command substitutions in $PS4

Hello.

[tested on current git head
f6d4def4e27b13fab174e948b94cd10550d3e10e]

Command substitution doesn't seem to work in $PS4 (used for xtrace prompt):

$ PS4='$(date +%T)> ' dash -xc :
dash: 1: Syntax error: end of file unexpected (expecting ")")

And with the old syntax:

$ PS4='`date +%T`> ' dash -xc :

And with more than one command:

$ PS4='`date +%T`> ' dash -xc ':;:'

dash doesn't return and seems to go in a forking loop, presumably because the date there triggers another PS4 expansion and so on recursively

If I prevent the recursion with:

@@ -776,7 +776,15 @@ evalcommand(union node *cmd, int flags)
 		int sep;
 
 		out = &preverrout;
+
+		/*
+		 * reset xflag temporarily for command substitutions performed
+		 * upon $PS4 expansion
+		 */
+		xflag = 0;
 		outstr(expandstr(ps4val()), out);
+		xflag = 1;
+
 		sep = 0;
 		sep = eprintlist(out, varlist.list, sep);
 		eprintlist(out, arglist.list, sep);

$ PS4='`date +%T`> ' ./src/dash -xc ':;sleep 1; date'

21:43:47> sleep 1
21:43:48> date
Mon Nov 17 21:43:48 GMT 2014

The command substitution still fails upon the first expansion only. I quickly gave up trying to find out why as I found the code there hard to follow.

Stephane

In FreeBSD sh, I have disabled command substitution in PS4 (POSIX does not require it anyway). It is treated as a syntax error.

Talking about syntax errors, dash has a problem there as well. If there is a syntax error in PS4 and set -x is enabled, it will not execute any simple commands (assignments to PS4 cause an error and are subsequently reverted). FreeBSD sh prints the error messages and subsequently uses the raw unexpanded text.

Jilles Tjoelker