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.

Table of Contents

1. Part 1: Using $?
   • What Is an Exit Status?
   • Inspecting $? in Practice
   • Common Exit Codes
   • Back-to-Back Commands
   • Masking Errors
2. 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
3. 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.

Inspecting $? in Practice

1. 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.”

2. Successful command:

   $ ls
   music videos photos documents
   $ echo $?
   0
   

3. File-not-found error:

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

Common Exit Codes

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

$ 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.

References

• Bash Reference Manual
• Advanced Bash-Scripting Guide
• Shell Exit Status Conventions