Bash Scripting: A Comprehensive Guide

Bash is a powerful scripting language that runs on Unix-like systems, including Linux and macOS. It's a versatile tool for automating tasks, managing files, and interacting with the system.

Basics

Shell Script: A text file containing a sequence of Bash commands.
Shebang: The first line of a script, indicating the interpreter to use (e.g., #!/bin/bash).
Execution: Make the script executable (chmod +x script.sh) and run it (./script.sh).

Essential Commands

echo: Prints text to the terminal.
read: Reads input from the user.
if-else: Conditional statements.
for: Loops through a list of items.
while: Loops as long as a condition is true.
case: Matches a value against patterns.
functions: Reusable blocks of code.

Variables

Declaration: variable_name=value
Usage: echo $variable_name
Types: String, integer, floating-point.

File Operations

Creating: touch filename
Deleting: rm filename
Copying: cp sourcefile destfile
Moving: mv sourcefile destfile
Listing: ls
Directing: cd directory

Input/Output

Reading: read variable
Writing: echo "text" > filename
Appending: echo "text" >> filename

Arithmetic Operations

Basic: expr 2 + 3
Shell Arithmetic: (( result = 2 + 3 ))

Control Flow

if-else:

Bash

if [ condition ]; then
    commands
elif [ condition ]; then
    commands
else
    commands
fi

for loop:

Bash

for variable in list; do
    commands
done

while loop:

Bash

while [ condition ]; do
    commands
done

case:

Bash

case variable in
    pattern1)
        commands ;;
    pattern2)
        commands ;;
    *)
        commands ;;
esac

Functions

Defining:

Bash

function function_name() {
    commands
}

Calling: function_name

Examples

Hello, World:

Bash

#!/bin/bash
echo "Hello, World!"

Simple calculator:

Bash

!/bin/bash
echo "Enter two numbers:"
read num1 num2
result=$((num1 + num2))
echo "The sum is: $result"

Regular Expressions

Pattern Matching: Used for complex string manipulation and searching.
Syntax: grep 'pattern' file
Example: Find lines containing email addresses: grep '[[:alnum:]]+@[[:alnum:]]+.[[:alnum:]]+' file

Pipes and Redirection

Pipes: Connect the output of one command to the input of another.
Example: List files, sort them by size, and print the top 10: ls -l | sort -n -k 5 | head -n 10
Redirection:
">": Overwrite output.
">>": Append output.
"<": Redirect input.

Command Substitution

Executing Commands Within Expressions: $(command) or command
Example: Set a variable to the current date: current_date=$(date)

Arrays

Declaration: array_name=(element1 element2)
Accessing: echo ${array_name[index]}
Example: Create an array of fruits: fruits=("apple" "banana" "orange")

Functions with Arguments

Passing Arguments: function_name arg1 arg2
Accessing Arguments: $1, $2, ...
Example:

Bash
function greet() {
    echo "Hello, $1!"
}
greet John

Conditional Expressions

Test Brackets: [ condition ] or [[ condition ]]
Operators: -f (file), -d (directory), -x (executable), -z (empty), -n (non-empty), =, !=, <, >, -eq, -ne, -lt, -le, -gt, -ge
Example: Check if a file exists: if [ -f file.txt ]; then ...

Debugging

Tracing: set -x
Error Checking: if [ $? -ne 0 ]; then ...
Logging: echo "Message" >> logfile

Best Practices

Clarity and Readability: Use meaningful variable names and comments.
Error Handling: Anticipate potential errors and provide informative messages.
Modularity: Break down complex scripts into smaller functions.
Efficiency: Optimize your scripts for performance.

Advanced Topics

Here Documents: Multi-line input within a script.
Getopts: Parsing command-line options.
Signal Handling: Responding to interrupts and signals.
Background Jobs: Running commands in the background.
Shell Options: Customize Bash behavior (e.g., set -e for strict error checking).

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Comments

[Jimmy McBride]

Great article! Loving seeing quality content about bash scripting like this. Keep it up!

[Joe]

Nice.
My man bash says Evaluation is done in fixed-width integers. You have to use special tools like bc to get floating point to do anything.

[EchoGlobal]

Bash scripting: where a missing semicolon can make you question all of your life choices. :-)

[Joe]

It's not that bad. If you put things on separate lines, you don't need any semicolons except in one form of the for statement.

By comparison, I once typed in a 20 line FORTRAN IV program (I'm old) and spent a whole night debugging it because I had mistyped one comma as a period where both were legal.
Something like y = funct(1.2) that should have been y = funct(1, 2) that passed one argument to the function instead of two. But that's how you learn good style and defensive programming! :)