Understanding Bash and Shell Scripting Commands: A Comprehensive Guide

Introduction

Bash (Bourne Again Shell) is the default command-line interpreter for most Linux distributions and macOS. Understanding Bash commands and shell scripting concepts is essential for anyone working with Unix-like operating systems. This guide covers fundamental commands, operators, and techniques commonly used in shell scripting.

Input/Output Redirection

Basic I/O Redirection (>, >>, <)

The shell provides various operators for redirecting input and output:

• > - Redirects output to a file (overwrites existing content)
• >> - Appends output to a file
• < - Takes input from a file

Example: Writing to a File

# Create or overwrite a file with content
echo "Hello" > file.txt

# Append content to a file
echo "World" >> file.txt

Common Misunderstanding

# This does NOT work as expected:
echo Hello > touch make.txt

This command doesn’t add “Hello” to a file named “make.txt”. Instead, it:

1. Treats touch as a literal string, not a command
2. Redirects “Hello” to a file named “touch”
3. Leaves “make.txt” as an unused argument

To properly create a file and add content:

# Create an empty file
touch make.txt

# Add content to the file
echo "Hello" > make.txt

Advanced Redirection (<<, <<<)

Here Document (<<)

The << operator (here document) allows you to provide a block of text as input to a command:

cat << EOF
This is a multi-line
text block that will be
passed to the cat command.
EOF

Here String (<<<)

The <<< operator passes a string directly as input to a command:

grep "search_term" <<< "This is a string to search within."

User Input with read

The read command captures user input in shell scripts:

Basic Usage

# Store user input in a variable
read variable_name

# Example with prompt
echo "Please enter your name:"
read name
echo "Hello, $name!"

Adding a Prompt

# Combine prompt and input in one command
read -p "Please enter your name: " name
echo "Hello, $name!"

Reading Multiple Values

# Read multiple variables from a single input
read first_name last_name
echo "Your first name is $first_name and your last name is $last_name."

Additional Options

# Silent mode (for passwords)
read -s -p "Enter your password: " password
echo "Password saved."

# Read with timeout (5 seconds)
read -t 5 -p "Quick! Enter something: " input

# Read into an array
read -a my_array -p "Enter values separated by spaces: "
echo "First value: ${my_array[0]}"

Text Processing Commands

Word Count (wc)

The wc (word count) command counts lines, words, and characters in files or input:

# Count lines, words, and characters in a file
wc filename.txt

# Count only lines (-l)
wc -l filename.txt

# Count only words (-w)
wc -w filename.txt

# Count only characters (-c)
wc -c filename.txt

# Find the length of the longest line (-L)
wc -L filename.txt

Practical Example

# Count files in current directory
ls | wc -l

Stream Editor (sed)

sed is a powerful stream editor for filtering and transforming text:

Basic Text Substitution

# Replace "Lewis" with "Max" in a file
sed 's/Lewis/Max/g' sedtest.txt

In this command:

• s specifies substitution
• /Lewis/Max/ defines the pattern and replacement
• g (global) replaces all occurrences in each line

In-place Editing with Backup

# Edit file in-place and create a backup with .ORIGINAL extension
sed -i.ORIGINAL 's/Lewis/Max/g' sedtest.txt

This creates sedtest.txt.ORIGINAL as a backup of the original file before making changes.

Using awk for Text Processing

awk is a powerful programming language for text processing:

# Print the first field (column) of each line
awk '{print $1}' filename.txt

If filename.txt contains:

apple 10 red
banana 20 yellow
cherry 30 darkred

The output will be:

apple
banana
cherry

Conditional Testing and Exit Status

Exit Status ($?)

After running a command, $? contains its exit status:

• 0 indicates success
• Non-zero values indicate different types of errors

# Run a command
ls

# Check its exit status
echo $?

# Use exit status in a conditional
cp source.txt destination.txt
if [ $? -eq 0 ]; then
    echo "Copy succeeded!"
else
    echo "Copy failed!"
fi

Comparison Operators

Integer Comparison

• -eq - Equal to
• -ne - Not equal to
• -gt - Greater than
• -lt - Less than
• -ge - Greater than or equal to
• -le - Less than or equal to

# Equality comparison
if [ $number1 -eq $number2 ]; then
    echo "The numbers are equal."
else
    echo "The numbers are not equal."
fi

