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It does this by searching for an executable or script in the directories listed in the environment variable PATH. The which command is part of most Unix-like computers. It is also part of the C Shell. [6] A which command first appeared in 3BSD. [7] Carlo Wood developed the GNU implementation used in most Linux-based operating systems. [8]
On POSIX systems, the file descriptor for standard input is 0 (zero); the POSIX <unistd.h> definition is STDIN_FILENO; the corresponding C <stdio.h> abstraction is provided via the FILE* stdin global variable. Similarly, the global C++ std::cin variable of type <iostream> provides an abstraction via C++ streams.
The Linux tee command was written by Mike Parker, Richard Stallman, and David MacKenzie. [5] The command is available as a separate package for Microsoft Windows as part of the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities. [6] The FreeDOS version was developed by Jim Hall and is licensed under the GPL. [7]
The Linux enhanced version of getopt has the extra safety of getopts plus more advanced features. It supports long option names (e.g. --help) and the options do not have to appear before all the operands (e.g. command operand1 operand2 -a operand3 -b is permitted by the Linux enhanced version of getopt but does not work with getopts).
The rule is that any redirection sets the handle to the output stream independently. So "2>&1" sets handle 2 to whatever handle 1 points to, which at that point usually is stdout. Then ">" redirects handle 1 to something else, e.g. a file, but it does not change handle 2, which still points to stdout.
On DOS, OS/2, and Windows operating systems, the %PATH% variable is specified as a list of one or more directory names separated by semicolon (;) characters. [5]The Windows system directory (typically C:\WINDOWS\system32) is typically the first directory in the path, followed by many (but not all) of the directories for installed software packages.
The Bourne shell (sh) is a shell command-line interpreter for computer operating systems.It first appeared on Version 7 Unix, as its default shell. Unix-like systems continue to have /bin/sh—which will be the Bourne shell, or a symbolic link or hard link to a compatible shell—even when other shells are used by most users.
If pushd is not provided with a path argument, in Unix it instead swaps the top two directories on the stack, which can be used to toggle between two directories. The popd command removes (or 'pops', in the stack analogy) the current path entry from the stack and returns to the path at the top of the stack as the new working directory.