Search Multiple Words / String Pattern Using grep Command on Bash shell

How do I grep for multiple patterns?

The syntax is:

1. Use single quotes in the pattern: grep 'pattern*' file1 file2
2. Use extended regular expressions: egrep 'pattern1|pattern2' *.py
3. Use this syntax on older Unix shells: grep -e pattern1 -e pattern2 *.pl

Here are all other possibilities:

 grep 'word1\|word2\|word3' /path/to/fill

### Search all text files ###

grep 'word*' *.txt

### Search all python files for ‘wordA’ or ‘wordB’ ###

grep 'wordA*'\''wordB' *.py
grep -E 'word1|word2' *.doc
grep -e string1 -e string2 *.pl
egrep "word1|word2" *.c

Examples

In this example, search warning, error, and critical words in a text log file called /var/log/messages, enter:

$ grep 'warning\|error\|critical' /var/log/messages

To just match words, add the -w option:

 $ grep -w 'warning\|error\|critical' /var/log/messages

Use the egrep command and you can skip the above syntax to search three words:

$ egrep -w 'warning|error|critical' /var/log/messages

OR

$ grep -e 'warning|error|critical' /var/log/messages

I recommend that you pass the -i (ignore case) and --color option as follows too:

$ egrep -wi --color 'warning|error|critical' /var/log/messages

Sample outputs:

To search all *.conf files under /etc/, enter:

egrep -wi --color 'foo|bar' /etc/*.conf

To search recursively (including sub-directories) listed, run:

egrep -Rwi --color 'foo|bar' /etc/

source : https://www.cyberciti.biz/faq/searching-multiple-words-string-using-grep/