Special variables (Bash)

Bash has so-called special variables, that provide information about the state of the shell or commands, not to be confused with positional arguments:

• $?: The exit status of the last command executed
• $$: The process ID (PID) of the current shell
• $!: The PID of the last background process
• $_: The last argument of the previous command
• $-: The current shell options (e.g., options set by set command)
• IFS: The Internal Field Separator, not positional but frequently used for splitting strings.

Exit status code - $?

The exit status or exit status code is a numerical value returned by a program or command (e.g., a comparison) when it finishes running, and stored in special variable $?. It provides for the system or scripts to understand the outcome of execution, typically indicating whether it ran successfully or encountered an error. A code of 0 usually indicates success, while any other code some shade of failure. I suspect these codes are not consistently applied, though:

• 0: A status code of 0 usually means that the program or command ran successfully, with no error
• 1: A generic error
• 2: Incorrect usage or missing argument
• 127: Command not found

Volatile

An exit code is a volatile thing: As soon as you execute a new command, the status of the previous command will be overwritten.

Example:

wp plugin is-installed blub

echo "exit status: $?"
echo "exit status: $?"

Generates output

1
0

where

• 1 is the exit status of the wp plugin command
• 0 is the exit status of the first echo command.

Advantages of using status codes

The use of status codes allow you to check the success or failure of a command independently of its output. This is especially useful when:

• The command produces a lot of output: You can focus only on whether it succeeded or failed without needing to parse or interpret the output
• Silent or verbose commands: Some commands might not output anything at all (or may only output errors), but you can still check their status with the exit code
• Automating error handling: Scripts can reliably act on the exit code to handle different situations (e.g., retrying a command, logging an error, or taking corrective action) without worrying about inconsistencies in output.

Examples

See Wp plugin is-installed (WP-CLI) for some examples.

See also

• Positional arguments
• Wp plugin is-installed (WP-CLI)