How do you read lines from a file in a shell script?

In Shell scripting, there are several common methods to read lines from a file. Below, I will introduce several commonly used methods along with examples:

Method 1: Using while Loop with read Command

This is one of the most commonly used methods, which reads each line of the file through a loop. Here is an example:

bash
while IFS= read -r line
do
  echo "Read line: $line"
done < "file.txt"

In this script, IFS= (Internal Field Separator) ensures proper handling of spaces within lines, read -r prevents backslash characters from being misinterpreted, and < "file.txt" redirects the contents of the file file.txt into the while loop.

Method 2: Using cat with Pipes

Another approach involves combining the cat command with pipes to read file lines:

bash
cat file.txt | while IFS= read -r line
do
  echo "Read line: $line"
done

This method functions similarly to the first one but may be slightly slower in certain scenarios, such as when dealing with very large input files.

Method 3: Using awk

awk is a powerful text processing utility that can also be used to read file lines:

bash
awk '{print "Read line: " $0}' file.txt

Here, $0 represents the current line's content, and awk processes the file line by line by default.

Method 4: Using sed

Although sed is primarily designed for text replacement, it can also be used to read and print each line of a file:

bash
sed -n 'p' file.txt

The -n option instructs sed not to automatically print each line, while the 'p' command directs sed to print the current line.

Each method has its own strengths and limitations, and the choice depends on the specific use case and personal preference. In practical work, we typically select the appropriate method based on file size, processing complexity, and performance requirements.