Linux allows you to manipulate input and output streams (I/O streams). Input streams send data to a program. Output streams return data from a program. By default, your keyboard is the input stream on Linux, and your console is the output stream. There is also an error output stream which is dedicated to outputting errors from a program.

Name Purpose File Descriptor Default
stdin Input 0 Keyboard
stdout Normal output 1 Terminal
stderr Error output 2 Terminal

A pipe labled 'stdin' connects a keyboard to `bionano_software.' To additional pipes are connected to the other side of `bionano_software`, one of which is named "stdout (1)" and the other "stderr (2)." Stdout connects to an image of a DNA strand, and stderr connects to an image of a caution sign."

Redirects

stdout: “>” or “>>”

The greater-than token (>) redirects the standard output of a program to a new location. grep usually prints its results to the console. If you wanted to redirect it to a text file, you’d do the following:

# Create or replace output.txt
grep -i 'claws al ghul' catman_begins.txt > output.txt
# Create or append to output.txt
grep -i 'claws al ghul' catman_begins.txt >> output.txt

Similar to the previous image, stdin connects a keyboard to a linux program, this time named 'grep.' stdout (1) connects to a txt file, and stderr (2) connects to the linux terminal.

stderr: “2>” or “2>>”

Prepending the output redirection tokens with a 2 redirects the standard error. Below is an example with the vlc media player.

# Create or replace error.log
vlc mr_smith_goes_to_pawshington.mpg 2> error.log
# Create or append to error.log
vlc mr_smith_goes_to_pawshington.mpg 2>> error.log

Similar to the previous image, stdin connects a keyboard to a linux program, this time named 'vlc media player.' stdout (1) connects to a movie, and stderr (2) connects to a caution sign which represents error.log.

Both stdout and stderr

You can combine the redirects to redirect both stdout and stderr at the same time:

bionano_software > output.log 2> error.log   # create
bionano_software >> output.log 2>> error.log # append

Similar to the previous image, stdin connects a keyboard to a linux program, this time named 'bionano_software.' stdout (1) connects to a dna strand that represents output.log, and stderr (2) connects to a caution sign which represents the error.log.

stdin: “<”

The less-than token is used to redirect standard input (stdin). It tells the program to take its input from the file following the token:

# Create a file or replace an existing file:
sort > sorted_cats.txt < cats.txt
# Another format example, albeit contrived:
sort < cats.txt > sorted_cats.txt
# Sort can take a filename as a direct argument without any input redirection, so the better syntax is:
sort cats.txt > sorted_cats.txt

This time, it is not a keyboard that is connected to the linux program via a pipe labeled stdin, but it is a group of unsorted cats. The linux program this time is named 'sort,' and its output from the stdout (1) pipe is a line of neatly sorted cats. The stderr (2) pipe connects to the linux terminal.

Combine streams

One I/O stream can be redirected to another I/O stream. This is the syntax for redirecting stderr to stdout. It would ensure that all output, standard or error, would end up at the destination:

ping -c 10 procatinator.com > output.txt 2>&1
ping -c 10 procatinator.com &> output.txt # shorthand

Discard output

/dev/null can be thought of as a black hole. It’s a special device that discards anything that is passed to it. The ampersand-greater-than (&>) will redirect both streams to the same place:

./automated_task.sh &> /dev/null # run the program, throwing away all output

A keyboard now once again connects to a linux program via stdin, the name of the linux program being 'automated_task.sh.' However, now both stdout and stderr connect to a black hole, representing /dev/null.