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04-IO-Redirection.md

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IO Redirection

In this section, we will take a look at IO Redirection.

  • IO Redirection
  • Standard Streams in Linux

There are three data streams created when you launch a linux commnad.

  • Standard Input (STDIN)

    • STDIN is the standard input stream which accepts text as an input.

  • Standard Output (STDOUT)

    • Text output is delivered as STDOUT or the standard out stream

  • Standard ERROR (STDERR)

    • Error messages of the command are sent through the standard ERROR stream (STDERR)

    io

With IO Redirection, the STDIN, STDOUT and STDERR can be redirected to a text file.

REDIRECT STDOUT

To redirect STDOUT to a file instead of printing it on the screen.

$ echo $SHELL > shell.txt

To append STDOUT to an exisiting file

$ echo $SHELL >> shell.txt

REDIRECT STDERR

To redirect just the ERROR message we need to use 2 followed by forward arrow > symbol and then the name of the filename in which the errors are written.

$ cat missing_file 2> error.txt

To append the STDERR to the exisiting file

$ cat missing_file 2>> error.txt

If you want to execute and not print ERROR messages on the screen even if it generates a standard ERROR. You can redirect to /dev/null

$ cat missing_file 2> /dev/null

Command Line Pipes

Command Line Pipes allow the linking of multiple commands.

  • In simple terms, pipes allows the first commands standard output to be used as the standard input for the second command.

  • The pipes are defined using vertical bar symbol (|).

    $ grep Hello sample.txt | less

    pipe

Another command to work with STDIN and STDOUT is the tee command.

  • Instead of the redirect operator, we can use the command line pipe (|) followed by tee command.

    $ echo $SHELL | tee shell.txt

  • Use tee with -a option, to append instead of overwritting it

    $ echo "This is the bash shell" | tee -a

    tee