uniq:tldr:90d6c

This command is used to find duplicate lines in a file.

Here's a breakdown of the command:

1. sort: This command sorts the lines in the file alphabetically. Sorting the lines is necessary for finding duplicate lines because it brings identical lines next to each other.

2. ${filename}: This is a placeholder for the name of the file that you want to check for duplicate lines. Replace ${filename} with the actual name of your file.

3. |: This is a pipe symbol, used to send the output of one command as input to another command.

4. uniq -d: uniq is a command used to filter out duplicate adjacent lines and only display unique lines. The -d option is used to instruct uniq to only show the duplicate lines.

Therefore, when you run sort ${filename} | uniq -d, it sorts the lines in the file alphabetically and then filters out and displays only the duplicate lines.