Bash Special Variables $! $0 $? $$

$0 - The Name of the Script

$0 represents the name of the Bash script as it was called.

If you execute a script with bash myscript.sh, then within myscript.sh, $0 will be myscript.sh.

$1, $2, $3, … - Positional Parameters

These variables correspond to the arguments passed to the script. $1 is the first argument, $2 is the second, and so on.

In the command bash myscript.sh arg1 arg2, $1 equals arg1, and $2 equals arg2.

$! - PID of the Last Background Process

This holds the process ID of the most recent background process. It’s particularly useful in managing parallel processing in scripts.

$# - Number of Parameters

$# provides the count of arguments passed to the script, enabling scripts to adapt based on the number of inputs.

$* and $@ - All Positional Parameters

Both $* and $@ expand to all positional parameters, but they behave differently when quoted. "${*}" merges all parameters into a single string, while "${@}" treats each as a separate word.

$? - Exit Status of the Last Command

The exit status of the most recently executed command is stored in $?, where 0 typically signifies success.

Special Cases

• $$ is the PID of the script itself.
• $- shows the current options set for the shell

dirname

Strips the last component from a file path.

If you have a file path /home/user/documents/report.txt, the dirname command will extract and return /home/user/documents, which is the directory part of the path.

dirname can be especially useful in scripting when you need to perform operations relative to the location of a script or file.