Special variables ($0, $1, $#, $@, $?, $) in Bash Scripting - Step-by-Step Execution

Concept Flow - Special variables ($0, $1, $#, $@, $?, $$)

Start Script

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Read $0: Script Name

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Read $1: First Argument

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Read $# : Number of Arguments

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Read $@ : All Arguments

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Execute Command

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Read $? : Last Command Exit Status

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Read $$ : Current Script PID

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End Script

The script starts by reading special variables: $0 for script name, $1 for first argument, $# for argument count, $@ for all arguments, $? for last command status, and $$ for script process ID.

Execution Sample

Bash Scripting

#!/bin/bash

# Example script

echo "Script name: $0"
echo "First arg: $1"
echo "Number of args: $#"
echo "All args: $@"
ls
echo "Last command exit status: $?"
echo "Script PID: $$"

This script prints special variables and runs 'ls' to show $? usage.

Execution Table

Step	Variable/Command	Value/Action	Output
1	$0	./example.sh	Script name: ./example.sh
2	$1	hello	First arg: hello
3	$#	2	Number of args: 2
4	$@	hello world	All args: hello world
5	ls	runs ls command	Lists files in current directory
6	$?	0	Last command exit status: 0
7	$$	12345	Script PID: 12345

💡 Script ends after printing all special variables and command output.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
$0	unset	./example.sh	./example.sh	./example.sh	./example.sh	./example.sh	./example.sh	./example.sh
$1	unset	hello	hello	hello	hello	hello	hello	hello
$#	unset	2	2	2	2	2	2	2
$@	unset	hello world	hello world	hello world	hello world	hello world	hello world	hello world
$?	unset	unset	unset	unset	unset	unset	0	0
$$	unset	12345	12345	12345	12345	12345	12345	12345

Key Moments - 3 Insights

Why does $? show 0 after running ls?

Because $? holds the exit status of the last command. 'ls' succeeded, so $? is 0 as shown in step 6.

What does $@ show compared to $1?

$1 is only the first argument (step 2), but $@ shows all arguments separated by spaces (step 4).

Why is $$ useful?

$$ gives the current script's process ID, useful for creating unique temp files or tracking the script, as shown in step 7.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $# at step 3?

A 2

B 1

C 0

D 3

💡 Hint

Check the 'Value/Action' column for step 3 in the execution_table.

At which step does the script run a command and capture its exit status?

A Step 4

B Step 6

C Step 5

D Step 7

💡 Hint

Look for the 'ls' command execution in the execution_table.

If the script was run without arguments, what would $1 and $# be?

A $1 is empty, $# is 1

B $1 is empty, $# is 0

C $1 is the script name, $# is 0

D $1 is the script name, $# is 1

💡 Hint

Refer to variable_tracker for how $1 and $# represent arguments.

Concept Snapshot

Special variables in bash:
$0 = script name
$1 = first argument
$# = number of arguments
$@ = all arguments
$? = last command exit status
$$ = current script PID
Use these to handle inputs and track script state.

Full Transcript

This lesson shows how bash special variables work. $0 holds the script name, $1 the first argument, $# the count of arguments, and $@ all arguments. The script runs a command 'ls' and $? captures its exit status. $$ shows the script's process ID. Watching these variables step-by-step helps understand how scripts get inputs and check command results.