/usr/share/zsh/functions/Misc/relative is in zsh-common 5.1.1-1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 | # Print the a relative path from the second directory to the first,
# defaulting the second directory to $PWD if none is specified.
emulate -L zsh || return 1
[[ $1 != /* ]] && print $1 && return
[[ -f $1 ]] && 3=$1:t 1=$1:h
[[ -d $1 && -d ${2:=$PWD} ]] || return 1
[[ $1 -ef $2 ]] && print ${3:-.} && return
# The simplest way to eliminate symlinks and ./ and ../ in the paths:
1=$(cd $1; pwd -r)
2=$(cd $2; pwd -r)
local -a cur abs
cur=(${(s:/:)2}) # Split 'current' directory into cur
abs=(${(s:/:)1} $3) # Split target directory into abs
# Compute the length of the common prefix, or discover a subdirectory:
integer i=1
while [[ i -le $#abs && $abs[i] == $cur[i] ]]
do
((++i > $#cur)) && print ${(j:/:)abs[i,-1]} && return
done
2=${(j:/:)cur[i,-1]/*/..} # Up to the common prefix directory and
1=${(j:/:)abs[i,-1]} # down to the target directory or file
print $2${1:+/$1}
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