Question

I’d like to replace the leading space or tab characters on each line of a file with a like-number of other characters (let’s use _ for simplicity).

E.g.

foo bar
 foo bar
  line 3

becomes

foo bar
_foo bar
__line 3

Note that non-leading whitepsace is not affected, otherwise this would be easy! I suspect it’s still easy, but I’m missing the trick.

Answer 1

This will work for you:

sed ':a;s/^\([[:space:]]*\)[[:space:]]/\1_/;ta' file

Example

$ sed ':a;s/^\([[:space:]]*\)[[:space:]]/\1_/;ta' <<<$'foo bar\n foo bar\n\t\tline 3\n _ x'
foo bar
_foo bar
__line 3
__ x