13. Error Handling

This chapter focuses on making scripts dependable: exit codes, shell options, traps, defensive checks, logging, and error redirection.

• Exit codes: Use command status values to decide what a script should do next.

• Shell options: Enable stricter behavior when failures should stop execution.

• Traps: Run cleanup or diagnostic code when signals or errors occur.

• Defensive scripting: Validate assumptions before commands cause damage.

• Isolation: Use subshells and process substitution when temporary context should not leak.

• Logging: Record useful information for debugging and maintenance.

Chapter Video

How to Debug a Bash Script (7:23) by ProgrammingKnowledge

Hosted on YouTube; property of its original creator, embedded here for educational reference only.

Learning Objectives

By the end of this chapter, you will be able to:

1. Interpret exit codes and propagate failures intentionally.

2. Use shell options and traps to control failure behavior.

3. Use subshells and process substitution deliberately in defensive scripts.

4. Apply defensive scripting practices to risky operations.

5. Log messages and redirect errors for diagnosis.

Chapter Flow

1. Exit Codes

2. Options & Traps for Error Handling

3. Shell Options and Subshells

4. Defensive Scripting

5. Logging & Errors

Glossary

#	Term	Explanation
1	Exit code	A numeric command result where 0 means success and nonzero usually means failure.
2	Error propagation	Passing failure information from one command or function to its caller.
3	$?	The exit status of the most recently completed foreground command.
4	set -e	A shell option that exits on some unhandled command failures.
5	set -u	A shell option that treats unset variables as errors.
6	set -o pipefail	An option that makes pipeline failure visible when any command fails.
7	Trace mode	Debug output enabled with set -x.
8	trap	A Bash builtin that runs commands when signals or selected events occur.
9	Signal	A process notification such as TERM, INT, or EXIT.
10	Cleanup handler	Code that removes temporary files or restores state before exit.
11	Defensive scripting	Writing checks that prevent unsafe assumptions from becoming failures.
12	Input validation	Checking external values before using them.
13	Precondition	A condition that must be true before a command or function can proceed.
14	Temporary file	A short-lived file used while a script is running.
15	Log level	A category such as info, warning, or error for recorded messages.
16	stderr	The standard stream for diagnostics and errors.
17	Error redirection	Routing error output to a file, stream, or command.
18	ShellCheck	A static analysis tool that flags common shell script problems.
19	Rollback	A recovery action that restores a previous safe state.
20	Idempotence	The property that repeating an operation has the same effect as doing it once.
21	shopt	A Bash builtin for enabling or disabling shell options.
22	Subshell	A child shell environment created for grouped commands or pipelines.
23	Process substitution	Bash syntax such as <(...) that exposes command output as a file-like stream.
24	SHELLOPTS	Environment variable that records enabled shell options.