Connecting commands together.
WHAT THE THREE STREAMS ARE
Standard input Standard output Standard error
WHY ERROR IS SEPARATE
So error messages are not mixed into data you are processing.
WHAT A SINGLE ANGLE BRACKET DOES
Sends output to a file, replacing its contents.
WHAT TWO ANGLE BRACKETS DO
Append instead of replacing.
WHY THAT DISTINCTION MATTERS
A single bracket on an existing file destroys it instantly.
WHAT A LEFT ANGLE BRACKET DOES
Feeds a file into a command as input.
WHAT A PIPE DOES
Sends one command's output into another's input.
WHY IT IS THE CENTRAL IDEA
Small tools combined do what no single tool provides.
WHAT COMMON COMBINATIONS LOOK LIKE
Listing something, filtering it, counting the result Reading a log, selecting a field, sorting, counting occurrences
WHAT TO REDIRECT ERRORS WITH
The number two before the angle bracket.
WHAT TO SEND BOTH STREAMS TO ONE PLACE
An ampersand before the angle bracket, in modern shells.
WHAT tee DOES
Writes to a file and passes the output onward.
WHEN THAT HELPS
When you want to see output and keep it.
WHAT TO DISCARD OUTPUT WITH
Redirecting to the null device.
WHERE THAT IS USED
Cron jobs, to suppress routine output.
WHAT TO NEVER SUPPRESS
Errors you have not read at least once.
