Exiting a Shell Script Based on Process Exit Codes

In shell scripting, handling errors efficiently is crucial to ensure scripts behave as expected. One common scenario is wanting the script to stop execution when any command fails. Here’s a breakdown of several methods to manage exit codes and halt your script on errors.

Understanding $? and Exit Codes

Every command in a shell script returns an exit code, with 0 indicating success and any non-zero value indicating an error. You can access the exit code of the last executed command using $?.

Example:

ls file.txt
if [[ $? -ne 0 ]]; then
  echo "Command failed, exiting..."
  exit 1
fi

In this snippet, if ls file.txt fails (e.g., the file doesn’t exist), the script prints an error message and exits with a code of 1.

Simplifying with set -e

Instead of manually checking $? after each command, you can use set -e at the beginning of your script. This option tells Bash to exit immediately if any command returns a non-zero status.

Example:

#!/bin/bash
set -e
echo "Starting script..."
ls file_that_does_not_exist.txt
echo "This line will not be executed if the above command fails."

With set -e, if ls fails, the script terminates immediately, skipping any remaining commands.

Handling Pipelines with set -o pipefail

A pipeline (e.g., command1 | command2) will only return the exit status of the last command. If you want the script to exit if any command in the pipeline fails, use set -o pipefail.

Example:

#!/bin/bash
set -e
set -o pipefail
grep "pattern" somefile.txt | awk '{print $1}'

In this example, if grep fails (for instance, if somefile.txt doesn’t contain the pattern), the script exits.

Advanced Control with PIPESTATUS

Bash provides the PIPESTATUS array to access the exit status of each command in a pipeline.

Example:

#!/bin/bash
false | true | echo "Only the last command succeeded"
echo "Exit statuses: ${PIPESTATUS[*]}"

This will output Exit statuses: 1 0 0, showing that the first command (false) failed, while the rest succeeded. You can use PIPESTATUS to detect which part of the pipeline failed.

Using OR (||) for Inline Error Handling

You can also use || to run a command only if the previous command fails. This can be handy for conditional exits.

Example:

command1 || exit 1
command2 || exit 1

Here, if either command1 or command2 fails, the script exits immediately with a status of 1.

• $?: Check the exit code after each command.
• set -e: Exit on any non-zero exit code.
• set -o pipefail: Exit on failure in any part of a pipeline.
• PIPESTATUS: Access each command’s exit status in a pipeline.
• || exit: Inline conditional exits based on command failures.

Each method offers flexibility in controlling script execution, making it easier to create reliable and error-resistant shell scripts.