Understanding grep command and its usage [Unix/Linux/macOS/Windows-Bash]

Undoubtably grep (pronounced as /ɡɹɛp/) is the most powerful and highly useful command when it comes to working with terminal for a Unix/Unix-like Operating System's, be it the Apple's macOS or FreeBSD, Solaris, HP-UX or Linux. You got to understand this command right if you are a computer science student or working in the software industry. Most of the servers are deployed on Unix/Linux operating systems that can only be accessed through the command-line interface and if there are tons of files and you need to look for a specific one that you are looking at has a huge text in it and you want to search for specific text content? grep is the command that can save you in such a scenario.

Table of contents:

What is a grep command?
Some background of grep command.
How to use grep?
grep command examples

1. What is grep command?

If you open your terminal or bash shell and type "man grep" it will print out a manual of this command, let's see how the manual defines this term.

✏️ grep searches for PATTERNS in each FILE. PATTERNS is one or more patterns separated by newline characters, and grep prints each line that matches a pattern. Typically PATTERNS should be quoted when grep is used in a shell command.

Ok, so if you run this command in a particular directory it will the text-contents of the files for specific lines that follows a pattern using a specific word or words or something as complex as a Regular Expression provided. So grep is a command-line utility that will helps you search file contents!

The word grep is derived from ed's (command line editor for UNIX systems) command g/re/p - "globally search for a regular expression and print matching lines" so yeah! regExp is something you should be good with to make the best use of this command.

2. Some background of grep command.

grep was written by Ken Thompson, yes, the man who write the B Programming Language and implemented the original Unix operating system at the Bell Labs USA. He write it initially for his private use, but once when his manager Doug McIlroy came up with a requirement to build something very similar, he worked on this utility and fixed some bugs it had and presented it as grep the next day! :) so grep was build overnight! (Check out this youtube video at 35min to know more about "the grep story - of how it came out of his directory to bin directory" - https://www.youtube.com/watch?v=EY6q5dv_B-o)

As stated earlier grep has been ideated from ed text editor's g/re/p command, the ed editor as you must have guess was also written by Ken Thompson. Though ed had support for regular expressions it was not good enough to work with a large text fix, so Thompson selected that code into grep. He chose the name because in ed, the command g/re/p would print all lines matching a specified pattern.

3. How to use grep?

grep comes bundled with the Unix/Linux based operating system, you do not need to install it unless you are using a Windows computer. You may require tools like Bash for Windows or Git Bash to use this command.

Let's have a look at the syntax of the command first, we will try to again make use of the grep manual,

SYNOPSIS
       grep [OPTION...] PATTERNS [FILE...]
       grep [OPTION...] -e PATTERNS ... [FILE...]
       grep [OPTION...] -f PATTERN_FILE ... [FILE...]

Alright, so lets see the first one, grep followed by option in braces that means optional and file again in braces that's optional again. So leaving them aside I can simply try grep followed by a pattern: example: grep my-Word-to-search?

Ok, I have a dirctory call countries that has a file called counties_details.txt that contains list of all countries and major languages spoken there, let me try out grep Sweden followed by the path to the file.

$ grep 'Sweden' countries_details.txt
Sweden  Swedish, small Sami- and Finnish-speaking minorities

Cool, lets try some regular expressions, all the lines that starts with letter S: ^S.*

