Improve bash/ext zsh-isms

bash/ext

@@ -1,36 +1,106 @@
 # Make Bash more like Zsh
 
 
-# Hooks
-# derived from http://glyf.livejournal.com/63106.html
+## Hooks
+# https://zsh.sourceforge.io/Doc/Release/Functions.html#Hook-Functions
 
 # TODO: support precmd_functions and chpwd_functions arrays
 
-# default: NOOPs
+### Defaults: NOOPs
+
 function preexec { :; }
+
 function precmd  { :; }
+
 function chpwd   { :; }
 
+### Implementations
+
+# Alternative cd function that calls chpwd afterwards builtin cd
+#
+# zsh: Executed whenever the current working directory is changed.
 __cd_invoke_chpwd() {
     builtin cd "$@"
+
     chpwd
+
+    # TODO: chpwd_functions array
 }
 
-__preexec_invoke_exec () {
+# zsh: Executed before each prompt. Note that precommand functions are not
+#      re-executed simply because the command line is redrawn, as happens, for
+#      example, when a notification about an exiting job is displayed.
+__prompt_command_invoke_precmd() {
+    precmd
+
+    # TODO: precmd_functions array
+}
+
+# to properly trigger only before command exec this must be the last prompt command
+__prompt_command_hook_preexec() {
+    trap '__debug_invoke_preexec' DEBUG
+}
+
+__prompt_command_unhook_preexec() {
+    # Unfortunately there's no way to remove just one trap from a signal
+    trap - DEBUG
+}
+
+# zsh: Executed just after a command has been read and is about to be executed. If
+#      the history mechanism is active (regardless of whether the line was discarded
+#      from the history buffer), the string that the user typed is passed as the
+#      first argument, otherwise it is an empty string. The actual command that will
+#      be executed (including expanded aliases) is passed in two different forms:
+#      the second argument is a single-line, size-limited version of the command
+#      (with things like function bodies elided); the third argument contains the
+#      full text that is being executed.
+__debug_invoke_preexec () {
+    # somehow PROMPT_COMMAND contents seems to be still affected by the
+    # just-removed trap. Ignore ours.
+    [[ "$BASH_COMMAND" == "__prompt_command_unhook_preexec" ]] && return
+    [[ "$BASH_COMMAND" == "__prompt_command_invoke_precmd" ]] && return
+    [[ "$BASH_COMMAND" == "__prompt_command_hook_preexec" ]] && return
+
+    # TODO: not sure this is still necessary
+    [[ -n "$COMP_LINE" ]] && return  # completion
+
     local this_command;
 
-    [ -n "$COMP_LINE" ] && return  # completion
-    [ "$BASH_COMMAND" = "$PROMPT_COMMAND" ] && return  # precmd
-
+    # TODO: hack: doesn't work when command is not added to history
     this_command=`history 1 | sed -e "s/^[ ]*[0-9]*[ ]*//g"`;
     preexec "$this_command"
 }
 
-# set up the hooks
-PROMPT_COMMAND="precmd"
-trap '__preexec_invoke_exec' DEBUG
+### Set up the hooks
+
+# Define a function to override the cd builtin
+#
+# - does not apply to `bash -c 'cd foo`
+# - does apply to shubshells: `(cd foo)` will call so side effects beware (env
+#   vars are conveniently scoped to subshell)
 cd() { __cd_invoke_chpwd "$@"; }
 
+# Call on each prompt. This transitively sets preexec to work for each prompt
+#
+# PROMPT_COMMAND is evaluated right before PS1 is displayed. This matches zsh
+# semantics for precmd.
+#
+# We forcefully set it instead of taking whatever was set:
+# - to control order (precmd eval + preexec hooking must be last)
+# - because of the trap issue
+PROMPT_COMMAND=(
+    # this also conveniently clears DEBUG traps
+    __prompt_command_unhook_preexec
+
+    # precmd hook processing
+    __prompt_command_invoke_precmd
+
+    # must be last to properly have DEBUG trigger only before command exec
+    __prompt_command_hook_preexec
+)
+
+
+## Other bits
 
 # clears a line that was not terminated by a LF fixing the dangling prompt
 # issue by marking it with a reversed %, like zsh