Userland: Add tee command (#166)

Author: https://github.com/vger92 🔰
Commit: https://github.com/SerenityOS/serenity/commit/1876606973b
Pull-request: https://github.com/SerenityOS/serenity/pull/166
Reviewed-by: https://github.com/awesomekling ✅

Userland/tee.cpp

@@ -0,0 +1,123 @@
+#include <AK/Vector.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <getopt.h>
+#include <signal.h>
+#include <stdio.h>
+#include <unistd.h>
+
+static Vector<int> collect_fds(int argc, char** argv, int start, bool aflag, bool* err)
+{
+    int oflag;
+    mode_t mode;
+    if (aflag) {
+        oflag = O_APPEND;
+        mode = 0;
+    } else {
+        oflag = O_CREAT | O_WRONLY | O_TRUNC;
+        mode = S_IROTH | S_IWOTH | S_IRGRP | S_IWGRP | S_IRUSR | S_IWUSR;
+    }
+
+    Vector<int> fds;
+    for (int i = start; i < argc; ++i) {
+        int fd = open(argv[i], oflag, mode);
+        if (fd < 0) {
+            perror("failed to open file for writing");
+            *err = true;
+        } else {
+            fds.append(fd);
+        }
+    }
+    fds.append(STDOUT_FILENO);
+    return fds;
+}
+
+static void copy_stdin(Vector<int>& fds, bool* err)
+{
+    for (;;) {
+        char buf[4096];
+        ssize_t nread = read(STDIN_FILENO, buf, sizeof(buf));
+        if (nread == 0)
+            break;
+        if (nread < 0) {
+            perror("read() error");
+            *err = true;
+            // a failure to read from stdin should lead to an early exit
+            return;
+        }
+
+        Vector<int> broken_fds;
+        for (int i = 0; i < fds.size(); ++i) {
+            auto fd = fds.at(i);
+            int twrite = 0;
+            while (twrite != nread) {
+                ssize_t nwrite = write(fd, buf + twrite, nread - twrite);
+                if (nwrite < 0) {
+                    if (errno == EINTR) {
+                        continue;
+                    } else {
+                        perror("write() failed");
+                        *err = true;
+                        broken_fds.append(fd);
+                        // write failures to a successfully opened fd shall
+                        // prevent further writes, but shall not block writes
+                        // to the other open fds
+                        break;
+                    }
+                } else {
+                    twrite += nwrite;
+                }
+            }
+        }
+
+        // remove any fds which we can no longer write to for subsequent copies
+        for (auto to_remove : broken_fds)
+            fds.remove_first_matching([&](int fd) { return to_remove == fd; });
+    }
+}
+
+static void close_fds(Vector<int>& fds)
+{
+    for (int fd : fds) {
+        int closed = close(fd);
+        if (closed < 0) {
+            perror("failed to close output file");
+        }
+    }
+}
+
+static void int_handler(int)
+{
+    // pass
+}
+
+int main(int argc, char** argv)
+{
+    bool aflag = false, iflag = false;
+    int c = 0;
+    while ((c = getopt(argc, argv, "ai")) != -1) {
+        switch (c) {
+        case 'a':
+            aflag = true;
+            break;
+        case 'i':
+            iflag = true;
+            break;
+        }
+    }
+
+    if (iflag) {
+        if (signal(SIGINT, int_handler) == SIG_ERR) {
+            perror("failed to install SIGINT handler");
+        }
+    }
+
+    bool err_open = false;
+    bool err_write = false;
+    auto fds = collect_fds(argc, argv, optind, aflag, &err_open);
+    copy_stdin(fds, &err_write);
+    close_fds(fds);
+
+    return (err_open || err_write) ? 1 : 0;
+}
+