tee

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. Provide a default value for the internationalization variables that are unset or null. (See XBD 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 that should be used to affect the format and contents of diagnostic messages written to standard error. [XSI) ] ![[Option Start]]( Determine the location of message catalogs for the processing of LC MESSAGES. ![[Option End]]( Default, except that if the -i option was specified, SIGINT shall be ignored. If any file operands are specified, the standard input shall be copied to each named file. 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 Description Defaults apply. The file operand is technically optional, but tee is no more useful than cat when none is specified. 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. Historical implementations ignore write errors. This is explicitly not permitted by this volume of POSIX.1-2017. 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 POSIX.1-2017 requires functionality equivalent to using O APPEND; see File Read, Write, and Creation .