Pipelines

Philosophy of the commandline: Small tools that do one thing very well, loosely connected together to make custom "pipelines" or workflows to do specific (or surprising) things.

stdin and stdout

Every program receives "standard in", and sends its output to "standard out". By default, stdin is taken from the keyboard, and stdout will display something to the screen. These mappings can be adjusted however using redirection using the special pipeline characters '>', '<', and '|'.

Redirecting stdout with >

date

Displays the date to the screen (no stdin used by date).

date > time.txt

Redirects the output of date and "saves as" time.txt.

cat time.txt

Display time.txt (to the screen by default)

Redirecting stdin with <

wc -l

"Word count" program can be used to simply count the number of lines of a text file (with the -l option). When the above command is run, wc "listens to stdin" which is the console/keyboard. The program appears to do nothing and the shell "hangs" waiting for input. Type a few lines in such as...

testing
one
two
three
<CTRL-D>

Finally, on a blank line, pressing Ctrl-D tells the shell "END OF FILE" -- or stop reading input, and wc will snap into action and output the number of lines it read from stdin.

wc -l < mytextfile

Tells wc to use mytextfile as stdin and thus shows how many lines are in that file.

Piping (stdin=>stdout) with |

ls | wc -l

Is sort of like:

ls >< wc -l  (this is invalid!)

In that stdout of ls is "piped" to be the stdin of wc. The result is a file count of the current directory. (NB: the ls command is smart and disables multiple column output if it's being redirect (ie not going straight to the console), to see this try:

ls | cat