sh: Implement simple parameter expansion in PS1 and PS2

This change follows a localized approach within getprompt() and avoids
full parser reentry. While this means we don't support advanced
expansions like ${parameter#pattern}, it provides POSIX-compliant basic
parameter expansion without the complexity of making the parser
reentrant. This is sufficient for the vast majority of use cases.

PR:		46441

bin/sh/parser.c

@@ -55,6 +55,8 @@
 #include "show.h"
 #include "eval.h"
 #include "exec.h"	/* to check for special builtins */
+#include "main.h"
+#include "jobs.h"
 #ifndef NO_HISTORY
 #include "myhistedit.h"
 #endif
@@ -2050,7 +2052,129 @@ getprompt(void *unused __unused)
 	 * Format prompt string.
 	 */
 	for (i = 0; (i < PROMPTLEN - 1) && (*fmt != '\0'); i++, fmt++) {
-		if (*fmt != '\\') {
+		if (*fmt == '$') {
+			const char *varname_start, *varname_end, *value;
+			char varname[256];
+			int namelen, braced = 0;
+
+			fmt++;  /* Skip the '$' */
+
+			/* Check for ${VAR} syntax */
+			if (*fmt == '{') {
+				braced = 1;
+				fmt++;
+			}
+
+			varname_start = fmt;
+
+			/* Extract variable name */
+			if (is_digit(*fmt)) {
+				/* Positional parameter: $0, $1, etc. */
+				fmt++;
+				varname_end = fmt;
+			} else if (is_special(*fmt)) {
+				/* Special parameter: $?, $!, $$, etc. */
+				fmt++;
+				varname_end = fmt;
+			} else if (is_name(*fmt)) {
+				/* Regular variable name */
+				do
+					fmt++;
+				while (is_in_name(*fmt));
+				varname_end = fmt;
+			} else {
+				/*
+				 * Not a valid variable reference.
+				 * Output literal '$'.
+				 */
+				ps[i] = '$';
+				if (braced && i < PROMPTLEN - 2)
+					ps[++i] = '{';
+				fmt = varname_start - 1;
+				continue;
+			}
+
+			namelen = varname_end - varname_start;
+			if (namelen == 0 || namelen >= (int)sizeof(varname)) {
+				/* Invalid or too long, output literal */
+				ps[i] = '$';
+				fmt = varname_start - 1;
+				continue;
+			}
+
+			/* Copy variable name */
+			memcpy(varname, varname_start, namelen);
+			varname[namelen] = '\0';
+
+			/* Handle closing brace for ${VAR} */
+			if (braced) {
+				if (*fmt == '}') {
+					fmt++;
+				} else {
+					/* Missing closing brace, treat as literal */
+					ps[i] = '$';
+					if (i < PROMPTLEN - 2)
+						ps[++i] = '{';
+					fmt = varname_start - 1;
+					continue;
+				}
+			}
+
+			/* Look up the variable */
+			if (namelen == 1 && is_digit(*varname)) {
+				/* Positional parameters - check digits FIRST */
+				int num = *varname - '0';
+				if (num == 0)
+					value = arg0 ? arg0 : "";
+				else if (num > 0 && num <= shellparam.nparam)
+					value = shellparam.p[num - 1];
+				else
+					value = "";
+			} else if (namelen == 1 && is_special(*varname)) {
+				/* Special parameters */
+				char valbuf[20];
+				int num;
+
+				switch (*varname) {
+				case '$':
+					num = rootpid;
+					break;
+				case '?':
+					num = exitstatus;
+					break;
+				case '#':
+					num = shellparam.nparam;
+					break;
+				case '!':
+					num = backgndpidval();
+					break;
+				default:
+					num = 0;
+					break;
+				}
+				snprintf(valbuf, sizeof(valbuf), "%d", num);
+				value = valbuf;
+			} else {
+				/* Regular variables */
+				value = lookupvar(varname);
+				if (value == NULL)
+					value = "";
+			}
+
+			/* Copy value to output, respecting buffer size */
+			while (*value != '\0' && i < PROMPTLEN - 1) {
+				ps[i++] = *value++;
+			}
+
+			/*
+			 * Adjust fmt and i for the loop increment.
+			 * fmt will be incremented by the for loop,
+			 * so position it one before where we want.
+			 */
+			fmt--;
+			i--;
+			continue;
+		} else if (*fmt != '\\') {
 			ps[i] = *fmt;
 			continue;
 		}

