How to grep patterns read from a file

grep is an awesome tool that allows you to search for strings, nunbers and patterns of them, its usage can be quite complicated since it has a very large number of command line options, but on each simplest form search is done this way, assume that we have the following file named data.txt

This is a simple example 
in a text file

Now running the following command which searches for lines contain This returns the found line

$ grep This data.txt 
This is a simple example

But what if we have a procedure that everyday appends new keywords and those keywords needs to be used as search patterns against a file? Lets see how we can do this in a very easy way!

Creating the patterns file

Create the following file and save it as patterns.txt

This 
cat

From this file each line will be used a search pattern vs the file we want to search for patterns

The -f option does the magic!

-f is the option that allows us to use a file as a search pattern source, it works like this

grep -f patterns.txt data.txt

And it returns the following since This exists in both files! isnt that cool?

$ grep -f patterns.txt data.txt 
This is a simple example

Things to note

grep is very versatile! common options like -i and -v works with -f , this allows to do neat tricks like, finding lines in data.txt that no pattern exists in patterns.txt , line in a text file was print because nor This or cat exists in data.txt

$ grep -v -f patterns.txt data.txt 
in a text file

What about regexes?

-f parameter works with regexes as well, update patterns.txt to match the following

This 
cat 
text [0-9]

Then update data.txt to match the following

This is a simple example 
in a text file 
text 9

Running again the grep command generates the following, text 9 printed because matches regex text [0-9]

$ grep -f patterns.txt data.txt 
This is a simple example 
text 9

Conclusion

grep is a powerful command, one very useful option is -f as we saw! this allows us to create powerful tools and scripts that can make our daily life easier! i hope you enjoyed this article!