nl

nl is used to number the lines in a file.

$ nl filename 

$ echo "hello
> world
> hi" > hello.txt
$ nl hello.txt
   1  hello
   2  world
   3  hi

Useful Options / Examples

Example command

nl -s

Through the -s option, any string can be added to act as a separator between line numbers and the line text.

$ nl -s"--> " hello.txt
   1--> hello
   2--> world
   3--> hi

Example command

nl -i

The -i flag can be used to override the default increment of line numbers.

$ nl -i4 hello.txt 
   1  hello
   5  world
   9  hi

Example command

nl -b

The -b option has several flags to style numbering:

• t only numbers lines that are non-empty

• a numbers all the lines

• n numbers no lines

• pBRE only numbers lines that match the basic regular expression, BRE

$ echo "hello
> world
>
> hello
> again" > hello.txt

$ nl -ba hello.txt
   1  hello
   2  world
   3
   4  hello
   5  again

In the example above, all lines (including empty lines) are numbered.

$ nl -t hello.txt
   1  hello
   2  world
   
   3  hello
   4  again

In the example above, only non-empty lines are numbered.

$nl -bph hello.txt
   1  hello
      world
      
   2  hello
      again

In the example above, only lines starting with “h” are numbered.