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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.

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Table of Contents

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

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Part 1: Using $?

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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.

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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

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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 CodeMeaning
0Success
1General error
2Misuse of shell built-ins
126Command found but not executable
127Command not found
130Script terminated by Ctrl-C
You can define custom exit codes (128 and above) to represent specific failure modes in your scripts.

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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

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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

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Part 2: Writing Scripts That Leverage $?

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

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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

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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

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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

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Custom Exit Codes and a terminate Function

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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.

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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

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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.

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References

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