Using errexit and arithmetic expressions in bash
I was having some issues with a Bash script that used set -o errexit and arithmetic expressions to increment a value:
set -o errexit
x=0
((x++))
((x++))
((x++))
When I tested this on my Mac, the script ran fine, but it failed when I tried to run it in GitHub Actions – the first increment caused the entire script to fail.
My Mac only has Bash 3.2, whereas GitHub Actions has Bash 5 – this was the root cause, and I learnt a couple of interesting things along the way.
The return status of arithmetic expressions depends on the value
The first question is why ((x++)) returns a non-zero exit code.
Via a Stack Overflow answer, I found the relevant section of the bash manpage (emphasis mine):
((expression))- The expression is evaluated according to the rules described below under ARITHMETIC EVALUATION. If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to let "expression".
I expanded my minimal example to print the return code and the current value of x, and I used Docker to run it on several versions of Bash (Bash 3, Bash 4, and Bash 5):
#!/usr/bin/env bash
set -o errexit
bash --version
x=0
((x++)); echo "\$?=$?, x=$x"
((x++)); echo "\$?=$?, x=$x"
((x++)); echo "\$?=$?, x=$x"
The output was always the same:
$?=1, x=1
$?=0, x=2
$?=0, x=3
So why did it work on my Mac and not GitHub Actions?
The behaviour of errexit changed in Bash 4.1
Via another Stack Overflow answer I’ve lost, it turns out that the definition of -e/errexit changed in Bash 4.1.
This is the Bash 4.0 manpage:
-e- Exit immediately if a simple command (see SHELL GRAMMAR above) exits with a non-zero status.
and this is the same section in Bash 4.1:
-e- Exit immediately if a pipeline (which may consist of a single simple command), a subshell command enclosed in parentheses, or one of the commands executed as part of a command list enclosed by braces (see SHELL GRAMMAR above) exits with a non-zero status.
I’m not entirely sure which of these is an arithmetic expression, but clearly stuff changed enough to change the behaviour of the overall script.
How to fix it
If I’m incrementing a variable from 0 with errexit, now I just do:
(x++) || true