$ grep '^S.*'
St. Kitts and Nevis     English
St. Lucia       English (official), French patois
St. Vincent and the Grenadines  English, French patois
San Marino      Italian
Saudi Arabia    Arabic
Senegal French (official); Wolof, Pulaar, Jola, Mandinka
Serbia  Serbian (official); Romanian, Hungarian, Slovak, and Croatian, Albanian
Seychelles      Seselwa Creole 92%, English 5%, French (all official)
Singapore       Mandarin 35%, English 23%, Malay 14.1%, Hokkien 11.4%, .....
Slovakia        Slovak 84% (official), Hungarian 11%, Roma 2%, Ukrainian 1% (2001)
Slovenia        Slovenian 91%, Serbo-Croatian 5% (2002)
South Africa    IsiZulu 23.8%, IsiXhosa 17.6%, Afrikaans 13.3%, Sepedi 9.4%, .
South Sudan     English (official), Arabic (official), regional languages ......
Spain   Castilian Spanish 74% (official); Catalan 17%, Galician 7%, Basque 2% 
Sri Lanka       Sinhala 74% (official and national), Tamil 18% (national), other 8%; English by about 10%
Suriname        Dutch (official), Surinamese (lingua franca), English widely spoken, Hindustani, Javanese
Swaziland       English, siSwati (both official)
Sweden  Swedish, small Sami- and Finnish-speaking minorities
Switzerland     German 64%, French 20%, Italian 7% (all official); Romansch 0.5% (national)
Syria   Arabic (official); Kurdish, Armenian, Aramaic, Circassian widely understood; French, English

4. 10+ grep Command Examples:

I will try to put down as many grep examples that I can,

Search for lines containing specific word: example: grep 'United' text-file-name

$ grep 'United' countries_details.txt
United Arab Emirates    Arabic (official), Persian, English, Hindi, Urdu
United Kingdom  English, Welsh, Scots Gaelic
United States   English 82%, Spanish 11%

Search for lines containing specific word in all files in a directory: example: grep 'and' *

$ grep 'and' *
Afghanistan     Dari Persian, Pashtu, other Turkic and minor languages
Angola  Portuguese, Bantu and other African languages
Antigua and Barbuda   English, local dialects

-i: Search with word insensitivity: example: grep -i 'denmark' text-file-name

$ grep -i 'denmark' countries_details.txt
Denmark Danish, Faroese, Greenlandic, German, English

-c: Get the count of the lines matched: example: grep -c 'United' text-file-name
```
$ grep -c 'United' countries_details.txt
3
```

-o: Print only the matching part the line: example: grep -o 'United' text-file-name
```
$ grep -c 'United' countries_details.txt
United
United
United
```

-l: Print only filenames of the lines matched: example: grep -l 'United' *
```
$ grep -l 'United' *
countries_details.txt
database_dump.txt
```

-n: Print matched text line with line numbers: example: grep -n 'United' *

$ grep -n 'United' *
186:United Arab Emirates    Arabic (official), Persian, English, Hindi, Urdu
187:United Kingdom  English, Welsh, Scots Gaelic
188:United States   English 82%, Spanish 11%

-v: Print lines that did not match the patterns: example: grep -v 'and' *

$ grep -v 'and' *
Samoa   Samoan, English
San Marino      Italian
Saudi Arabia    Arabic

-e: Using multiple (regex) Expressions at once with grep: example: grep -e 'United' -e 'Sweden' -e 'Australia'

$ grep -e 'United' -e 'Sweden' -e 'Australia' countries_details.txt
Australia       English 79%, native and other languages
Sweden  Swedish, small Sami- and Finnish-speaking minorities
United  Arab Emirates    Arabic , Persian, English, Hindi, Urdu
United  Kingdom  English, Welsh, Scots Gaelic
United  States   English 82%, Spanish 11%

-w: Whole word match: example: grep -w 'English,' file-name

$ grep -w 'English,'countries_details.txt
Argentina       Spanish (official), English, Italian, German, French
Brazil  Portuguese (official), Spanish, English, French
Greece  Greek 99% (official), English, French
Iceland Icelandic, English, Nordic languages, German widely spoken
India   Hindi 30%, English, Bengali, 
Gujarati, Malayalam, Marathi, Oriya, Punjabi, Tamil, Telugu, Urdu, Kannada, Sanskrit 
Indonesia       Bahasa Indonesia (official), English, 
Dutch, Javanese, and more than 580 other languages and dialects
Jamaica English, Jamaican Creole
Lebanon Arabic (official), French, English, Armenian
Lesotho English, Sesotho (both official); Zulu, Xhosa
New Zealand     English, Maori (both official)
United Kingdom  English, Welsh, Scots Gaelic

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