duplicating pipe content


Copy standard input to each FILE, and also to standard output

$ echo "example" | tee [FILE]

Append to the given FILEs, do not overwrite

$ echo "example" | tee -a [FILE]

Print standard input to the terminal, and also pipe it into another program for further processing

$ echo "example" | tee [/dev/tty] | [xargs printf "[%s]"]

Create a directory called "example", count the number of characters in "example" and write "example" to the terminal

$ echo "example" | tee >(xargs mkdir) >(wc -c)


#define _GNU_SOURCE/* See feature_test_macros(7) */ #include <fcntl.h>

ssize_t tee(intfd_in, intfd_out, size_tlen ", unsigned int " flags );


tee() duplicates up to len bytes of data from the pipe referred to by the file descriptor fd_in to the pipe referred to by the file descriptor fd_out. It does not consume the data that is duplicated from fd_in; therefore, that data can be copied by a subsequent splice(2).

flags is a bit mask that is composed by ORing together zero or more of the following values:

1.9i SPLICE_F_MOVE Currently has no effect for tee(); see splice(2).

SPLICE_F_NONBLOCK Do not block on I/O; see splice(2) for further details.

SPLICE_F_MORE Currently has no effect for tee(), but may be implemented in the future; see splice(2).

SPLICE_F_GIFT Unused for tee(); see vmsplice(2).


Upon successful completion, tee() returns the number of bytes that were duplicated between the input and output. A return value of 0 means that there was no data to transfer, and it would not make sense to block, because there are no writers connected to the write end of the pipe referred to by fd_in.

On error, tee() returns -1 and errno is set to indicate the error.


EAGAIN SPLICE_F_NONBLOCK was specified in flags or one of the file descriptors had been marked as nonblocking (O_NONBLOCK), and the operation would block.

EINVAL fd_in or fd_out does not refer to a pipe; or fd_in and fd_out refer to the same pipe.

ENOMEM Out of memory.


The tee() system call first appeared in Linux 2.6.17; library support was added to glibc in version 2.5.


This system call is Linux-specific.


Conceptually, tee() copies the data between the two pipes. In reality no real data copying takes place though: under the covers, tee() assigns data to the output by merely grabbing a reference to the input.


The example below implements a basic tee(1) program using the tee() system call. Here is an example of its use:

$ date|./a.out out.log | cat Tue Oct 28 10:06:00 CET 2014 $ catout.log Tue Oct 28 10:06:00 CET 2014

Program source

#define _GNU_SOURCE #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <errno.h> #include <limits.h>
int main(int argc, char *argv[]) { int fd; int len, slen;
if (argc != 2) { fprintf(stderr, "Usage: %s <file> \ n", argv[0]); exit(EXIT_FAILURE); }
fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644); if (fd == -1) { perror("open"); exit(EXIT_FAILURE); }
do { /* * tee stdin to stdout. */ len = tee(STDIN_FILENO, STDOUT_FILENO, INT_MAX, SPLICE_F_NONBLOCK);
if (len < 0) { if (errno == EAGAIN) continue; perror("tee"); exit(EXIT_FAILURE); } else if (len == 0) break;
/* * Consume stdin by splicing it to a file. */ while (len > 0) { slen = splice(STDIN_FILENO, NULL, fd, NULL, len, SPLICE_F_MOVE); if (slen < 0) { perror("splice"); break; } len -= slen; } } while (1);
close(fd); exit(EXIT_SUCCESS); }


This page is part of release 5.02 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man -pages/.


splice(2), vmsplice(2), pipe(7)

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