userland: add "grep" command

userland/Makefile

@@ -15,6 +15,7 @@ BIN_TARGET_NAMES := \
 	env \
 	eyes \
 	fib \
+	grep \
 	halt \
 	imgview \
 	init \

userland/grep.c

@@ -0,0 +1,69 @@
+// grep - print lines matching a pattern
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+int main(int argc, char* const argv[]) {
+    if (argc < 2) {
+        dprintf(STDERR_FILENO, "Usage: grep PATTERN [FILE]...\n");
+        return EXIT_FAILURE;
+    }
+
+    const char* pattern = argv[1];
+
+    for (int i = 2; i < argc; i++) {
+        const char* filename = argv[i];
+
+        struct stat st;
+        if (stat(filename, &st) < 0) {
+            perror("stat");
+            return EXIT_FAILURE;
+        }
+
+        char* buf = malloc(st.st_size + 1); // +1 for null terminator
+        if (!buf) {
+            perror("malloc");
+            return EXIT_FAILURE;
+        }
+
+        int fd = open(filename, O_RDONLY);
+        if (fd < 0) {
+            perror("open");
+            goto done;
+        }
+
+        size_t cursor = 0;
+        ssize_t nread;
+        while ((nread = read(fd, buf + cursor, st.st_size - cursor)) > 0)
+            cursor += nread;
+        if (nread < 0) {
+            perror("read");
+            goto done;
+        }
+        close(fd);
+        fd = -1;
+        buf[cursor] = 0;
+
+        static const char* sep = "\n";
+        char* saved_ptr;
+        for (char* line = strtok_r(buf, sep, &saved_ptr); line;
+             line = strtok_r(NULL, sep, &saved_ptr)) {
+            if (strstr(line, pattern)) {
+                if (argc > 3)
+                    printf("%s:%s\n", filename, line);
+                else
+                    puts(line);
+            }
+        }
+
+    done:
+        free(buf);
+        if (fd >= 0)
+            close(fd);
+    }
+
+    return EXIT_SUCCESS;
+}