Determine if relative or absolute path in shell program

Question

As stated in the title, I need to determine when a program is ran if the path is relative or absolute:

./program #relative
dir/dir2/program #relative
~User/dir/dir2/program #absolute
/home/User/dir/dir2/program #absolute

This are my test cases. How exactly could I go about doing this in a shell program?

Or more generally, how to check if a path, $0 in this case, is relative or absolute?

Psychodynamics

The shell will resolve ~ before it runs the script, so you only need to check for a leading / – Dispersion

@Dispersion - only if it is provided as is. In case of echo "~User" it will not. Update: sorry, I've checked the initial question, it was about '${0}', so this is not the case for tilde. – Picnic

@loshadvtapkah If you quote the expression then it means literally the relative path in the directory ./~User -- if that's not what you mean, then the quoting is completely a red herring. – Vere

Answer 1

if [[ "$0" = /* ]]
then
   : # Absolute path
else
   : # Relative path
fi

Columnist

why do you put [[ ]]. – Schoolgirl

@DragosRizescu - With [, I would have to escape the asterisk. With (( the asterisk would stop working at all. The point is, you don't want to match the asterisk against files that happen to exist in your root directory; you want to match it against the other argument of the equals operator. – Columnist

For zsh single = doesn't work. I had to use the [[ "$0" == /* ]] – Tequila

@Tequila - Apparently, the OP and myself are bash users. csh, zsh and some others differ from Bourne shell and bash in using the double equals sign (==) for comparison, because the single equals sign is taken by the assignment operator. This question might need a more specific tag. – Columnist

Answer 2

A general solution for any $path, rather than just $0

POSIX One Liner

[ "$path" != "${path#/}" ] && echo "absolute" || echo "relative"

Magus

Answer 3

case "$directory" in
   /*)
      echo "absolute"
      ;;
   *)
      echo "relative"
      ;;
esac

Sketch

Answer 4

If you don't want to expand the directories in /* you can also use regex:

[[   "$foo" =~ ^/ ]] && echo "absolute" || echo "relative"
[[ ! "$foo" =~ ^/ ]] && echo "relative" || echo "absolute"

Plastered

Answer 5

if [ ${path:0:1} == / ]
then
     echo Absolute path
else
     echo Non-absolute path
fi

Bare

Please use the edit link to format this code and explain how it works and don't just give the code, as an explanation is more likely to help future readers. See also How to Answer. source – Gobo