } bash: foo: readonly function $ unset -f foo bash: unset: foo: cannot unset: readonly function $ foo Hello As you can see, making the function read-only not only protects it from getting overridden, but also protects it from being unset (removed completely).
Currently (as of bash-5.0.11), trying to modify a read-only function would not terminate the shell if one is using the errexit shell option (set -e). Chet, the bash maintainer, says that this is an oversight and that it will be changed with the next release.
Update: This was fixed during October 2019 for bash-5.1-alpha, so any bash release 5.1 or later would exit properly if an attempt to modify a read-only function is made while the errexit shell option is active.
bash: f: readonly function and non-zero status code, but does not exit if errexit option enabled. bash, but I will ask on one of the bash mailing lists to be sure. bash only exits when set -e is in effect when POSIX requires it to (and readonly -f is not POSIX). Greg points out that the bash manual never mentions "failure in function declaration" as reason for errexit to trigger an exit (unless a function declaration counts as a compound command, which he's pretty sure it does not). The thread is ongoing here: lists.gnu.org/archive/html/help-bash/2019-09/msg00039.html errexit or set -e in your question. Little remark to @Kusalananda's answer. According to what bash maintainer said that changing a read-only function shall trigger the termination of the shell when using set -e.
I'm using Ubuntu 20.10 with a much newer bash version. Unfortunately it seems to be broken.
$ bash --version GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu) test.sh
#!/usr/bin/env bash set -e test() { echo "I'm test() function" } readonly -f test test test() { # should exit here echo "I'm test() function (duplicated)" } test Output
$ chmod +x test.sh; ./test.sh I'm test() function ./test.sh: line 14: test: readonly function I'm test() function I've tested this script inside a docker container using latest bash version and it works as expected.
GNU bash, version 5.1.8(1)-release (x86_64-pc-linux-musl) 
typeset -r:typeset -rf f.readonly -f f