How to search for multiple patterns?

To search for multiple patterns in a file using the grep command, you can use the -E option for extended regular expressions or the -e option to specify multiple patterns. Here are a couple of examples:

Using -E for Extended Regular Expressions

You can use the pipe | to separate multiple patterns:

grep -E "pattern1|pattern2|pattern3" filename.txt

Using -e Option

You can specify each pattern with the -e option:

grep -e "pattern1" -e "pattern2" -e "pattern3" filename.txt

Example

If you want to search for lines containing either "ERROR" or "WARNING" in a log file, you can use:

grep -E "ERROR|WARNING" server_log.txt

or

grep -e "ERROR" -e "WARNING" server_log.txt

Both commands will return lines from server_log.txt that contain either "ERROR" or "WARNING".