Grep command searches file contents to find lines that contain a pattern. The pattern is described in Regular Expression (regex) language. Here are three youtube video links about Grep and regular expressions.
Ryan’s Tutorials website has a nice introduction
to Grep and Regular Expression. Note egrep is a shortcut to grep -E in
Ubuntu. The man page of grep describes that “in GNU grep there is no difference
in available functionality between basic and extended syntaxes”.
$which egrep
/bin/egrep
$file /bin/egrep
/bin/egrep: POSIX shell script, ASCII text executable
$vim /bin/egrep # file content is
#!/bin/sh
exec grep -E "$@"
Here is a very simple grep example:
$grep "Jane Williams" filename.txt
The grep command has many options. A few common ones are:
The filename.txt in the above example could be either ./* or ./*.txt.
For example, this command will search current directory and subdirectories.
$grep -winr "Jane Williams" ./
Here are a few more options:
You can use pipes to search the results of another command.
$history | grep "git commit"
The grep -P option interprets the pattern as a Perl-compatible regex. The
Python and Perl “essentially has the same regex syntax”. So with the -P
option turned on, the grep command will accept Python style special characters
in pattern. Here is a list of common regex special characters in Python:
Here is an example of using the -P option. This is to search number of
methods in Flask class.
$grep -P -n "def\w+\(self" .../app.py | wc -l
Update on 8/28/2020:
Today I want to see how many blog posts I have written this year and compare the number with last year.
I want to search date: 2020-, but find grep does not like the character -. A Google search
with phrases “grep search dash” finds
an stackoverflow post.
It turns out that the dash character needs to escape twice like this date: 2020\\-. So the command
to tally blog posts becomes like this,
grep -r "date: 2020\\-" | wc -l
grep -r "date: 2019\\-" | wc -l
The result is 26 v.s. 26. The 2019 only covers 9 months starting from 4/1 and 2020 covers 8 months so far, so it is close.
Note I do not have to include - character in the search pattern. It is just something
good to know.
Added on 5/4/2021
If you want to restrict the search to a certain file, you can use the --include option. For example,
the command below restricts the search to cli.py file. You can also use \*.h to restrict search
to .h files. This is also from a
stackoverflow Q&A.
$grep -winr --include cli.py "load_dotenv" ./
$grep -win "load_dotenv" ./cli.py # or this for this example