Add numbers to the beginning of every line in a file

cat -n thefile will do the job, albeit with the numbers in a slightly different format.

Don't use cat or any other tool which is not designed to do that. Use the program:

nl - number lines of files

Example:

$ nl --number-format=rz --number-width=9 foobar
$ nl -n rz -w 9 foobar # short-hand

Because nl is made for it ;-)

AWK's printf, NR and $0 make it easy to have precise and flexible control over the formatting:

~ $ awk '{printf("%010d %s\n", NR, $0)}' example.txt
0000000001 This is
0000000002 the text
0000000003 from the file.

You're looking for the nl(1) command:

$ nl -nrz -w9  /etc/passwd
000000001   root:x:0:0:root:/root:/bin/bash
000000002   daemon:x:1:1:daemon:/usr/sbin:/bin/sh
000000003   bin:x:2:2:bin:/bin:/bin/sh
...

-w9 asks for numbers nine digits long; -nrz asks for the numbers to be formatted right-justified with zero padding.