Bash Substitution: Filename, Variable, Command, and Arithmetic

Bash offers powerful substitution mechanisms that allow you to dynamically replace parts of your commands and scripts. This post will explore filename/globbing, variable, command, and arithmetic substitution, providing you with the tools to create more versatile Bash scripts.

Types of Substitution:

1. Filename/Globbing Substitution:

   • Expands patterns to match filenames.

   • Uses wildcards like *, ?, and [].

   • Example: ls *.txt (lists all .txt files in the current directory).

2. Variable Substitution:

   • Replaces a variable with its value.

   • Uses the dollar sign ($).

   • Example: echo $HOME (displays the user's home directory).

3. Command Substitution:

   • Replaces a command with its output.

   • Uses backticks (`) or $(...).

   • Example: echo "Current date: $(date)" (displays the current date).

4. Arithmetic Substitution:

   • Performs arithmetic operations.

   • Uses $((...)).

   • Example: echo "Sum: $((10 + 5))" (displays the sum of 10 and 5).

Simple Bash Script Example:

#!/bin/bash

# Filename/Globbing Substitution
echo "Text files in current directory:"
ls *.txt

# Variable Substitution
username=$USER
echo "Current user: <span class="math-inline">username"
\# Command Substitution
current\_date\=</span>(date)
echo "Current date: <span class="math-inline">current\_date"
\# Arithmetic Substitution
result\=</span>((20 - 5))
echo "Result of 20 - 5: $result"

#Combine them all
echo "The user $USER, ran this script on $(date), and the result of 10+5 is $((10+5)), inside of the directory $(pwd)"

Practice Session 1: With Example Commands

1. Create several files with different extensions (e.g., file1.txt, file2.log, data.csv).

touch file1.txt file2.log data.csv

2. Use filename substitution to list all .txt files.

ls *.txt

3. Define a variable my_dir with the value of the current directory.

my_dir=$(pwd)

4. Display the value of my_dir.

echo $my_dir

5. Use command substitution to display the current time in seconds since the epoch.

echo "Seconds since epoch: $(date +%s)"

6. Use arithmetic substitution to calculate and display the product of 7 and 8.

echo "Product: $((7 * 8))"

7. Use all 4 substitutions in one echo command. For example, print the name of all log files, the current user, the date, and the result of 2+2.

echo "Log files: $(ls *.log), user: $USER, date: <span class="math-inline">\(date\), 2\+2\=</span>((2+2))"

Practice Session 2: Without Example Commands

1. Create a set of files with different extensions in your current directory.

2. Use filename substitution to list all files with a specific extension.

3. Define a variable with a string value of your choice.

4. Display the variable's value within a sentence.

5. Use command substitution to display the output of the uptime command.

6. Use arithmetic substitution to calculate and display the result of a mathematical expression.

7. Combine all four substitution types in a single echo command to display a dynamic message.

8. Create a variable that contains a file name, and then use globbing to list all files that start with the variable content.

9. Create a variable that contains a number, and then use arithmetic substitution to multiply that number by 10.

10. Use command substitution to get the current date, and then use variable substitution to display it in a sentence.