bin/sh/tests/parser/Makefile

@@ -86,6 +86,12 @@ ${PACKAGE}FILES+=	only-redir2.0
 ${PACKAGE}FILES+=	only-redir3.0
 ${PACKAGE}FILES+=	only-redir4.0
 ${PACKAGE}FILES+=	pipe-not1.0
+${PACKAGE}FILES+=	ps1-expand1.0
+${PACKAGE}FILES+=	ps1-expand2.0
+${PACKAGE}FILES+=	ps1-expand3.0
+${PACKAGE}FILES+=	ps1-expand4.0
+${PACKAGE}FILES+=	ps1-expand5.0
+${PACKAGE}FILES+=	ps2-expand1.0
 ${PACKAGE}FILES+=	set-v1.0 set-v1.0.stderr
 ${PACKAGE}FILES+=	var-assign1.0
 

bin/sh/tests/parser/ps1-expand1.0

@@ -0,0 +1,7 @@
+# Test simple variable expansion in PS1
+testvar=abcdef
+output=$(testvar=abcdef PS1='$testvar:' ENV=/dev/null ${SH} +m -i </dev/null 2>&1)
+case $output in
+*abcdef*) exit 0 ;;
+*) echo "Expected 'abcdef' in prompt output"; exit 1 ;;
+esac

bin/sh/tests/parser/ps1-expand2.0

@@ -0,0 +1,7 @@
+# Test braced variable expansion in PS1
+testvar=xyz123
+output=$(testvar=xyz123 PS1='prefix-${testvar}-suffix:' ENV=/dev/null ${SH} +m -i </dev/null 2>&1)
+case $output in
+*xyz123*) exit 0 ;;
+*) echo "Expected 'xyz123' in prompt output"; exit 1 ;;
+esac

bin/sh/tests/parser/ps1-expand3.0

@@ -0,0 +1,8 @@
+# Test special parameter $$ (PID) in PS1
+output=$(PS1='pid:$$:' ENV=/dev/null ${SH} +m -i </dev/null 2>&1)
+# Check that output contains "pid:" followed by a number (not literal $$)
+case $output in
+*pid:\$\$:*) echo "PID not expanded, got literal \$\$"; exit 1 ;;
+*pid:[0-9]*) exit 0 ;;
+*) echo "Expected PID after 'pid:' in output"; exit 1 ;;
+esac

bin/sh/tests/parser/ps1-expand4.0

@@ -0,0 +1,8 @@
+# Test special parameter $? (exit status) in PS1
+output=$(PS1='status:$?:' ENV=/dev/null ${SH} +m -i </dev/null 2>&1)
+# Should start with exit status 0
+case $output in
+*status:\$?:*) echo "Exit status not expanded, got literal \$?"; exit 1 ;;
+*status:0:*) exit 0 ;;
+*) echo "Expected 'status:0:' in initial prompt"; exit 1 ;;
+esac

bin/sh/tests/parser/ps1-expand5.0

@@ -0,0 +1,8 @@
+# Test positional parameter $0 in PS1
+output=$(PS1='shell:$0:' ENV=/dev/null ${SH} +m -i </dev/null 2>&1)
+# $0 should contain the shell name/path
+case $output in
+*shell:\$0:*) echo "Positional parameter not expanded, got literal \$0"; exit 1 ;;
+*shell:*sh*:*) exit 0 ;;
+*) echo "Expected shell name after 'shell:' in output"; exit 1 ;;
+esac

bin/sh/tests/parser/ps2-expand1.0

@@ -0,0 +1,12 @@
+# Test variable expansion in PS2 (continuation prompt)
+testvar=continue
+# Send incomplete command (backslash at end) to trigger PS2
+output=$(testvar=continue PS2='$testvar>' ENV=/dev/null ${SH} +m -i <<EOF 2>&1
+echo \\
+done
+EOF
+)
+case $output in
+*continue\>*) exit 0 ;;
+*) echo "Expected 'continue>' in PS2 output"; exit 1 ;;
+esac