cross-posted from: https://lemmy.run/post/10868
Beginner’s Guide to
grep
grep
is a powerful command-line tool used for searching and filtering text in files. It allows you to find specific patterns or strings within files, making it an invaluable tool for developers, sysadmins, and anyone working with text data. In this guide, we will cover the basics of usinggrep
and provide you with some useful examples to get started.Installation
grep
is a standard utility on most Unix-like systems, including Linux and macOS. If you’re using a Windows operating system, you can install it by using the Windows Subsystem for Linux (WSL) or through tools like Git Bash, Cygwin, or MinGW.Basic Usage
The basic syntax of
grep
is as follows:
grep [options] pattern [file(s)]
options
: Optional flags that modify the behavior ofgrep
.pattern
: The pattern or regular expression to search for.file(s)
: Optional file(s) to search within. If not provided,grep
will read from standard input.Examples
Searching in a Single File
To search for a specific pattern in a single file, use the following command:
grep "pattern" file.txt
Replace
"pattern"
with the text you want to search for andfile.txt
with the name of the file you want to search in.Searching in Multiple Files
If you want to search for a pattern across multiple files, use the following command:
grep "pattern" file1.txt file2.txt file3.txt
You can specify as many files as you want, separating them with spaces.
Ignoring Case
By default,
grep
is case-sensitive. To perform a case-insensitive search, use the-i
option:
grep -i "pattern" file.txt
Displaying Line Numbers
To display line numbers along with the matching lines, use the
-n
option:
grep -n "pattern" file.txt
This can be helpful when you want to know the line numbers where matches occur.
Searching Recursively
To search for a pattern in all files within a directory and its subdirectories, use the
-r
option (recursive search):
grep -r "pattern" directory/
Replace
directory/
with the path to the directory you want to search in.Using Regular Expressions
grep
supports regular expressions for more advanced pattern matching. Here’s an example using a regular expression to search for email addresses:
grep -E "\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}\b" file.txt
In this case, the
-E
option enables extended regular expressions.Conclusion
grep
is a versatile tool that can greatly enhance your text searching and filtering capabilities. With the knowledge you’ve gained in this beginner’s guide, you can start usinggrep
to quickly find and extract the information you need from text files. Experiment with different options and explore more advanced regular expressions to further expand your skills withgrep
. Happy grepping!
Did you know the whole grep program was written within a day, by non other then Ken Thompson https://youtube.com/watch?v=NTfOnGZUZDk&feature=share7
I did not.
Thank you for sharing it. Something you learn everyday, eh 😀.
This is cool, but I’d love to see some examples outputs so I can get a sense of what it actually does.
Sure, will try to include output in future. Appreciate the feedback.
I have a concise guide at https://learnbyexample.github.io/cli-computing/searching-files-and-filenames.html#grep and a more detailed book at https://learnbyexample.github.io/learn_gnugrep_ripgrep/introduction.html. Both have examples and exercises.
You can live without vim, but not without cat and grep.
Very true and I always combine them when I dont need to, using
cat file.txt | grep foo
instead of justgrep foo file.txt
Yeah, IDK why, just feels kinda natural.
If you are serious about working in a terminal, then I highly recommend learning modern replacements for the old tools.
In this case
ripgrep
(orrg
) https://github.com/BurntSushi/ripgrep is phenomenal. Especially for searching recursively in a large directory tree it is unbelievably quicker than regulargrep
.It won’t be installed on any random machine, so
grep
is still useful, but if you regularly need to text search in files then there are better tools.Yeap, but most of the time you end up trying to figure out issue on remote system, where you don’t have
ripgrep
always installed, but if you have that available on the system you are working on.ripgrep
is always a better alternative.
Good information and barely scratches the surface of grep usage. It can get a lot more complicated but also do a lot more than you think.
Two of my most used grep invocations are:
- Diff two files, showing lines:
grep -xvFf (file2) (file1)
- Show lines that do not contain a string:
grep -rivE '^#' (file)
( only shows uncommented lines for most Linux configuration files)
- Diff two files, showing lines:
Others have already mentioned
man grep
orgrep help
But, in case you don’t know about it, there are two great utilities to get examples and help for almost any given command:
tldr
andcheat
are great.https://github.com/cheat/cheat
https://github.com/tldr-pages/tldr
Just
cheat grep
ortldr grep
and you’re good to go :)Excellent guide, thanks for the write up!
One thing I’d like to point out is that you can pipe output from an application into
grep
and then be able to use all the information above.For instance if I want to know the full name of my wireguard interface I can just pass (pipe) the output of
ifconfig
intogrep
:ifconfig | grep wire
Thank you
Very nice guide, thanks for taking the time!
Thank you.
Fantastic summary of one of the most universally used cli tool. One thing to add is that the name
grep
comes from theed
commandg/re/p
which stands for “global, regular expression, print” since thats what it does; search everything (global) for a given regular expression (even if the “regular expression” is just a specific string to match) and prints it out. Keeping the name origin in mind helps me to remember what it does.Beginner’s Guide to
grep
man grep
“Using regular expressions” is misleading. A beginner could think that by default
grep
is looking for a literal string, but it does not.Would you mind explaining this regular expression a little, and how it’s grabbing email addresses? What is “\b”? What does “+” do? And the {2,}?
+ means that of the prepending selection of characters (those in the []-brackets) at least one character has to Match.
The curly braces are some advanced Form of that and means for {n,m} anything between n and m characters of the prepending selection. In this Case it would be at least 2 as m isn’t given. This will Match any Domain ending With at least 2 characters.
Notice however that this is a very Basic regex pattern to Match Email addresses.
Though related, it’s often conflated with regular expressions as a concept. On that note, services like RegExr let you test patterns and their effects visually.
Another good one is rubular.com