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The reason for this is to distinguish between a file named '1' and stdout, i.e. cat file 2 >1 vs cat file 2 > & 1. In the first case, stderr is redirected to a file named ' 1 ' and in the second, stderr is redirected to stdout. Another useful capability is to redirect one standard file handle to another.
>> file means stdout will be appended at the end of file. >>& file means both stdout and stderr will be appended at the end of file. < file means stdin will be read from file. << string is a here document. Stdin will read the following lines up to the one that matches string. Redirecting stderr alone isn't possible without the aid of a sub-shell.
In the case of an interactive shell, that is usually the text terminal which initiated the program. The file descriptor for standard output is 1 (one); the POSIX <unistd.h> definition is STDOUT_FILENO; the corresponding C <stdio.h> variable is FILE* stdout; similarly, the C++ <iostream> variable is std::cout.
Example usage of tee: The output of ls -l is redirected to tee which copies them to the file file.txt and to the pager less. The name tee comes from this scheme - it looks like the capital letter T. The tee command is normally used to split the output of a program so that it can be both displayed and saved in a file. The command can be used to ...
The C programming language provides many standard library functions for file input and output.These functions make up the bulk of the C standard library header <stdio.h>. [1] The functionality descends from a "portable I/O package" written by Mike Lesk at Bell Labs in the early 1970s, [2] and officially became part of the Unix operating system in Version 7.
Yes (stdin, stdout, stderr, stdout+stderr) Yes (via registry, TCMD.INI / 4NT.INI file, startup parameters, environment variables, SETDOS command) Yes (automatic via registry and TCSTART / 4START as well as TCEXIT / 4EXIT, or explicitly via /K startup option) Yes (via CALL command or /C and /K startup options) Yes No VMS DCL [22] OpenVMS
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A file descriptor open when an exec call is made remains open in the new process image, unless was fcntl ed with FD_CLOEXEC or opened with O_CLOEXEC (the latter was introduced in POSIX.1-2001). This aspect is used to specify the standard streams (stdin, stdout and stderr) of the new program.