Strict Mode

Bash is incredibly flexible for automation, but its default behavior can mask failures. Unlike languages with structured exception handling, Bash continues executing even when commands fail, often returning a misleading exit code of zero. Enabling Strict Mode ensures your scripts fail fast, catch unset variables, and correctly handle pipeline errors.

Enabling Strict Mode

Three shell options form the core of Strict Mode:

set -e
Exit immediately if any command returns a non-zero status.
set -u
Treat unset variables as an error and exit immediately.
set -o pipefail
Return the exit status of the first failed command in a pipeline.

You can combine these flags in a single command:

set -euo pipefail

Before diving deeper, let’s review how Bash’s logical operators influence control flow.

OR (`||`) and AND (`&&`) Operators

Use && and || to chain commands based on exit statuses:

command1 && command2
Runs command2 only if command1 succeeds (exit code 0).
command1 || command2
Runs command2 only if command1 fails (non-zero exit code).

The `set -e` Flag

Without set -e, a failing command won’t stop the script:

#!/usr/bin/env bash
ehco "Hello World"
exit 0

$ ./safe.sh
./safe.sh: line 3: ehco: command not found
$ echo $?
0

Enabling set -e preserves the failing command’s exit code:

#!/usr/bin/env bash
set -e
ehco "Hello World"
exit 0

$ ./safe.sh
./safe.sh: line 4: ehco: command not found
$ echo $?
127

Linux reserves certain exit codes for common errors:

The image shows a list of reserved exit codes used in scripting, detailing their numbers and meanings, such as "Success" for code 0 and "General error" for code 1.

Exit Code	Description
0	Success
1	General error
2	Misuse of shell builtins
127	Command not found

For a full list, see the Bash manual.

The `set -u` Flag

By default, using an unset variable expands to an empty string. With set -u, Bash treats it as an error:

#!/usr/bin/env bash
set -u

name="JuanCarlos"
echo "My full name is ${name} ${last_name}. Nice to meet you."
exit 0

$ ./unset-variable.sh
./unset-variable.sh: line 4: last_name: unbound variable
$ echo $?
1

Always initialize variables or test their existence to avoid unexpected script exits.

The `set -o pipefail` Flag

By default, pipelines only report the exit status of their last command, potentially hiding earlier failures:

#!/usr/bin/env bash
cat non-existent-file.txt | sort | uniq
exit 0

$ ./pipefail-v1.sh
cat: non-existent-file.txt: No such file or directory
$ echo $?
0

The image illustrates a process flow in strict mode scripting, showing two commands connected by a pipe, with "Stderr" and an output display labeled "2".

To catch errors anywhere in the pipeline, combine all three flags and handle failures explicitly:

#!/usr/bin/env bash
set -euo pipefail

readonly PIPE_ERROR=156

terminate() {
  local -r msg="$1"
  local -r code="${2:-160}"
  echo "$msg" >&2
  exit "$code"
}

cat non-existent-file.txt | sort || \
  { terminate "Error in piped command" "$PIPE_ERROR"; }

exit 0

$ ./pipefail_v4.sh
cat: non-existent-file.txt: No such file or directory
Error in piped command
$ echo $?
156

With Strict Mode enabled (-e, -u, -o pipefail), your Bash scripts will:

Fail fast on errors
Prevent usage of uninitialized variables
Detect broken pipelines

—resulting in more reliable and maintainable automation.

Introduction

Conventions

Refresher

Streams

Expansions - Part One

Globs

Expansions - Part Two

Special Shell Variables

Arrays

Good Practices applied

awk

sed

Enabling Strict Mode

OR (`||`) and AND (`&&`) Operators

The `set -e` Flag

The `set -u` Flag

The `set -o pipefail` Flag

Links and References

Watch Video

​Enabling Strict Mode

​OR (||) and AND (&&) Operators

​The set -e Flag

​The set -u Flag

​The set -o pipefail Flag

​Links and References

Watch Video

Enabling Strict Mode

OR (`||`) and AND (`&&`) Operators

The `set -e` Flag

The `set -u` Flag

The `set -o pipefail` Flag

Links and References