Linux Basics: Using Bash Exit Status

A command reports an exit status separately from the text it prints. In Bash on Debian and Ubuntu, zero normally means success and a nonzero value signals another outcome. Checking that status helps you decide whether to continue a sequence of commands.

Read the previous status

true
printf '%s\n' "$?"
false
printf '%s\n' "$?"

0
1

true returns zero, while false returns one. Bash expands $? to the status of the most recently completed command.

Check it immediately. Even a successful printf replaces the previous status, so another command between the operation and the check can hide the result you wanted.

A nonzero value needs context: grep uses one for no matching lines, while larger values indicate an error. Check the relevant command’s documentation.

Continue only after success

Run mktemp -d, then use cd to enter the temporary directory it prints.

mkdir notes && printf 'Directory created\n'

&& runs the command on its right only when the command on its left succeeds. If the directory already exists, this mkdir fails and the message is skipped.

Choose a fallback

[ -f notes/todo.txt ] || printf 'No regular todo file yet\n'

|| runs the right-hand command only after a nonzero status. Here, the test checks whether the path identifies a regular file. Spaces around [ and ] are required.

For pipelines, Bash normally reports the last command’s status. A successful final command can hide an earlier failure. Do not assume a pipeline’s zero status proves every stage succeeded.

Reference

Debian Bash manual

Acknowledgements

This post was created with assistance from AI.

Linux Basics — lesson 12 of 25. Series index

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