read

Basic Usage: Reading a Single Line

At its core, read takes input from the standard input (typically your keyboard) and assigns it to a variable.

read myVariable
echo "You entered: $myVariable"

This script prompts the user for input. Whatever the user types (followed by Enter) is stored in the myVariable variable and then printed to the console.

Handling Multiple Variables

read can simultaneously assign input to multiple variables, separating input based on whitespace.

read name age city
echo "Name: $name, Age: $age, City: $city"

If the user enters “John Doe 30 New York”, name will be “John Doe”, age will be “30”, and city will be “New York”. Note that this relies on whitespace as the delimiter.

Specifying a Delimiter

For finer control over input separation, use the -d option to specify a custom delimiter.

read -d ',' var1 var2 var3 <<< "apple,banana,orange"
echo "Var1: $var1, Var2: $var2, Var3: $var3"

This example uses a comma as the delimiter. The <<< operator provides the input string directly to read.

Reading from a File

While primarily used for terminal input, read can also read from files using input redirection.

while IFS= read -r line; do
  echo "$line"
done < myfile.txt

This script reads myfile.txt line by line. IFS= read -r is crucial: IFS= prevents word splitting, and -r prevents backslash escapes from being interpreted.

Prompting the User with a Message

You can add a prompt to the user input using the -p option:

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

This adds “Enter your name:” to the prompt, enhancing user experience.

Setting a Timeout

For situations requiring timed input, the -t option sets a timeout in seconds.

read -t 5 -p "Enter your password (5 seconds): " password
if [ -z "$password" ]; then
  echo "Timeout!"
else
  echo "Password entered: $password"
fi

If the user doesn’t enter a password within 5 seconds, the script prints “Timeout!”.

Ignoring Leading and Trailing Whitespace

The -r option, as mentioned earlier, is for preserving backslashes. Combining it with parameter expansion to remove leading/trailing whitespace offers further control:

read -r input
input="${input#"${input%%[![:space:]]*}"}"  #remove leading whitespace
input="${input%"${input##*[![:space:]]}"}"  #remove trailing whitespace
echo "Cleaned Input: $input"

This example uses parameter expansion to remove leading and trailing whitespace from the input.

Handling Escape Sequences

By default, read interprets backslash escape sequences (like \n for newline). The -r option prevents this interpretation, preserving the literal backslash.

These examples demonstrate the versatility of the read command. From simple single-line input to complex file processing and timeout handling, read is an essential tool in any Linux user’s or script writer’s arsenal.