The tee utility shall copy standard input to standard output, making a
copy in zero or more files. The tee utility shall not buffer output.
If the -a option is not specified, output files shall be written (see
FileRead, Write, and Creation .
The following environment variables shall affect the execution of tee:
Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization
Variables for the precedence of internationalization variables used to
determine the values of locale categories.)
If set to a non-empty string value, override the values of
all the other internationalization variables.
Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as opposed to
multi-byte characters in arguments).
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to standard error.
Determine the location of message catalogs for the
processing of LC_MESSAGES.
If a write to any successfully opened file operand fails, writes to other
successfully opened file operands and standard output shall continue,
but the exit status shall be non-zero. Otherwise, the default actions
specified in Utility DescriptionDefaults apply.
The following sections are informative.
The buffering requirement means that tee is not allowed to use
ISO C standard fully buffered or line-buffered writes. It does not mean
that tee has to do 1-byte reads followed by 1-byte writes.
It should be noted that early versions of BSD ignore any invalid options and
accept a single '-' as an alternative to -i. They also print a
message if unable to open a file:
"tee: cannot access %s\n", <pathname>
Historical implementations ignore write errors. This is explicitly not permitted
by this volume of IEEE Std 1003.1-2001.
Some historical implementations use O_APPEND when providing append mode; others
use the lseek() function to seek to the end-of-file after opening the
file without O_APPEND. This volume of IEEE Std 1003.1-2001
requires functionality equivalent to using O_APPEND; see File Read, Write,
and Creation .
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html