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Standard output is a stream to which a program writes its output data. The program requests data transfer with the write operation. Not all programs generate output. For example, the file rename command (variously called mv, move, or ren) is silent on success. Unless redirected, standard output is
This is the primary way to output data from a program by directly using a system call. The destination is identified by a numeric code. The data to be written, for instance a piece of text, is defined by a pointer and a size, given in number of bytes. write thus takes three arguments: The file code (file descriptor or fd).
In the following example, standard output is written to file, but errors are redirected from stderr to stdout, i.e. sent to the screen: command 2 > & 1 > file. To write both errors and standard output to file , the order should be reversed.
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 ...
A kernel panic message from a Linux system An OpenSolaris kernel panic. Kernel panic in Ubuntu 13.04 "Raring Ringtail" (Linux kernel 3.8) in Oracle VM VirtualBox
Output of dmesg viewed with grep. The output of dmesg can amount to many complete screens. For this reason, this output is normally reviewed using standard text-manipulation tools such as more, tail, less or grep. [6] Size of the dmesg buffer is limited and the output is often captured in a permanent system logfile via a logging daemon, such as ...
An important aspect of this, setting Unix pipes apart from other pipe implementations, is the concept of buffering: for example a sending program may produce 5000 bytes per second, and a receiving program may only be able to accept 100 bytes per second, but no data is lost. Instead, the output of the sending program is held in the buffer.
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.