How does the POSIX tee command work?

Question

tee newOutputFile < existingInputFile > newOutputFile2

How exactly will teeaccept the arguments? Would that be so?

• Tee will process first newOutputFile < existingInputFile So the contents of existingInputFile will be written to newOutputFile
• newOutputFile > newOutputFile2 Thus, the contents of newOutputFile will be written to newOutputFile 2

I am trying to write a shell that processes this particular command. However, I am confused as to what order to pass in the arguments tee. The way I encoded my program will be

tee newOutputFile2 < existingInputFIle

Answer 1

The command teeis a regular Unix program, like shor sortor cat.

-, < existingInputFile > newOutputFile2, tee ( fork, , tee ). , existingInputFile, newOutputFile2. , tee, argv[0] ( tee) argv[1] ( newOutputFile) , .

, existingInputFile; tee, , tee. , newOutputFile2; ( ) tee, , tee - . , tee, , -.

tee , , .

---

, . , execvp, ( tee) ( newOutputFile2). , -. dup2 ?

, . tee newOutputFile < existingInputFile > newOutputFile2 tee, newOutputFile. (tee ) , , , , . , tee, , , , :

some_command arg1 arg2 | tee some_command.log | another_command its_arg1 its_arg2 > output.file

dup2() , , :

// Redirect standard input from existingInputFile using dup2()
char *i_filename = "existingInputFile";
int fd = open(i_filename, O_RDONLY);
if (fd < 0)
{
    fprintf(stderr, "unable to open file %s for reading (%d: %s)\n",
            i_filename, errno, strerror(errno));
    exit(1);
}
dup2(fd, STDIN_FILENO);
close(fd);  // Crucial!

, fd . , . .

:

// Redirect standard input from existingInputFile
close(0);
char *i_filename = "existingInputFile";
int fd = open(i_filename, O_RDONLY);
if (fd < 0)
{
    fprintf(stderr, "unable to open file %s for reading (%d: %s)\n",
            i_filename, errno, strerror(errno));
    exit(1);
}
assert(fd == 0);

// Redirect standard output to NewOutputFile2
close(1);
char * o_filename = "newOutputFile2";
fd = open(o_filename, O_WRONLY|O_CREAT|O_TRUNC, 0644); // Classically 0666
if (fd < 0)
{
    fprintf(stderr, "unable to open file %s for writing (%d: %s)\n",
            o_filename, errno, strerror(errno));
    exit(1);
}
assert(fd == 1);

, open() , , 0, , open() 0 -1 ( 0 ). , , , 1 open() 1 -1 ( 1 ). , -, 2>/dev/null 2>&1 - .

, 0644 :

O_IRUSR|O_IWUSR|O_IRGRP|O_IROTH

( |O_IWGRP|O_IWOTH, (0666), umask ). , , , O_Ixyyy.