Search code examples
macosunixexpand

Why can unexpand on MacOS change internal spaces without the "-a" argument?

The man page for unexpand states:

By default, only leading blanks and tabs are reconverted to maximal strings of tabs.

However, it seems to definitely be converting internal blanks as well:

echo "  HELLO  THERE" | unexpand -t 2 | tr '\t' '_'

yields

_HELLO_ THERE

instead of the expected

_HELLO  THERE

This is the output I'd expect if I included the -a option, but the output is the same with or without that option.

Solution

  • From the POSIX spec, emphasis added:

    When -t is specified, the presence or absence of the -a option shall be ignored; conversion shall not be limited to the processing of leading <blank> characters.

    The GNU coreutils version of expand (but not the BSD one used by Apple) has a --first-only option which counteracts this behavior.