Identifying Duplicate Lines in a File Using Command Line

In this article, we will delve into how to identify duplicate lines in a file using the command line. This is a common task for system administrators and developers who often need to clean up data or debug code. We’ll cover several methods, including using sort, uniq, cut, grep, and awk.

Using sort and uniq

The sort and uniq commands are often used together to identify duplicate lines in a file.

Here’s how you can use them:

sort -k1,1 file.txt | uniq -d

In this command, sort -k1,1 file.txt sorts the file based on the first field. The sorted output is then piped (|) into uniq -d, which prints only the duplicate lines.

• sort: This command sorts lines in text files.
• -k1,1: This option tells sort to sort based on the first field.
• uniq: This command filters out repeated lines in a file.
• -d: This option tells uniq to print only duplicate lines.

Using cut, sort, and uniq

If you want to identify duplicate lines based on a specific field, you can use the cut command in addition to sort and uniq:

cut -d " " -f1 file.txt | sort | uniq -d

In this command, cut -d " " -f1 file.txt extracts the first field from each line. The extracted fields are then sorted and passed to uniq -d to print the duplicates.

• cut: This command removes sections from each line of files.
• -d " ": This option sets the delimiter to a space.
• -f1: This option selects the first field.

Using grep and a loop

You can also use a loop and the grep command to identify duplicate lines:

for dup in $(cut -d " " -f1 file.txt | uniq -d); do
 grep -- "$dup" file.txt
done

In this command, cut -d " " -f1 file.txt | uniq -d finds the duplicate first fields. The for loop then iterates over these duplicates, and grep -- "$dup" file.txt prints the corresponding lines from the file.

• grep: This command searches the given file for lines containing a match to the given strings or words.

Using awk

awk is a powerful text-processing command. Here’s how you can use it to identify duplicate lines:

#!/usr/bin/awk -f
{
 lines[$1][NR] = $0;
}
END {
 for (url in lines) {
 if (length(lines[url]) > 1) {
 for (lineno in lines[url]) {
 print lines[url][lineno];
 }
 }
 }
}

In this script, lines[$1][NR] = $0; stores lines grouped by the first field. The END block then iterates over these groups and prints any groups that contain more than one line.

• awk: This command scans and processes text.
• NR: This variable holds the number of input records awk has processed since the beginning of the program.
• $0: This variable holds the entire line.

To run this script on a file, save it as script.awk and run awk -f script.awk file.txt.

These methods should give you a solid start on identifying duplicate lines in a file using the command line. Remember, the best method to use will depend on your specific needs and the structure of your data. Happy coding!

For more information on these commands, you can check their manual pages by typing man <command> in the terminal. For example, man sort will provide a detailed manual on the sort command.