Grep Basics

grep stands for Global Regular Expression Print. It is one of the most powerful and commonly used terminal commands — it searches through files and prints every line that contains a matching pattern.

We have server.log:

info: server started
info: listening on port 8080
error: disk full
warning: high memory usage
info: request received
error: connection timeout
info: server shutdown

The basic format is:

grep "pattern" filename

For example, to find every line that contains the word error:

grep "error" server.log

Output:

error: disk full
error: connection timeout

Only the lines containing error are printed. All other lines are hidden.

Searching in multiple files:

grep "error" *.log   # search all .log files

Searching for a phrase with spaces:

grep "connection timeout" server.log

Case sensitivity:

By default, grep is case sensitive. So searching for error will not match Error or ERROR:

grep "error" server.log    # matches: error
grep "Error" server.log    # matches: Error
grep "ERROR" server.log    # matches: ERROR