Bash, the powerful command-line interface for Linux and Unix systems, offers a suite of tools known as filters that streamline text processing. In this post, we’ll focus on three essential filters: head, tail, and wc. Let’s uncover how these tools can help you extract, analyze, and manipulate text data with ease.
What are Filters?
Filters are programs that take input from standard input (stdin), process it, and then output the results to standard output (stdout). This allows you to chain commands together using pipes (|) for efficient data flow.
head: Getting the First Glimpse
The head command displays the first few lines of a file or input. By default, it shows the first 10 lines, but you can customize this with the -n option.
head -n 5 file.txt # Display the first 5 lines of file.txttail: Seeing the End
The tail command does the opposite, showing the last few lines. It also defaults to 10 lines and accepts the -n option. Additionally, tail has a powerful -f (follow) option for monitoring files that are being updated in real time.
tail -n 3 logfile.txt # Display the last 3 lines of logfile.txt
wc: Counting Words, Lines, and Characters
The wc command provides counts for words, lines, and characters in a file or input. Use options like -l (lines), -w (words), and -c (characters) to specify what you want to count.
wc -l myfile.txt # Count the number of lines in myfile.txt
Combining Filters for Precision
Filters are even more powerful when combined. For example, to display lines 11 through 15 of a file:
head -n 15 file.txt | tail -n 5
Example: Monitoring Log Files
tail -f /var/log/syslog # Continuously monitor system log
Here Document Example
while read -r time; do
echo "Current time: $time"
sleep 1
done << EOF
$(date)
$(date)
$(date)
EOF
This script outputs the current time three times, with a one-second delay between each output. The << EOF creates a “here document” that provides input to the while loop.