Variables and Arguments

Scripts become useful when they can remember values and adapt to input.

Variables

#!/bin/bash

name="Ada"
echo "Hello, $name!"

• Assign with name="Ada" — no spaces around the =
• Read the value with $name or ${name}
• Use quotes when a value contains spaces: greeting="Hello world"

Command substitution

Capture the output of a command into a variable:

#!/bin/bash

now=$(date)
files=$(ls | wc -l)
echo "The time is $now"
echo "This directory has $files items"

Positional arguments

Scripts can accept arguments like any other command:

#!/bin/bash

echo "Script name:  $0"
echo "First arg:    $1"
echo "Second arg:   $2"
echo "All args:     $@"
echo "Arg count:    $#"

./args.sh hello world
# Script name:  ./args.sh
# First arg:    hello
# Second arg:   world
# All args:     hello world
# Arg count:    2

if [ -z "$1" ]; then
    echo "Usage: $0 <filename>"
    exit 1
fi

A practical example

#!/bin/bash

dir=${1:-$HOME}     # use arg 1, or default to $HOME

echo "Contents of $dir:"
ls -lah "$dir"

${1:-$HOME} means "use $1 if it is set and not empty, otherwise use $HOME".

Try it

1. Write a script that greets a name passed as $1
2. Add a default name with ${1:-World}
3. Run it with and without an argument