Reading Input and Handling Script Arguments

Two ways to give a script information

1. Ask while it runs. The script pauses and asks a question. This uses the read command.
2. Pass it at launch. You type extra words after the script name. These are called arguments.

Asking with read

#!/bin/bash
read -p "What is your name? " name
echo "Nice to meet you, $name"

read waits for the user to type a line and stores it in the variable. -p shows a prompt first. More options:

read -s -p "Password: " pass
read -t 5 -p "Quick, answer: " answer
read -n 1 -p "Continue? (y/n) " key

• -s hides what is typed, which is good for passwords.
• -t 5 gives up after 5 seconds.
• -n 1 reads a single key without waiting for Enter.

Arguments at launch

Anything typed after the script name is available inside the script as numbered variables.

./greet.sh Alex London

• $0 is the script name (./greet.sh)
• $1 is the first argument (Alex)
• $2 is the second argument (London)

#!/bin/bash
echo "Hello $1 from $2"

Counting and listing arguments

• $# is the number of arguments.
• $@ is all the arguments, with each one kept as a separate word when quoted as "$@".

#!/bin/bash
echo "You passed $# arguments"
for arg in "$@"; do
    echo "Argument: $arg"
done

Validating arguments

A good script checks its input and explains how to use it when something is missing.

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

-ne means "not equal", so this runs when the number of arguments is not exactly 1. The message shows the correct usage, and exit 1 stops the script with a failure code.

Default values

name="${1:-friend}"
echo "Hello, $name"

${var:-default} uses default when the variable is empty or not set. Here, running the script with no argument prints Hello, friend.

Flags with getopts

Many programs accept flags such as -v or -n 5. The built-in getopts reads them.

#!/bin/bash
verbose=0
count=1
while getopts "vn:" opt; do
    case "$opt" in
        v) verbose=1 ;;
        n) count="$OPTARG" ;;
    esac
done

The string "vn:" lists the allowed flags. A letter followed by a colon, here n:, takes a value. That value is stored in $OPTARG. A flag without a colon, here v, is just on or off.

Exit codes

Every program ends with an exit code, a number that tells whether it worked.

exit 0
exit 1

• 0 means success.
• Any other number means a problem. Use different numbers for different problems, for example 1 for bad usage and 2 for a missing file.

The caller reads the code from $?. Tools such as if and && use it to decide what to do next.