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Showing posts from April 16, 2017

Relevant Linux Features: Pipe

Relevant Linux Features: Pipe The pipe is implemented with the  " | " symbol. It takes the output (stdout) of the command on the left and sends it as input (stdin)  for the command on the right: In the example below, docker run --help is the first command. Its output is used as input to the more command, which displays the output, one screen at a time: Note: stderr (standard error) is NOT passed through the pipe. I.e. we are not able to pass stderr through the pipe as we can stdout . Licensed under a Creative Commons Attribution 4.0 International License .

Relevant Linux Features: Command Substitution

Command substitution - $( command ) In command substitution , the shell runs command , however instead of displaying the output of command , it stores the output in a variable. You can then pass that variable as input to another command . The syntax of command substitution is $(command) or the older `command` , using back-ticks. Let's say you want to remove the most recent container running. You can use docker ps -a which lists all containers, starting with the most recent, then copy the Container ID into the docker rm <Container ID> command:   Alternatively, you can use Command Substitution and let the system do some of the work for you. The following command runs docker ps -lq which gets the ID of the most recent container, it passes that ID to the docker rm command: $ docker rm $(docker ps -lq) :   Licensed under a Creative Commons Attribution 4.0 International License .

Relevant Linux Features: Standard I/O

Relevant Linux Features: Standard I/O stdin, stdout, stderr: Linux recognizes three input/output streams: STDIN standard input by default input to a command comes from the keyboard STDIN can be redirected to come from other than the keyboard, e.g. a file or a device use the "<" symbol to redirect input, e.g. command < file e.g. to send the contents of file, text001 as INPUT to the command pr and offset each line by 5 spaces: $ pr --indent=5 < text001 STDOUT standard output by default, output of a command is sent to the terminal STDOUT can be redirected to go to other than the terminal, e.g. a file or a device use the ">" symbol to redirect output, e.g. command > file e.g. send the output of command, ls to a file, text001 instead of the terminal: $ ls > text001 STDERR standard error by default, error from a command is sent to the terminal STD

Relevant Linux Features: Redirection

Relevant Linux Features: Redirection Linux allows I/O to be redirected away from the default source or target . The default source of STDIN is the keyboard , i.e. by default a command expects to get its input from the keyboard. To send input to a command, from a file, use the " < " redirection symbol. The default target of STDOUT is the terminal or screen , i.e. by default a command expects to send its output to the screen. To redirect it elsewhere, use the " > " symbol: Note: " command > file " sends the output to a file, " file ". If " file " already exists, any existing content is overwritten. To instead append the output to the end of the file, use " >> " instead, i.e. " command >> file ". The default target of STDERR is the screen , i.e. by default a command expects to send its error output to the screen . To redirect it elsewhere, u