tee – read from standard input and write to standard output and files

A UNIX Command

$tee name
my name is nice.
my name is nice.
$cat name
my name is nice.

UNIX Explanation

Copy standard  input to each  FILE, and also  to standard
output.  A  special variant of  the tee for the  shell is
called script and  permits duplicating all input commands
submitted to a shell into a file.

Related Source Code Exposition

static bool
tee_files (int nfiles, const char **files)
FILE **descriptors;
char buffer[BUFSIZ];
ssize_t bytes_read;
int i;
bool ok = true;
char const *mode_string =
? (append ? "ab" : "wb")
: (append ? "a" : "w"));

descriptors = xnmalloc (nfiles + 1, sizeof *descriptors);

/* Move all the names `up' one in the argv array to make room for
the entry for standard output. This writes into argv[argc]. */
for (i = nfiles; i >= 1; i--)
files[i] = files[i - 1];

if (O_BINARY && ! isatty (STDIN_FILENO))
xfreopen (NULL, "rb", stdin);
if (O_BINARY && ! isatty (STDOUT_FILENO))
xfreopen (NULL, "wb", stdout);

/* In the array of NFILES + 1 descriptors, make
the first one correspond to standard output. */
descriptors[0] = stdout;
files[0] = _("standard output");
setvbuf (stdout, NULL, _IONBF, 0);

for (i = 1; i <= nfiles; i++)
descriptors[i] = (STREQ (files[i], "-")
? stdout
: fopen (files[i], mode_string));
if (descriptors[i] == NULL)
error (0, errno, "%s", files[i]);
ok = false;
setvbuf (descriptors[i], NULL, _IONBF, 0);

while (1)
bytes_read = read (0, buffer, sizeof buffer);
#ifdef EINTR
if (bytes_read < 0 && errno == EINTR)
if (bytes_read <= 0)

/* Write to all NFILES + 1 descriptors.
Standard output is the first one. */
for (i = 0; i <= nfiles; i++)
if (descriptors[i]
&& fwrite (buffer, bytes_read, 1, descriptors[i]) != 1)
error (0, errno, "%s", files[i]);
descriptors[i] = NULL;
ok = false;

if (bytes_read == -1)
error (0, errno, _("read error"));
ok = false;

/* Close the files, but not standard output. */
for (i = 1; i <= nfiles; i++)
if (!STREQ (files[i], "-")
&& descriptors[i] && fclose (descriptors[i]) != 0)
error (0, errno, "%s", files[i]);
ok = false;

free (descriptors);

return ok;

Source Code Highlight

Copy the standard input into  each of the NFILES files in
FILES  and  into the  standard  output.   Return true  if

Featured Image

Related Knowledge

The  `tee'  command  copies  standard input  to  standard
output and also to any files given as arguments.  This is
useful when  you want not only  to send some  data down a
pipe, but also to save a copy.

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