From LQWiki

The nl command numbers the lines in its input files.

In its simplest invocation, nl concatenates the contents of the file names given as arguments to standard output, with sequential line numbers. A file name of '-' (a single hyphen) as with most Unix commands, means to read from standard input.

A simple example:

$ cat > sample
first line
second line
^D
$ nl sample
     1 first line
     2 second line
$

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