Bash Special Variables

Variable Description

$? The exit status of the last executed command. 0 indicates success, and any non-zero value indicates an error.

$$ The process ID (PID) of the current shell.

$! The process ID (PID) of the last background command executed.

$# The number of positional parameters (arguments) passed to the script or function.

$@ Expands to all positional parameters passed to the script or function as separate entities.

$* Expands to all positional parameters as a single string, separated by the first character of the IFS.

$0 The name of the shell script or function.

$1, $2, ... Positional parameters representing the arguments passed to the script or function, where $1 is the first argument, $2 is the second, and so on.

: Bash Special Variables

Example Usage

Here is an example demonstrating the usage of these variables:

echo "Script Name: $0"
echo "Number of Arguments: $#"
echo "All Arguments (individually): $@"
echo "All Arguments (as a single string): $*"
echo "First Argument: $1"
echo "Exit Status of Last Command: $?"
echo "Process ID of Current Shell: $$"

# Simulate a background process
sleep 5 &
echo "Process ID of Last Background Command: $!"

Execute Parsed bash command

To execute a bash command after some shell parsing

Say we have want to manually run a command from our crontab

crontab -l | tail -n 1

To execute that in bash we would do as follows

bash -c "$(crontab -l | tail -n 1)"

Awk Guide for Substrings

substr(string, start, length)

string: The input string.
start: The starting position (1-based index).
length: (Optional) The number of characters to extract. If omitted, extracts to the end of the string.

Examples

Command Output Explanation

Command	Output	Explanation
`bash echo Hello, World! \| awk '{print substr($0, 1, 5)}'`	`Hello`	First 5 characters
`bash echo Hello, World! \| awk '{print substr($2, 2, 2)}'`	`or`	Use space as delimiter, next 2 chars from second field
`bash echo Hello, World! \| awk '{print substr($0, 8)}'`	`World!`	Char 8 to the end of the line
`bash echo "apple,banana,cherry" \| awk -F, '{print substr($2, 1, 3)}'`	`ban`	Use comma as delimiter, 3 chars from second field
`bash echo "abcdef" \| awk '{print substr($0, 1, 3) "-" substr($0, 4, 3)}'`	`abc-def`	Combine substrings formed by awk

: Common [awk]{.title-ref} Substring Examples

Notes

The default field separator is a space. Use the -F option to specify a custom delimiter.
Conditions can be added to extract substrings only when specific criteria are met, e.g., if ($2 > 100){.bash}.