Question

Given the plain text file with lines

bli foo bla
 abc
 dfg
bli foo bla
 hik
 lmn

what sed or awk magic transforms it to

bli foo_01 bla
 abc
 dfg
bli foo_02 bla
 hik
 lmn

so that every occurence of ‘foo’ is replaced by ‘foo_[occurence number]’.

Answer 1

This is another way to express radoulov’s answer

awk '/foo/ {sub(/foo/, "&_" sprintf("%02d",++c))} 1' infile

You should take care that you don’t match “foobar” while looking for “foo”:

gawk '/\<foo\>/ {sub(/\<foo\>/, "&_" sprintf("%02d",++c))} 1'