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. Faria yes. But the last two commands are equivalent, they will send both error and output to the same file.
Show 7 more comments. Additionally, certain commands such as time and strace write output to stderr by default, and may or may not provide a method of redirection specific to that command Basic theory behind redirection is that a process spawned by shell assuming it is an external command and not shell built-in is created via fork and execve syscalls, and before that happens another syscall dup2 performs necessary redirects before execve happens.
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