08-01 — Shell Scripting: Basics¶

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What is Shell Scripting?¶

So far you've been typing commands one at a time. A shell script is just a text file full of commands that run one after another automatically. Scripts let you:

Automate repetitive tasks (backups, deployments)
Combine multiple commands into one reusable tool
Schedule tasks to run without you being present

If you find yourself typing the same sequence of commands more than twice, it's worth making a script.

Your First Script¶

Create a file called hello.sh:

#!/bin/bash
echo "Hello, World!"
echo "Today is: $(date)"
echo "Running as: $(whoami)"

The very first line #!/bin/bash is called the shebang. It tells Linux which program should read and run this script (/bin/bash in this case). Always include this line.

Now make the script executable and run it:

chmod +x hello.sh    # make it runnable
./hello.sh           # run it

The ./ means "run this file from the current directory". You need it because Linux doesn't look in the current directory for programs by default (for security reasons).

Output:

Hello, World!
Today is: Thu Jun 20 14:32:01 UTC 2024
Running as: jahid

Notice $(date) — the $() syntax runs a command and substitutes its output inline. So "Today is: $(date)" becomes "Today is: Thu Jun 20...".

Variables¶

Variables store values so you can reuse them.

#!/bin/bash

NAME="Jahid"
AGE=25

echo "Hello, $NAME"
echo "You are $AGE years old"

Rules for variables: - No spaces around the = sign. NAME = "Jahid" is wrong. NAME="Jahid" is correct. - When assigning: write NAME="Jahid" (no dollar sign) - When using the value: write $NAME (with dollar sign)

Command substitution — capture command output into a variable¶

CURRENT_DATE=$(date +%Y-%m-%d)
echo "The date is: $CURRENT_DATE"

Whatever date +%Y-%m-%d prints gets stored in the variable.

Special built-in variables¶

These are automatically set by bash:

Variable	What it contains
`$0`	The name of the script itself
`$1`, `$2`, `$3`...	Arguments passed to the script
`$#`	The total number of arguments
`$@`	All arguments as a list
`$?`	The exit code of the last command (0 = success, non-zero = error)
`$$`	The PID of the current script
`$USER`	Your username
`$HOME`	Your home directory

Example using arguments:

#!/bin/bash
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "Total arguments: $#"

Run it with: ./myscript.sh hello world

Output:

Script name: ./myscript.sh
First argument: hello
Second argument: world
Total arguments: 2

Reading User Input¶

#!/bin/bash

read -p "Enter your name: " NAME
echo "Hello, $NAME!"

read waits for the user to type something and press Enter. -p is the "prompt" text shown to the user.

read -sp "Enter password: " PASS
echo ""    # print a blank line after (the password was hidden)

-s means "silent" — the input is hidden (like a real password prompt).

Arithmetic¶

Bash can do basic math using $(( )):

A=10
B=3

echo $((A + B))    # 13
echo $((A - B))    # 7
echo $((A * B))    # 30
echo $((A / B))    # 3  (integer division — decimal is dropped)
echo $((A % B))    # 1  (the remainder: 10 ÷ 3 = 3 with remainder 1)

Note: bash only does integer math. For decimals, use bc:

echo "scale=2; 10 / 3" | bc    # prints 3.33

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