[builtins/read] Implement basic version of read -p

We're not handling 'read -e -p' yet, which involves GNU readline

Addresses issue #788

frontend/flag_def.py

@@ -112,6 +112,7 @@
 READ_SPEC.ShortFlag('-n', args.Int)
 READ_SPEC.ShortFlag('-a', args.String)  # name of array to read into
 READ_SPEC.ShortFlag('-d', args.String)
+READ_SPEC.ShortFlag('-p', args.String)  # prompt
 
 
 MAPFILE_SPEC = FlagSpec('mapfile')

osh/builtin_misc.py

@@ -180,6 +180,9 @@ def Run(self, cmd_val):
       if arg.s:  # silent
         bits |= pyos.TERM_ECHO
 
+      if arg.p is not None:  # only if tty
+        mylib.Stderr().write(arg.p)
+
     if bits == 0:
       status = self._Read(arg, names)
     else:

spec/builtin-io.test.sh

@@ -660,6 +660,21 @@ fooba
 ## END
 ## N-I dash/zsh/ash stdout-json: ""
 
+#### read -p (not fully tested)
+
+# hm DISABLED if we're not going to the terminal
+# so we're only testing that it accepts the flag here
+
+case $SH in (dash|mksh|zsh) exit ;; esac
+
+echo hi | { read -p 'P'; echo $REPLY; }
+echo hi | { read -p 'P' -n 1; echo $REPLY; }
+## STDOUT:
+hi
+h
+## END
+## stderr-json: ""
+## N-I dash/mksh/zsh stdout-json: ""
 
 #### read usage
 read -n -1