Conceptual overview:-
grep is a command-line utility used to search for a specific string of text within one or more files. It can be used to search for a specific word or phrase in a file or to search for a pattern using regular expressions. The output of the grep command will show the lines in the input file(s) that contain the search term.
A regular expression, or regex, is a sequence of characters that defines a search pattern. Regex can be used to match a wide range of patterns, from simple strings to complex patterns that include multiple characters, digits, and special characters.
grep uses POSIX-extended regular expressions by default, but it can also use other types of regular expressions, such as basic regular expressions (BRE) and extended regular expressions (ERE).
Example:-
Search for a specific word in a file: This command will search for the word "example" in the file "file.txt" and output any lines that contain that word.
Copy codegrep "example" file.txt
Command options:-
Command options, also known as command-line options or flags, are additional parameters that can be passed to a command in order to modify its behavior. These options are usually specified by a single dash (-) followed by a single letter, or a double dash (--) followed by a word or phrase.
grep has a wide range of options that can be used to modify its behaviour. Here are some of the most commonly used options and a brief explanation of what they do:
-i or --ignore-case: Ignore case distinctions and match both upper and lowercase variations of the search term.
-v or --invert-match: Invert the sense of matching, to select non-matching lines.
-c or --count: Only show a count of matching lines, instead of the lines themselves.
-n or --line-number: Prefix each line of output with the line number within the input file.
-r or --recursive: Search for the search term in all files in the specified directory and its subdirectories.
-l or --files-with-matches: Only show the names of files that contain at least one matching line, instead of the matching lines themselves.
-w or --word-regexp: Only show lines that match the search term as a whole word, not as a substring.
-o or --only-matching: Show only the matching part of the lines, instead of the entire line.
-h or --no-filename: Never show the file name in the output.
-q or --quiet or --silent : Quiet mode: don't write anything to standard output.
-s or --no-messages : Suppress error messages about nonexistent or unreadable files.
-a or --text : Process a binary file as if it were text; this is equivalent to the --binary-files=text option.
-A or --after-context : Show n lines of trailing context after matching lines.
-B or --before-context : Show n lines of leading context before matching lines.
-C or --context : Show n lines of output context.
-I or --binary-files=TYPE : Ignore binary files.
Examples:-
-ior--ignore-case: Perform a case-insensitive search
grep -i "word" file.txt
-vor--invert-match: Invert the sense of matching, to select non-matching lines
grep -v "word" file.txt
-cor--count: Print only a count of the matching lines
grep -c "word" file.txt
-nor--line-number: Prefix each line of output with the 1-based line number within its input file.
grep -n "word" file.txt
-wor--word-regexp: Select only those lines containing matches that form whole words
grep -w "word" file.txt
-ror--recursive: Search for a string recursively in all files under a directory
grep -r "word" /path/to/directory
-Ror-r -d recurse: Same as -r, but follow all symlinks
grep -R "word" /path/to/directory
-lor--files-with-matches: Only the names of files that contain selected lines, without writing the lines
grep -l "word" file1.txt file2.txt file3.txt
-oor--only-matching: Show only the part of a matching line that matches PATTERN
grep -o "word" file.txt
-Eor--extended-regexp: Interpret PATTERN as an extended regular expression (ERE)
grep -E "word|anotherword" file.txt
-A or --after-context : Show n lines of trailing context after matching lines.
grep -A 3 "pattern" file.txt
-B or --before-context : Show n lines of leading context before matching lines.
grep -B 2 "pattern" file.txt
Regular Expressions:-
A regular expression is a sequence of characters that defines a search pattern. Regular expressions can be used to match patterns in text, such as specific characters, words, or phrases. grep can use regular expressions to search for patterns in a file or a group of files. Here are some examples of regular expressions that can be used with grep:
. - Matches any single character
* - Matches zero or more occurrences of the preceding character or group
+ - Matches one or more occurrences of the preceding character or group
? - Matches zero or one occurrence of the preceding character or group
^ - Matches the start of a line
$ - Matches the end of a line
[] - Matches any one of the characters inside the square brackets
[^] - Matches any one character not inside the square brackets
{} - Matches the preceding character or group a specified number of times
() - Defines a group of characters to be matched as a unit
| - Matches either the preceding or following character or group
\ - Escapes the following character, so it is treated as a literal instead of a special
Examples:-
- Search for any word that starts with "ex":
grep "^ex" file.txt
- Search for any word that ends with "ple":
Cgrep "ple$" file.txt
- Search for any word that contains the letter "a" followed by any single character:
grep "a." file.txt
- Search for any word that contains the letter "a" or "b":
grep "[ab]" file.txt
- Search for any word that contains the letter "a" and "b":
grep "a.*b" file.txt
- Search for any word that contains numbers and alphabets:
grep "[a-zA-Z0-9]" file.txt
- Search for any word that starts with a letter and ends with a number:
grep "[a-zA-Z]*[0-9]" file.txt
- Search for any word that starts with a letter, has any number of characters in the middle, and ends with a number:
grep "[a-zA-Z].*[0-9]" file.txt
- Search for any word that contains exactly 3 letters:
grep "^...$" file.txt
- Search for any word that contains exactly 3 letters and ends with "ple":
grep "^...ple$" file.txt