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In computing, redirection is a form of interprocess communication, and is a function common to most command-line interpreters, including the various Unix shells that can redirect standard streams to user-specified locations. The concept of redirection is quite old, dating back to the earliest operating systems (OS).
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 inherited from the parent process.
Note that these methods prevent the process from being sent a 'stop' signal on logout, but if input/output is being received for these standard I/O files (stdin, stdout, or stderr), they will still hang the terminal. [1] See Overcoming hanging, below. nohup is often used in combination with the nice command to run processes on a lower priority.
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 capture intermediate output before the data is altered by another command or program. The tee command reads standard input, then writes its content to standard output. It simultaneously copies the ...
Wrapper scripts often use this command to run a program (either directly or through an interpreter or virtual machine) after setting environment variables or other configuration. By using exec, the resources used by the shell program do not need to stay in use after the program is started. [2] The exec command can also perform a redirection.
Bash can execute the vast majority of Bourne shell scripts without modification, with the exception of Bourne shell scripts stumbling into fringe syntax behavior interpreted differently in Bash or attempting to run a system command matching a newer Bash builtin, etc. Bash command syntax includes ideas drawn from the Korn Shell (ksh) and the C ...
The command ls -l is executed as a process, the output (stdout) of which is piped to the input (stdin) of the process for grep key; and likewise for the process for less. Each process takes input from the previous process and produces output for the next process via standard streams .
>> file means stdout will be appended at the end of file. >>& file means both stdout and stderr will be appended at the end of file. < file means stdin will be read from file. << string is a here document. Stdin will read the following lines up to the one that matches string. Redirecting stderr alone isn't possible without the aid of a sub-shell.