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The discussion in the comments of this answer made me wonder how hard it would be to write a built-in only version of dirname.

The discussion in the comments of this answer made me wonder how hard it would be to write a built-in only version of dirname.

Update: Made a gist .

#!/bin/sh
p=1ドル
skip78=
stripslashes() {
 i=1ドル
 while [ "$i" != "$o" ]; do
 o=$i
 i=${i%/}
 done
 eval "2ドル=\$i"
}
[ "$p" = '//' ] || {
 case "$p" in
 *[!/]*|'')
 stripslashes "$p" p
 case "$p" in
 */*)
 p=${p%/*}
 ;;
 *)
 p=.
 skip78=skip78
 ;;
 esac
 ;;
 *)
 p=/
 skip78=skip78
 ;;
 esac
}
[ -n "$skip78" ] || {
 { [ "$p" != '//' ] || [ -n "2ドル" ]; } && {
 stripslashes "$p" p
 [ -z "$p" ] && p=/
 }
}
printf -- %s\\n "$p"

#!/bin/sh
p=1ドル
alt=2ドル
skip78=
stripslashes() {
 i=1ドル
 while [ "$i" != "$o" ]; do
 o=$i
 i=${i%/}
 done
 eval "2ドル=\$i"
}
[ "$p" = '//' ] || {
 case "$p" in
 *[!/]*|'')
 stripslashes "$p" p
 case "$p" in
 */*)
 p=${p%/*}
 ;;
 *)
 p=.
 skip78=skip78
 ;;
 esac
 ;;
 *)
 p=/
 skip78=skip78
 ;;
 esac
}
[ -n "$skip78" ] || {
 { [ "$p" != '//' ] || [ -z "$alt" ]; } && {
 stripslashes "$p" p
 [ -z "$p" ] && p=/
 }
}
printf -- %s\\n "$p"