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content/posts/bash-variables.en.md

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+---
+title: Bash Special Variables $! $0 $? $$
+seoTitle: Bash Special Variables $! $0 $? $$
+description: A comprehensive guide to special variables in Bash, including $0, $1, $!, and more, essential for effective script writing.
+toc: true
+categories: [Linux, Bash, CheatSheet]
+series: [CheatSheet, Snippets]
+tags:  [Linux, Bash, Programming, Snippets]
+date: 2023-02-14
+lastMod: 2023-12-14
+featuredImage: https://picsum.photos/700/229?grayscale
+draft: false
+---
+
+
+## `$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.