Bash: The `read` Command

Basic Usage

echo "Hello World" | while read var1 var2; do
    echo "var1: $var1"
    echo "var2: $var2"
done

# Result:
# var1: Hello
# var2: World

By default, read uses the characters in the IFS variable (space, tab, newline) to split the input line into words and assigns them sequentially to the variables provided.

The `-p` (prompt) Option

The -p option allows you to display a prompt message to the user before reading the input. This is a crucial feature for creating interactive scripts.

read -p "What is your name? " name
echo "Hello, $name!"

# A sample interaction:
# What is your name? John
# Hello, John!

This is the most common and user-friendly way to get input from a user in a Bash script.

The `-r` (raw) Option

The -r option prevents read from interpreting backslash escapes. It is highly recommended to always use -r to avoid unexpected behavior.

# Without -r, \t is interpreted as a tab character
echo "This is a line with a tab: \t" | while read line; do
    echo "Output: '$line'"
done

# Result (tab is removed):
# Output: 'This is a line with a tab:  '

# With -r, \t is treated as a literal string
echo "This is a line with a tab: \t" | while read -r line; do
    echo "Output: '$line'"
done

# Result:
# Output: 'This is a line with a tab: \t'

The `-d` (delimiter) Option

The -d option allows you to specify a single character as the delimiter for the entire input. The read command will stop reading when it encounters this delimiter instead of a newline.

echo "alpha:beta:gamma" | while read -r -d: word; do
    echo "Word: $word"
done

# Result:
# Word: alpha
# Word: beta
# Word: gamma

The `-d` option is especially useful for processing data that is not separated by newlines, such as comma-separated values (CSV) on a single line.

The Relationship between `-d`, `IFS`, and Word Splitting

Understanding the interaction between these is crucial for advanced scripting.

# The `read` command trims characters from the beginning and end of the read string if they are in the IFS variable.
# The -d option only changes the end-of-input delimiter.
# This example uses a pipeline to show the behavior clearly.

IFS=' '
echo "  First Second, Third     Fourth   ," | while read -r -d, word; do
  echo "=$word="
done

# Result:
# =  First Second=
# = Third     Fourth=

In this case, read -d, reads each segment until a comma is found. Since the default IFS includes spaces, read trims leading and trailing spaces from each segment. However, it does not split words in the middle of a segment; for example, "First Second" is not broken into two words. The comma is not part of the `IFS` so it is not trimmed.

Reading a File Line by Line

The most common use case for read is to process a file line by line within a while loop. This is the most efficient and reliable method.

while read -r line; do
    echo "Processing line: $line"
done < file.txt

By default, read uses the newline character as its delimiter, making it perfect for this task. The -r option is used to ensure any backslashes in the file are treated literally.