shell_variables.md

In Linux, several special variables (also called shell variables or environment variables) are commonly used when working with the command line and scripting. Here are the most important ones you should know:

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1. Positional Parameters (Command-line Arguments)

Variable	Description
$0	The name of the script or command being executed.
$1, $2, ..., $9	The first, second, ..., ninth argument passed to the script.
${10}, ${11}, ...	Arguments beyond the ninth (must be enclosed in {}).
$*	All arguments passed to the script as a single string.
$@	All arguments passed to the script as separate strings.
$#	The number of arguments passed to the script.

Example: Positional Parameters in a Script

#!/bin/bash
echo "Script Name: $0"
echo "First Argument: $1"
echo "Second Argument: $2"
echo "All Arguments (\$*): $*"
echo "All Arguments (\$@): $@"
echo "Total Arguments: $#"

Run the script:

./script.sh Hello World

Output:

Script Name: ./script.sh
First Argument: Hello
Second Argument: World
All Arguments ($*): Hello World
All Arguments ($@): Hello World
Total Arguments: 2

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2. Exit Status and Process Control

Variable	Description
$?	Exit status of the last executed command.
$$	Process ID (PID) of the current script or shell.
$!	Process ID of the last background job.
$-	Current shell options (set using set -o).

Example: Check PID

echo "Current Process ID: $$"
sleep 10 &
echo "Background Process ID: $!"

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3. Shell Environment Variables

Variable	Description
$HOME	User's home directory (/home/username).
$USER	Current logged-in username.
$PATH	Directories where executables are searched for.
$SHELL	Default shell (/bin/bash, /bin/zsh, etc.).
$PWD	Current working directory.
$OLDPWD	Previous working directory.
$EDITOR	Default text editor (vim, nano, etc.).
$LANG	System language settings.

Example: Display User Information

echo "User: $USER"
echo "Home Directory: $HOME"
echo "Current Shell: $SHELL"

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4. Process Management

Variable	Description
$PPID	Parent process ID (PID of the parent shell).
$UID	User ID of the current user.
$EUID	Effective user ID (useful for checking root access).

Example: Check User and Parent Process

echo "User ID: $UID"
echo "Effective User ID: $EUID"
echo "Parent Process ID: $PPID"

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5. Random and Time-related Variables

Variable	Description
$RANDOM	Generates a random number (0 - 32767).
$SECONDS	Number of seconds since the script started.
$LINENO	Current line number in a script.

Example: Generate Random Number

echo "Random Number: $RANDOM"
echo "Script Running Time: $SECONDS seconds"

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6. Input/Output Variables

Variable	Description
$IFS	Internal Field Separator (default: space, tab, newline).
$PS1	Primary shell prompt string ($ for normal users, # for root).
$PS2	Secondary shell prompt (> used in multi-line commands).

Example: Change Shell Prompt

export PS1="MyShell> "

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7. Special Variables for Debugging

Variable	Description
$_	Last argument of the previous command.
$BASH_SOURCE	Script filename.
$FUNCNAME	Current function name (inside a function).

Example: Reuse Last Argument

echo "Hello World"
echo $_   # Output: World

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Conclusion

These special variables help in scripting, automation, and debugging. You should experiment with them in scripts to understand their behavior better.

Would you like specific examples for any of these? 🚀