Source: Dev.to

Bash Scripting Overview

A bash script is a plain‑text file that contains a sequence of commands executed by the Bash (Bourne‑Again SHell) interpreter, line by line.
Saving commands in a script lets you repeat the same steps many times simply by running the script file.

• Bash scripting is a powerful way to automate system tasks, manage resources, and perform routine operations on Unix/Linux systems.
• The terms bash and shell are often used interchangeably, though the shell is the command‑line interface that runs the Bash interpreter.

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The Shell Prompt

shatakshi@truamateur$

• When a shell is waiting for a command it displays $ (regular user) or # (root).
• Common built‑in commands:

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Script Naming & Shebang

• By convention Bash scripts end with .sh, but the extension is not required.
• Every Bash script should start with a shebang line, which tells the kernel which interpreter to use:

#!/bin/bash

The shebang is simply the absolute path to the Bash executable.

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Example Script: run_all.sh

1. Create the file

# Open VS Code (or any editor) and create the file
code run_all.sh

2. Add the following content

#!/bin/bash
# Display the current date
echo "Today is $(date)"

# Ask the user for a directory path
echo -e "\nEnter the path to a directory:"
read the_path

# List the contents of the supplied path
echo -e "\nYour path contains the following files and folders:"
ls "$the_path"

3. Explanation of each line

4. Make the script executable

chmod u+x run_all.sh   # add execute permission for the file owner

5. Run the script

./run_all.sh

Note: You must be in the directory that contains run_all.sh (or provide the full path).

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Variables

• Bash variables store strings; there are no explicit data types.
• Variable names may start with a letter (A‑Z, a‑z) or an underscore (_) and may contain letters, numbers, or underscores.
• Variable names are case‑sensitive.
• Do not use reserved keywords (if, elif, then, …) as variable names.

Setting & Using Variables

name=shatakshi          # assignment (no spaces around =)
my_name=$name           # use $ to expand the value

Example: Prompt for a name

#!/bin/bash
echo "What's your name?"
read entered_name
echo -e "\nHi $entered_name, hope you are having a good time learning Bash scripting."

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Command‑Line Arguments

When a script is invoked, arguments are available as $1, $2, …

#!/bin/bash
echo "Hello, $1"

Running ./script.sh Alice prints Hello, Alice.

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Input / Output Redirection

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Conditional Statements

#!/bin/bash
if [[ $a -gt 60 && $b -lt 100 ]]; then
    echo "Both conditions are true"
elif [[ $a -gt 60 ]]; then
    echo "Only a is greater than 60"
else
    echo "Neither condition is true"
fi

• [[ … ]] is the preferred test construct.
• Logical operators: && (AND), || (OR).
• The block is terminated with fi.

Example: Positive / Negative / Zero

#!/bin/bash
echo "Please enter a number:"
read num

if   [ "$num" -gt 0 ]; then
    echo "$num is positive"
elif [ "$num" -lt 0 ]; then
    echo "$num is negative"
else
    echo "$num is zero"
fi

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Loops

While Loop

#!/bin/bash
i=1
while [[ $i -le 10 ]]; do
    echo "$i"
    (( i += 1 ))   # increment i
done

For Loop (Brace Expansion)

#!/bin/bash
for i in {1..5}; do
    echo "$i"
done

The loop prints numbers 1 through 5.

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Debugging

• Enable execution tracing with set -x at the start of a script. Each command is printed prefixed by +.

#!/bin/bash
set -x   # turn on debugging

# Your script commands go here

• Bash sets an exit status ($?) after each command. A non‑zero value indicates an error and can be tested:

some_command
if [ $? -ne 0 ]; then
    echo "some_command failed"
fi

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Summary

• Bash scripts automate repetitive command‑line tasks.
• Begin every script with a shebang (#!/bin/bash).
• Use variables, user input, arguments, conditionals, and loops to build logic.
• Make scripts executable with chmod u+x.
• Debug with set -x and check exit codes ($?).

With these fundamentals you can start writing your own Bash scripts to streamline everyday system administration and development workflows.

Bash Scripting Tutorial

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Understanding $?

$? holds the exit status of the last executed command.

• 0 – success
• Non‑zero – an error occurred

#!/bin/bash
# Your script goes here

if [ $? -ne 0 ]; then
    echo "Error occurred."
fi

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Example: Command that fails

#!/bin/bash
ls /some/folder/that/doesnt/exist

if [ $? -ne 0 ]; then
    echo "Error occurred"
fi

Output

ls: cannot access '/some/folder/that/doesnt/exist': No such file or directory
Error occurred

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Using read with a timeout

The read builtin almost always succeeds, but you can force a timeout to test the error path.

#!/bin/bash
echo "Enter a number (you have 3 seconds):"
read -t 3 num

if [ $? -ne 0 ]; then
    echo "Error occurred"
fi

Output (when you don’t type anything for 3 seconds)

Enter a number (you have 3 seconds):
Error occurred

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Debugging with echo

Printing variable values helps you see what’s being passed around.

#!/bin/bash
# Your script goes here

echo "Value of variable x is: $x"

# More code goes here

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Exiting on any failure – set -e

If you want the script to stop immediately when any command fails, enable the errexit option.

#!/bin/bash
set -e   # Exit on any command returning a non‑zero status

# Your script goes here
read -t 3 num   # If this times out, the script exits here
echo "You entered: $num"   # This line is never reached on timeout

Note: With set -e, the script terminates as soon as read -t 3 num fails, so the subsequent $? check is never reached.

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TL;DR

• Use $? to inspect the exit status of the previous command.
• Wrap checks in if [ $? -ne 0 ]; then … fi.
• Add -t to read for a timeout‑based error test.
• set -e makes the script abort on the first failure, simplifying error handling.

Thanks for reading! I hope this quick guide helps you understand Bash scripting a bit better.