`command -v` portability

The "which" utility is not guaranteed to be installed, and if it is, its behavior is not portable.

Conversely, the "command -v" shell builtin is required to exist in all POSIX 2008 compliant shells, and is thus guaranteed to work everywhere.

Examples of open-source shells likely to be installed as /bin/sh on Linux, which implement the 12-year-old standard: ash, bash, busybox, dash, ksh, mksh and zsh.

A side benefit of using the POSIX portable option is that it does not require an external disk executable to be forked. This therefore represents a mild speedup.

scripts/xdg-email.in

@@ -98,6 +98,9 @@ open_kde()
                                         --group "PROFILE_$profile" \
                                         --key EmailClient \
                                   | cut -d ' ' -f 1)
+            if [ -z "${client%%*.desktop}" ] && ! which "$client" >/dev/null 2>&1; then
+                client=`desktop_file_to_binary "$client"`
+            fi
 
             if echo "$client" | grep -Eq 'thunderbird|icedove'; then
                 run_thunderbird "$client" "$1"