Thus, the above pipeline of commands counts the number of directories in the current directory and writes that number to file4 which is created if it does not already exist, or is overwritten if it already exists.
This can be easily confirmed by using the head command or the cat command one of the most popular commands for reading files to read the file, i. Here the cat command writes an integer representing the total number of directories to standard output, which is, of course, the monitor screen. Not all programs have a standard output, and thus nothing normally appears on the display screen when they are used.
Examples include cp , which copies files and directories, mkdir , which creates directories and mv , which renames and moves files and directories. The input counterpart of standard output is standard input.
Often abbreviated stdin , it is the source of input s for a program. Just as standard output is by default the display screen, standard input is by default any text entered from the keyboard. However, the input to most command line programs can likewise be redirected, and it can come from files whose names are names provided as arguments or from the output of another program.
Standard output and standard input are important parts of the traditional terminology for Unix-like operating systems. Although such terminology can be confusing for new users, experienced users find it very useful both because of the precision that it adds to explanations and because it is widely used including in the online man and info documentation typically installed on Unix-like operating systems.
Created February 10, Updated April 2, All Rights Reserved. The other end is determined by the shell that launched the command.
That end will be connected to the terminal window, connected to a pipe, or redirected to a file or other command, according to the command line that launched the command.
In Linux, stdin is the standard input stream. This accepts text as its input. Text output from the command to the shell is delivered via the stdout standard out stream. Error messages from the command are sent through the stderr standard error stream. So you can see that there are two output streams, stdout and stderr , and one input stream, stdin. Because error messages and normal output each have their own conduit to carry them to the terminal window, they can be handled independently of one another.
Streams in Linux—like almost everything else—are treated as though they were files. You can read text from a file, and you can write text into a file. Both of these actions involve a stream of data. Each file associated with a process is allocated a unique number to identify it. This is known as the file descriptor. Whenever an action is required to be performed on a file, the file descriptor is used to identify the file.
These values are always used for stdin , stdout, and stderr :. In a similar vein, when talking about stdin , stdout , and stderr it is convenient to trot out the accepted axiom that a process neither knows nor cares where its three standard streams are terminated. Should a process care whether its output is going to the terminal or being redirected into a file?
Can it even tell if its input is coming from the keyboard or is being piped into it from another process? Actually, a process does know—or at least it can find out, should it choose to check—and it can change its behavior accordingly if the software author decided to add that functionality. The ls command behaves differently if its output stdout is being piped into another command. And ls does the same thing if its output is being redirected:.
You can react to the errors if you need to, as they occur. It also stops the error messages from contaminating the file that stdout has been redirected into. The first line of the script echoes text to the terminal window, via the stdout stream.
This will generate an error message that is delivered via stderr. We can see that both streams of output, stdout and stderr , have been displayed in the terminal windows. The error message that is delivered via stderr is still sent to the terminal window. We can check the contents of the file to see whether the stdout output went to the file.
You can use one of the numeric file descriptors to indicate which standard output stream you wish to redirect. The error message is redirected and the stdout echo message is sent to the terminal window:. Surely, if we can redirect either stdout or stderr to a file independently of one another, we ought to be able to redirect them both at the same time, to two different files?
Yes, we can. This command will direct stdout to a file called capture. Because both streams of output—standard output and standard error—are redirected to files, there is no visible output in the terminal window. We are returned to the command line prompt as though nothing has occurred.
The only other combination we can do is to send both stdout and stderr to the same file. Both the stdout and stderr streams have been redirected to a single destination file. We discussed how a command can detect if any of the streams are being redirected, and can choose to alter its behavior accordingly. Can we accomplish this in our own scripts? And it is a very easy technique to understand and employ. The clever part is the test within the square brackets.
The -t terminal option returns true 0 if the file associated with the file descriptor terminates in the terminal window.
If stdin is connected to a terminal window the test will prove true. For me, on OSX If not, check in the header files included in stdio. In the standard library code, where else?
In a Linux machine around here it's in libc. The C standard explicitly states that stdin is a macro defined in stdio. It is not allowed to be defined anywhere else. This macro can of course point at implementation details that are implemented elsewhere, such as in a "stdio. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Ask Question. Asked 10 years, 3 months ago. Active 3 years, 11 months ago. Viewed 9k times. I found this line in stdio. Improve this question. Add a comment. Active Oldest Votes.
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