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Top Forums Shell Programming and Scripting Redirecting STDERR to file and screen, STDOUT only to file Post 302697711 by alister on Friday 7th of September 2012 09:06:58 AM
Old 09-07-2012
The y-shaped hose and bucket analogy paints a memorable image. Nicely done. The only thing I'd add, explicitly (it's implied in your explanation), is that pipe redirection occurs before other, left-to-right redirections.

Quote:
Originally Posted by bakunin
The only uncertainty left is that i am not sure if the exact sequence of the messages will be preserved...
Even without the pipe and redirections, you can't depend on your average script/executable to emit messages in the exact order that they're generated, since typically a mix of unbuffered (stderr) and buffered (stdout) streams are used.

Of much greater importance is that script 2>&1 1>/some/file | tee -a /some/file involves multiple processes writing to the same file without any form of communication. Writes from script will clobber writes from tee, or vice versa. If they at least shared a file descriptor, while one message could still split another in two, there would never be any overwriting.

Regards,
Alister

Last edited by alister; 09-07-2012 at 10:47 AM..
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tee(1)							      General Commands Manual							    tee(1)

NAME
tee - Displays the output of a program and copies it into a file SYNOPSIS
tee [-ai] file... STANDARDS
Interfaces documented on this reference page conform to industry standards as follows: tee: XCU5.0 Refer to the standards(5) reference page for more information about industry standards and associated tags. OPTIONS
Adds the output to the end of file instead of writing over it. Ignores the SIGINT signal. OPERANDS
Standard input is stored into, or appended to, the file specified. [Tru64 UNIX] The tee command can accept up to 20 file arguments. DESCRIPTION
The tee command reads standard input and writes to both standard output, and each specified file. The tee command is useful when you wish to view program output as it is displayed, and also want to save it in a file. The tee command does not buffer output, so you may wish to pipe the output of tee to more if more than one full screen of data is anticipated. NOTES
If a write to any file fails, the exit status of tee will be non-zero. Writes to all other specified files may be successful, and opera- tion will continue until standard input is exhausted. EXIT STATUS
The following exit values are returned: Successful completion. An error occurred. EXAMPLES
To view and save the output from a command at the same time, enter: lint program.c | tee program.lint This displays the standard output of the command lint program.c at the terminal, and at the same time saves a copy of it in the file program.lint. If program.lint already exists, it is deleted and replaced. To display and append to a file, enter: lint program.c | tee -a program.lint This displays the standard output of lint program.c at the terminal and at the same time appends a copy of it to the end of pro- gram.lint. If the file program.lint does not exist, it is created. ENVIRONMENT VARIABLES
The following environment variables affect the execution of tee: Provides a default value for the internationalization variables that are unset or null. If LANG is unset or null, the corresponding value from the default locale is used. If any of the internationalization vari- ables contain an invalid setting, the utility behaves as if none of the variables had been defined. If set to a non-empty string value, overrides the values of all the other internationalization variables. Determines the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multibyte characters in arguments). Determines the locale for the for- mat and contents of diagnostic messages written to standard error. Determines the location of message catalogues for the processing of LC_MESSAGES. SEE ALSO
Commands: cat(1), echo(1), script(1) Standards: standards(5) tee(1)
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