Special shell Question mark

In Bash and other POSIX-compliant shells, the special variable $? holds the exit status of the last executed command, script, or function. Checking this value is essential for robust error handling in shell scripts.

Part 1: Using $?
Part 2: Writing Scripts That Leverage $?
References

Part 1: Using `$?`

What Is an Exit Status?

Every command returns an integer exit status.

0 means success.
Non-zero indicates failure or a specific error condition.

The image explains that the special shell variable $? stores the exit status of a command, script, or function.

If you redirect both stdout and stderr (e.g., > /dev/null 2>&1), you won’t see any output, but $? still reflects success or failure.

Inspecting `$?` in Practice

Script with a typo:

#!/usr/bin/env bash
ehco "Hello!"

$ ./wrong_echo.sh > /dev/null 2>&1
$ echo $?
127

Exit code 127 means “command not found.”

Successful command:

$ ls
music videos photos documents
$ echo $?
0

File-not-found error:

$ ls some_file.txt
ls: cannot access 'some_file.txt': No such file or directory
$ echo $?
2

Common Exit Codes

The image shows a table of special shell variables with exit code numbers and their meanings, such as "0" for success and "1" for a general error."

Exit Code	Meaning
0	Success
1	General error
2	Misuse of shell built-ins
126	Command found but not executable
127	Command not found
130	Script terminated by Ctrl-C

You can define custom exit codes (128 and above) to represent specific failure modes in your scripts.

Back-to-Back Commands

When you execute multiple commands, $? always reflects the last exit status:

$ ./script.sh
permission denied: ./script.sh
$ echo $?
126

$ ls script.sh
script.sh
$ echo $?
0

Masking Errors

A trailing exit 0 can hide earlier failures:

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

$ ./wrong_echo.sh
./wrong_echo.sh: line 2: ehco: command not found
$ echo $?
0

Part 2: Writing Scripts That Leverage `$?`

To ensure your script stops on errors and reports accurate statuses, apply one of these techniques.

Technique 1: `if` After Each Command

#!/usr/bin/env bash

ehco "Hello!"
if [[ $? -ne 0 ]]; then
    echo "Error: Failed to run command."
    exit 1
fi

echo "Command ran successfully!"
exit 0

$ ./wrong_echo-v2.sh
./wrong_echo-v2.sh: line 2: ehco: command not found
Error: Failed to run command.
$ echo $?
1

Technique 2: OR Operator (`||`)

#!/usr/bin/env bash
ehco "Hello!" || { echo "Error: Failed to run command."; exit 1; }
exit 0

$ ./wrong_echo-v3.sh
./wrong_echo-v3.sh: line 1: ehco: command not found
Error: Failed to run command.
$ echo $?
1

Technique 3: `set -e`

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

$ ./wrong_echo-sete.sh
./wrong_echo-sete.sh: line 2: ehco: command not found
$ echo $?
127

Using set -e in an interactive shell will terminate your session on the first error.

$ set -e
$ ehco "Hello"
-bash: ehco: command not found

Custom Exit Codes and a `terminate` Function

Initial Script: `server_appender.sh`

#!/usr/bin/env bash

readonly CONF_FILE="./fqdn.properties"
readonly SERVER_NAMES="server1 server2 server3"
readonly DEFAULT_USER="mummshad"

fqdn=$(cat "${CONF_FILE}")

for server in ${SERVER_NAMES}; do
    echo "${DEFAULT_USER}@${server}.${fqdn}"
done

exit 0

If fqdn.properties is empty, this outputs malformed hostnames.

Adding an Empty-File Check

#!/usr/bin/env bash

readonly CONF_FILE="./fqdn.properties"
readonly SERVER_NAMES="server1 server2 server3"
readonly DEFAULT_USER="mummshad"

if [[ ! -s "${CONF_FILE}" ]]; then
    echo "Error: ${CONF_FILE} is empty"
    exit 1
fi

fqdn=$(cat "${CONF_FILE}")

for server in ${SERVER_NAMES}; do
    echo "${DEFAULT_USER}@${server}.${fqdn}"
done

exit 0

$ ./server_appender.sh
Error: ./fqdn.properties is empty
$ echo $?
1

Defining a `terminate` Function

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

readonly CONF_FILE="./fqdn.properties"
readonly SERVER_NAMES="server1 server2 server3"
readonly DEFAULT_USER="mummshad"
readonly ERROR_FILE=150

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

if [[ ! -s "${CONF_FILE}" ]]; then
    terminate "FQDN file is empty" "${ERROR_FILE}"
fi

fqdn=$(cat "${CONF_FILE}")

for server in ${SERVER_NAMES}; do
    echo "${DEFAULT_USER}@${server}.${fqdn}"
done

exit 0

$ ./custom_exit_code_final.sh
Error: FQDN file is empty
$ echo $?
150

By combining set -e, custom exit codes, and a reusable terminate function, your Bash scripts will halt on failures and report meaningful statuses.

Introduction

Conventions

Refresher

Streams

Expansions - Part One

Globs

Expansions - Part Two

Special Shell Variables

Arrays

Good Practices applied

awk

sed

Special shell Question mark

Table of Contents

Part 1: Using `$?`

What Is an Exit Status?

Inspecting `$?` in Practice

Common Exit Codes

Back-to-Back Commands

Masking Errors

Part 2: Writing Scripts That Leverage `$?`

Technique 1: `if` After Each Command

Technique 2: OR Operator (`||`)

Technique 3: `set -e`

Custom Exit Codes and a `terminate` Function

Initial Script: `server_appender.sh`

Adding an Empty-File Check

Defining a `terminate` Function

References

Watch Video

​Table of Contents

​Part 1: Using $?

​What Is an Exit Status?

​Inspecting $? in Practice

​Common Exit Codes

​Back-to-Back Commands

​Masking Errors

​Part 2: Writing Scripts That Leverage $?

​Technique 1: if After Each Command

​Technique 2: OR Operator (||)

​Technique 3: set -e

​Custom Exit Codes and a terminate Function

​Initial Script: server_appender.sh

​Adding an Empty-File Check

​Defining a terminate Function

​References

Watch Video

Table of Contents

Part 1: Using `$?`

What Is an Exit Status?

Inspecting `$?` in Practice

Common Exit Codes

Back-to-Back Commands

Masking Errors

Part 2: Writing Scripts That Leverage `$?`

Technique 1: `if` After Each Command

Technique 2: OR Operator (`||`)

Technique 3: `set -e`

Custom Exit Codes and a `terminate` Function

Initial Script: `server_appender.sh`

Adding an Empty-File Check

Defining a `terminate` Function

References