add simple head implementation

files/usr/share/man/head.man

@@ -0,0 +1,14 @@
+SYNOPSIS
+    head [-NUM] [FILE]...
+
+DESCRIPTION
+    Output the first part of each FILE.
+
+    Print the first 10 lines of each FILE to standard output.
+    With more than one FILE, precede each with a header giving the file name.
+
+    With no FILE read standard input.
+
+OPTIONS
+    -h, --help  shows command help.
+    -NUM        print only the first NUM lines.

programs/CMakeLists.txt

@@ -11,6 +11,7 @@ set(PROGRAM_LIST
     edit.c
     env.c
     false.c
+    head.c
     id.c
     init.c
     ipcrm.c

programs/head.c

@@ -0,0 +1,111 @@
+/// @file head.c
+/// @brief `head` program.
+/// @copyright (c) 2014-2024 This file is distributed under the MIT License.
+/// See LICENSE.md for details.
+
+#include <err.h>
+#include <fcntl.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <strerror.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/unistd.h>
+
+static int head(int fd, const char *fname, size_t n) {
+    // Count the printed lines
+    int i = 0;
+    // Prepare the buffer for reading.
+    char buffer[BUFSIZ];
+    size_t leftover = 0;
+    char *line = buffer;
+    ssize_t bytes_read = 0;
+
+    while ((bytes_read = read(fd, buffer + leftover, sizeof(buffer) - leftover)) > 0) {
+        char *lineend;
+        while (i < n && (lineend = memchr(line, '\n', sizeof(buffer) - (line - buffer)))) {
+            lineend++; // Include the newline
+            write(STDOUT_FILENO, line, lineend-line);
+            line = lineend;
+            i++;
+        }
+
+        if (i >= n)
+            break;
+
+        leftover = (leftover + bytes_read) - (line - buffer); // Bytes left in the buffer
+        memmove(buffer, line, leftover); // Move the leftover to the front of buffer
+        line = buffer;
+    }
+
+    close(fd);
+    if (bytes_read < 0) {
+        fprintf(STDERR_FILENO, "head: %s: %s\n", fname, strerror(errno));
+        return EXIT_FAILURE;
+    }
+
+    return 0;
+}
+
+int main(int argc, char **argv)
+{
+    int ret = 0;
+    int n = 10;
+    char **file_args_start = argv;
+
+    // Find help argument
+    for (int i = 1; i < argc; ++i) {
+        if ((strcmp(argv[i], "--help") == 0) || (strcmp(argv[i], "-h") == 0)) {
+            printf("Print the first part of files.\n");
+            printf("Usage:\n");
+            printf("    head [-<num>] [FILE]...\n");
+            return 0;
+        }
+    }
+
+    // Detect number of lines argument
+    if (argv[1][0] == '-') {
+        char *nptr = argv[1]+1;
+        char *endptr;
+        errno = 0;
+        n = strtol(nptr, &endptr, 10);
+           if (*endptr || errno == ERANGE) {
+               errx(EXIT_FAILURE, "head: invalid number of lines: `%s`", nptr);
+           }
+
+           if (endptr == nptr) {
+               errx(EXIT_FAILURE, "head: line number option requires an argument");
+           }
+        file_args_start = &argv[1];
+        argc--;
+    }
+
+    // No argument was provided -> read from stdin
+    if (argc == 1) {
+        return head(STDIN_FILENO, "stdin", n);
+    }
+
+    for (int i = 1; i < argc; i++) {
+        const char* fname = file_args_start[i];
+        int fd;
+        if (strcmp(fname, "-") == 0) {
+            fd = STDIN_FILENO;
+        } else {
+            fd = open(fname, O_RDONLY, 0);
+            if (fd < 0) {
+                fprintf(STDERR_FILENO, "head: %s: %s\n", fname, strerror(errno));
+                ret = EXIT_FAILURE;
+                continue;
+            }
+        }
+
+        // Print file header if multiple files are given
+        if (argc > 2) {
+            printf("==> %s <==\n", fname);
+        }
+
+        ret = ret || head(fd, fname, n);
+    }
+    return ret;
+}