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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.

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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.

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

Use && and || to chain commands based on exit statuses:
The image explains the use of "OR" (||) and "AND" (&&) operators in scripting. It shows that command2 runs if the first command is unsuccessful with "OR", and if successful with "AND".
  • command1 && command2
    Runs command2 only if command1 succeeds (exit code 0).
  • command1 || command2
    Runs command2 only if command1 fails (non-zero exit code).

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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 CodeDescription
0Success
1General error
2Misuse of shell builtins
127Command not found
For a full list, see the Bash manual.

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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.

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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.

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