wip(commands): re-implement core commands as bash functions

Re-implement all libexec executables as bash functions, with the goal of
being able build a single standalone execuable bash script for
tmuxifier.

lib/commands/tmux-version.sh

@@ -0,0 +1,63 @@
+# Enable extended globbing for version string cleanup.
+shopt -s extglob
+
+# Output current Tmux version, or compare against a target version.
+#
+# Usage:
+#   tmuxifier-tmux-version             # Outputs current Tmux version
+#   tmuxifier-tmux-version "1.9"       # Outputs "=", "<", or ">"
+#
+# Arguments:
+#   $1 - Optional target version to compare against
+#
+# Output:
+#   Without arguments: The current Tmux version string
+#   With target version: One of "=", "<", or ">" indicating if the current
+#                        Tmux version is equal to, less than, or greater than
+#                        the target version.
+#
+# Returns:
+#   0 - Always succeeds
+tmuxifier-tmux-version() {
+  # Provide tmuxifier help
+  if calling-help "$@"; then
+    echo "usage: tmuxifier tmux-version [<target-version>]
+
+Outputs current Tmux version. If given optional target-version it outputs one
+of three possible characters indicating if the current Tmux version number is
+equal to, less than, or greater than the <target-version>.
+
+The three possible outputs are \"=\", \"<\", and \">\"."
+    return
+  fi
+
+  local version
+  version="$(tmux -V)"
+  version="${version/tmux /}"
+
+  # Fix for tmux next-* versions
+  version="${version/next-/}"
+
+  if [ -z "$1" ]; then
+    echo "$version"
+    return
+  fi
+
+  if [ "$version" == "master" ]; then
+    # When version string is "master", tmux was compiled from source, and we
+    # assume it's later than whatever the <target-version> is.
+    echo '>'
+  else
+    # Fix for "1.9a" version comparison, as vercomp() can only deal with
+    # purely numeric version numbers.
+    version="${version//+([a-zA-Z])/}"
+
+    local result
+    vercomp "$version" "$1" && result=$? || result=$?
+    case $result in
+      0) echo '=' ;;
+      1) echo '>' ;;
+      2) echo '<' ;;
+    esac
+  fi
+}