Ubuntu 22.04 – What is Bash and its config

Jan 24, 2025 | New

Bash (Bourne Again SHell) is one of the most commonly used command-line interpreters or shells in Unix-based systems, such as Linux and macOS. It provides an interface for users to interact with the operating system by executing commands, running scripts, and managing processes.

Purpose: Bash acts as an intermediary between the user and the operating system. It allows users to issue commands, automate tasks, and interact with files and processes.

Location: The Bash binary is typically located at /bin/bash.

Key Features of Bash

  • Command Execution: You can execute system commands directly by typing them into the terminal.
  • Scripting: Bash supports writing scripts—text files containing a series of commands—to automate tasks.
  • Job Control: Bash allows you to manage foreground and background processes.
  • Command History: It stores previously executed commands, accessible with the history command or arrow keys.
  • Command Aliases: You can define shortcuts for frequently used commands using aliases.
  • Tab Completion: Bash offers command and file name auto-completion when you press Tab.
  • Shell Variables: It supports variables for storing data temporarily.
  • Redirection and Pipes: Input/output can be redirected using operators like >, <, and |.

Structure of a Bash Command

A basic command follows this structure:

command [options] [arguments]
  • Command: The program or utility you want to run (e.g., ls, cd, echo).
  • Options: Flags or switches that modify the command’s behavior (e.g., -l, -a).
  • Arguments: Additional input to the command, like file names or directories.

Example:

ls -l /home

This lists the contents of the /home directory in a detailed format.

Bash Configuration Files

Bash reads configuration files to set up the environment and behaviors:

  • Global Configuration:
    • /etc/profile: System-wide settings.
    • /etc/bash.bashrc: System-wide settings for interactive shells.
  • User-Specific Configuration:
    • ~/.bashrc: Settings for interactive shells (custom aliases, environment variables).
    • ~/.bash_profile or ~/.profile: Settings for login shells.
    • ~/.bash_logout: Commands to run when logging out.

Typical .bashrc Usage:

The .bashrc file is used to configure:

  • Aliases (e.g., alias ll='ls -l').
  • Custom environment variables.
  • Command prompts (PS1 variable).
  • Functions.

Common Bash Commands

Here’s a quick list of commonly used commands:

  • File and Directory Management:
    • ls: List directory contents.
    • cd: Change directory.
    • mkdir: Create a new directory.
    • rm: Remove files or directories.
    • cp: Copy files or directories.
    • mv: Move or rename files or directories.
  • Text Processing:
    • cat: Concatenate and display file contents.
    • echo: Print text or variables to the terminal.
    • grep: Search for patterns in text files.
    • awk: Text processing and data extraction.
    • sed: Stream editor for text manipulation.
  • System Information:
    • pwd: Print the current working directory.
    • whoami: Display the current username.
    • top: Show active processes and system performance.
    • df: Display disk usage.
    • uname: Show system information.
  • Process Management:
    • ps: Display running processes.
    • kill: Terminate processes by PID.
    • jobs: Show background jobs.
    • bg/fg: Resume jobs in the background or foreground.

Bash Scripting

Bash scripts allow you to automate tasks by writing a series of commands in a file.

Example of a Simple Script:

Create a file called example.sh:

#!/bin/bash

# A simple Bash script
echo "Hello, World!"
echo "Today is $(date)"

Run the script:

chmod +x example.sh   # Make the script executable
./example.sh          # Execute the script

Key Scripting Concepts:

  • Variables:
name="Alice"echo "Hello, $name!"
  • Conditionals:
if [ -f myfile.txt ]; then  echo "File exists" else    echo "File does not exist" 
fi
  • Loops:
for i in {1..5}; do
    echo "Looping: $i"
done
  • Functions: Bash allows the creation of reusable functions within scripts. For example:
greet() {
    echo "Hello, $1!"
}

greet "Bash"

Pipelines and Redirection

  • Pipes (|): Bash allows the output of one command to be used as the input for another command. This is called piping.
cat file.txt | grep "search_term"

This command will search for "search_term" in file.txt.

  • Redirection (>, >>):
    • > redirects the output to a file, overwriting it if it exists.
echo "Hello" > file.txt
    • >> appends the output to a file.bashCopyEditecho "World" >> file.txt
echo "World" >> file.txt
  • Input Redirection (<): Redirect input from a file to a command.
sort < file.txt

6. Job Control

Bash supports job control, allowing you to manage processes. You can run processes in the background or foreground and stop/continue them.

  • Running in the Background: To run a command in the background, append an & to it:b
sleep 60 &
  • Listing Jobs: Use jobs to list background jobs.
jobs
  • Bringing Jobs to the Foreground: Use fg to bring a background job back to the foreground.
fg $1
  • Stopping Jobs: Use Ctrl+Z to stop a job, and then use bg to resume it in the background.

Bash vs Other Shells

Bash is not the only shell available. Other common shells include:

  • Zsh (Z Shell): Known for better auto-completion and customization features.
  • Fish (Friendly Interactive Shell): Focuses on user-friendliness with advanced features and syntax highlighting.
  • Tcsh: An enhanced version of the C shell (csh), offering additional features.

While each shell has its strengths, Bash is widely used due to its compatibility, performance, and long-standing popularity.

Conclusion

Bash is a powerful and versatile shell that supports interactive usage as well as complex scripting. Whether you’re automating tasks, managing files, or configuring your system, Bash provides the tools necessary for efficiency and flexibility. By learning Bash, you can streamline your interactions with your operating system and gain deeper control over its functionality.