Writing to both stdout & a file

Related searches

I have a parent process which forks out a child to perform execv().

I need the output given by execv() to stdout to be displayed onscreen as also copied to a log file.

How do I write the same output to both stdout & a file, without using pipes or tees?

If you don't want to use a tee, before you write the data, write it to a file, then send it to stdout.

You should write a logging function that does this for you to make it cleaner.

How to print logging messages to both stdout and to a file in Python, Printing to both stdout and to a file prints output to the standard output stream as well as to a specified file. Use the logging module to print logging messages to� Again, this pipes stderr/stdout together to tee, which writes it both to stdout and my logfile. I know how to write stdout and stderr to a logfile in Python. Where I'm stuck is how to duplicate these back to the screen: subprocess.Popen("cat file", shell=True, stdout=logfile, stderr=logfile)

Pipe it through tee.

bash - Command that writes to both stdout and stderr, I'd use something like ls file not-a-file. where file exists in the current directory and not-a-file doesn't. Obviously you could substitute ls for any� When you use the construction: 1>stdout.log 2>&1 both stderr and stdout get redirected to the file because the stdout redirection is set up before the stderr redirection. If you invert order you can get stdout redirected to a file and then copy stderr to stdout so you can pipe it to tee.

You can use dup2() - this link provides an example

Show only stderr on screen but write both stdout and stderr to file , Even without any redirection, or with nothing but >logfile 2>&1 , you're not guaranteed to see output in order of generation. For starters, the stdout from the� >>file.txt: Open file.txt in append mode and redirect stdout there. 2>&1: Redirect stderr to "where stdout is currently going". In this case, that is a file opened in append mode. In other words, the &1 reuses the file descriptor which stdout currently uses.

You can do this entirely within your program, but you will still need to use anonymous pipes as created by the pipe() system call.

Basically, you will need a subprocess that performs the equivalent of tee, which is quite easy:

int child_pipe[2];
pid_t pid_exec_child, pid_output_child;

pipe(child_pipe);

pid_exec_child = fork();
if (pid_exec_child == 0)
{
    dup2(child_pipe[1], STDOUT_FILENO);
    close(child_pipe[0]);
    close(child_pipe[1]);
    execve(/* ... */);
    _exit(127);
}

close(child_pipe[1]);

pid_output_child = fork();
if (pid_output_child == 0)
{
    /* This child just loops around, reading from the other child and writing everything
     * to both stdout and the log file. */
    int logfd = open("logfile", O_WRONLY);
    char buffer[4096];
    ssize_t nread;

    while ((nread = read(child_pipe[0], buffer, sizeof buffer) != 0)
    {
        size_t nwritten_total;
        ssize_t nwritten;

        if (nread < 0)
        {
            if (errno == EINTR)
                continue;

            perror("read");
            _exit(1);
        }

        /* Copy data to stdout */
        nwritten_total = 0;
        while (nwritten_total < nread)
        {
            nwritten = write(STDOUT_FILENO, buffer + nwritten_total, nread - nwritten_total);

            if (nwritten < 0)
            {
                if (errno == EINTR)
                    continue;

                perror("write(stdout)");
                _exit(2);
            }

            nwritten_total += nwritten;
        }

        /* Copy data to logfile */
        nwritten_total = 0;
        while (nwritten_total < nread)
        {
            nwritten = write(logfd, buffer + nwritten_total, nread - nwritten_total);

            if (nwritten < 0)
            {
                if (errno == EINTR)
                    continue;

                perror("write(logfile)");
                _exit(3);
            }

            nwritten_total += nwritten;
        }
    }
    _exit(0);
}

close(child_pipe[0]);

/* Parent continues here */

Of course, it is probably easier to just exec tee in that second child instead...

(Note that the child processes use _exit(), since they have inherited the standard I/O state from the parent).

How to print to stderr and stdout in Python?, stdin, and whenever exceptions occur it is written to sys.stderr. We can redirect the output of our code to a file other than stdout. But you may be� DIR SomeFile.txt 2>&1. This is useful if you want to write both stdout and stderr to a single log file. DIR SomeFile.txt > output.txt 2>&1. To use the contents of a file as the input to a program, instead of typing the input from the keyboard, use the <operator. SORT < SomeFile.txt.

Also, you can use fifo's. mkfifo my.fifo; in execv: program > my.fifo; and open fifo as a regular file, reading from it. This way you can have your stdout parsed, but it has minor drawbacks with shared access.

Writing to a File with Python's print() Function, Instead, it sends text to a location called the standard output stream, also known as stdout . All UNIX systems have three main pipes - standard input pipe ( stdin ), � To redirect stderr to stdout and have error messages sent to the same file as standard output, use the following: command > file 2>&1. > file redirect the stdout to file, and 2>&1 redirect the stderr to the current location of stdout. The order of redirection is important.

The output echo generates is written in the stdout stream. Then, the piping redirects the content of stdout to stdin for the grep command. That’s how grep knows what content to perform the operation on. If you want to pipe both the stderr and stdout to the next command, then use the “|&” instead.

stdout and stderr both quack the same, but are used for very different purposes. Every process is initialized with three open file descriptors, stdin , stdout , and stderr . stdin is an abstraction for accepting input (from the keyboard or from pipes) and stdout is an abstraction for giving output (to a file, to a pipe, to a console).

Surely, if we can redirect either stdout or stderr to a file independently of one another, we ought to be able to redirect them both at the same time, to two different files? Yes, we can. This command will direct stdout to a file called capture.txt and stderr to a file called error.txt.

Comments
  • As written, your question will trigger the "use tee" response from a great many people. Consider editing to make it clear that you wish to do this internally...
  • Ad duplicate (though slightly different usage) of this: stackoverflow.com/questions/1760726/compose-output-streams Though I like my answer best.
  • possible duplicate of Copy STDOUT to file without stopping it showing onscreen
  • @Ether, the languages differ.
  • is there a way to do this without pipes or tees, using just the dup() family?
  • That link provides an example where stdout is redirected to a file and then redirected back to normal stdout. The poster was after a way for a normal print statement to be written to both stdout and a file.
  • @Byron - the fork is not there for printing purposes, just part of the logic of the script that has an example. the post down the same thread has pure logic with no fork: linuxforums.org/forum/linux-programming-scripting/…
  • As a'r said this doesn't answer the original question of writing to both stdout and file.