nl (Unix)

nl is a Unix utility for numbering lines, either from a file or from standard input, reproducing output on standard output.

It has a number of switches:

a - number all lines
t - number lines with printable text only
n - no line numbering
string - number only those lines containing the regular expression defined in the string supplied.

The default applied switch is t.

nl also supports a number of command line options.

Example

$ nl tf
    1  echo press cr
    2  read cr
    3  done

The following example numbers only the lines that begin with a capital letter A (matching on the regular expression /^A/). filename is optional.

$ nl -b p^A filename
       apple
    1  Apple
       BANANA
    2  Allspice
       strawberry

It can be useful as an alternative to grep -n:

$ cat somefile
aaa
bbb
ccc
ddd
$ nl somefile | grep ccc
    3 ccc