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: duplicate an open file descriptor – System Interfaces Reference, The Single UNIX Specification, Version 4 from The Open Group; Redirection Definition by The Linux Information Project (LINFO) I/O Redirection in The Linux Documentation Project; Redirection in Windows; Creating a Child Process with Redirected Input and Output in Windows
/A Append the pipeline content to the output file(s) rather than overwriting them. Note: When tee is used with a pipe, the output of the previous command is written to a temporary file. When that command finishes, tee reads the temporary file, displays the output, and writes it to the file(s) given as command-line argument.
Process substitution can also be used to capture output that would normally go to a file, and redirect it to the input of a process. The Bash syntax for writing to a process is >(command). Here is an example using the tee, wc and gzip commands that counts the lines in a file with wc -l and compresses it with gzip in one pass:
This is because the redirection form opens the file as the stdin file descriptor which command can fully access, while the cat form simply provides the data as a stream of bytes. Another common case where cat is unnecessary is where a command defaults to operating on stdin, but will read from a file, if the filename is given as an argument.
In the calling process the lowest numbered unused file descriptor will be used for the new file descriptor number. When using the dup2() system call it performs the same task as dup() with the exception of using the file descriptor number specified in the newfd variable of the call, in that newfd is adjusted to refer to the oldfd file description.
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
The > in the third example is a redirection operator, telling the command-line interpreter to send the output of the command not to its own standard output (the screen) but to the named file. This will overwrite the file. Using >> will redirect the output and append it to the file.
The program is also used to mount the new file system. At the time the file system is mounted, the handler is registered with the kernel. If a user now issues read/write/stat requests for this newly mounted file system, the kernel forwards these IO-requests to the handler and then sends the handler's response back to the user.