set Command Cheat Sheet

set is a shell builtin that allows you to change the values of shell options and set the positional parameters.

It is critical for writing robust and safe bash scripts.

Synopsis

set [options] [arguments]

Exit immediately if a command exits with a non-zero status.

set -e

Without this, a script keeps running even if cd /important/dir fails, potentially deleting the wrong files.

Exit if you try to use an undefined variable (e.g., $UNDEFINED).

set -u

* Prevents disasters like rm -rf /$DIR being interpreted as rm -rf / if $DIR is empty.*

If any command in a pipeline fails, the whole pipe fails. (Normally, only the last command's exit code matters).

set -o pipefail

Put this at the top of your scripts:

#!/bin/bash
set -euo pipefail

Print each command to stderr before executing it (Trace mode).

set -x
ls -la
# Output: + ls -la

To turn off:

set +x

Print shell input lines as they are read.

set -v

You can overwrite arguments ($1, $2...) currently in the shell.

set -- apple banana cherry
echo $1
# Output: apple

Useful for parsing strings or resetting script arguments.

No Overwrite (-C): Prevent output redirection from overwriting existing files.

set -C
echo "test" > existing_file.txt
# Error: cannot overwrite existing file

Export All (-a): Mark variables which are modified or created to be exported.
vi Mode: set -o vi (Enable vi shortcuts on command line).
emacs Mode: set -o emacs (Default).

Current Settings: Running set without arguments lists all shell variables and functions.
shopt: set covers POSIX options. shopt covers Bash-specific options (like globstar).