# Inequality comparison
if [ $number1 -ne $number2 ]; then
    echo "The numbers are not equal."
fi

String Comparison

• = or == - Equal to
• != - Not equal to
• -z - String is empty
• -n - String is not empty

# String equality
if [ "$str1" = "$str2" ]; then
    echo "Strings are equal."
fi

# Check if string is empty
if [ -z "$str" ]; then
    echo "String is empty."
fi

Script Safety and Error Handling

The set Command

The set command can modify shell behavior. A common pattern for safer scripts is:

set -euo pipefail

This combination:

• -e - Exit immediately if a command fails
• -u - Treat unset variables as errors
• -o pipefail - Make a pipeline fail if any command in it fails

Example of its effect:

#!/bin/bash
set -euo pipefail

echo "Starting script..."

# This will cause the script to exit immediately if it fails
some_command_that_might_fail

# This line won't be reached if the above command fails
echo "Script continuing..."

# This will cause an error if UNDEFINED_VARIABLE is not set
echo $UNDEFINED_VARIABLE

Variable Manipulation

Case Conversion

Convert strings to lowercase or uppercase:

# Convert to lowercase
lowercase_var=${variable,,}

# Convert first parameter to lowercase
lowercase_param=${1,,}

# Convert to uppercase
uppercase_var=${variable^^}

Array Operations

Working with arrays in Bash:

# Define an array
MY_FIRST_LIST=("apple" "banana" "cherry")

# Access all elements
echo "All fruits: ${MY_FIRST_LIST[@]}"

# Access a specific element (zero-indexed)
echo "Second fruit: ${MY_FIRST_LIST[1]}"

# Get array length
echo "Number of fruits: ${#MY_FIRST_LIST[@]}"

# Loop through array
for fruit in "${MY_FIRST_LIST[@]}"; do
    echo "Fruit: $fruit"
done

Command Substitution

Command substitution allows you to use the output of a command as part of another command:

# Capture command output in a variable using $()
current_date=$(date +%Y-%m-%d)
echo "Today is $current_date"

# Example: Extract system uptime without "up " prefix
up=$(uptime -p | cut -c4-)
echo "System uptime: $up"

Environment Variables and Configuration

The PATH Variable

$PATH is an environment variable that tells the shell where to look for executable files:

# View your current PATH
echo $PATH

# Add a directory to PATH
export PATH=$PATH:/new/directory/path

A typical PATH might look like:

/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/sbin:/sbin:/bin

Shell Configuration Files

The ~/.bashrc file contains initialization commands run when starting a new Bash shell:

# Common additions to .bashrc
alias ll='ls -la'
export PATH="$PATH:$HOME/bin"

# Define a function
function greet() {
    echo "Hello, $1!"
}

To apply changes without restarting the shell:

source ~/.bashrc

File and Directory Operations

Creating Links (ln)

The ln command creates links between files:

Hard Links

Hard links point to the same inode (data) as the original file:

# Create a hard link
ln original.txt hard_link.txt

Characteristics of hard links:

• Share the same inode
• Cannot link across different filesystems
• Cannot link to directories (usually)
• If the original file is deleted, the hard link still works

Symbolic Links (Symlinks)

Symbolic links point to the path of the target file:

# Create a symbolic link
ln -s original.txt symbolic_link.txt

Characteristics of symlinks:

• Have their own inode
• Can link across filesystems
• Can link to directories
• If the original file is deleted, the symlink becomes broken

Fuzzy Finder (fzf)

fzf is an interactive command-line fuzzy finder:

# Basic usage with pipe
find . -type f | fzf

# Search through command history
history | fzf

# Search through git commits
git log --oneline | fzf

Add to your .bashrc for better integration:

# Add key binding for fzf
export FZF_DEFAULT_COMMAND='find . -type f -not -path "*/\.git/*"'
bind -x '"\C-f": "fzf"'

Conclusion

This guide covered essential Bash commands and shell scripting concepts. By understanding these fundamentals, you’ll be better equipped to navigate Unix-like systems and create efficient shell scripts. As you continue to work with the command line, you’ll discover more powerful ways to combine these tools to solve complex problems.

Further Resources

• GNU Bash Manual
• Advanced Bash-Scripting Guide
• Bash Cheat Sheet