How to use the command 'egrep' (with examples)

The ’egrep’ command is a Linux utility used to search for patterns within files using extended regular expressions. It supports special characters such as ‘?’, ‘+’, ‘{}’, ‘()’, and ‘|’. This article will demonstrate various use cases of the ’egrep’ command along with their corresponding code, motivation, explanation, and example output.

Use case 1: Search for a pattern within a file

Code:

egrep "search_pattern" path/to/file

Motivation: This use case is handy when you want to search for a specific pattern or string within a single file. It can be useful for code analysis or log file parsing purposes.

Explanation:

• ’egrep’: The command itself.
• "search_pattern": The pattern or string you want to search for.
• path/to/file: The path to the file in which you want to search for the pattern.

Example output: If we run the following command: egrep "error" /var/log/syslog, it will search for the pattern “error” in the “/var/log/syslog” file. If there are any lines containing the word “error” in the syslog file, it will display them as the output.

Use case 2: Search for a pattern within multiple files

Code:

egrep "search_pattern" path/to/file1 path/to/file2 path/to/file3

Motivation: When you have multiple files and want to search for a pattern simultaneously within them, this use case becomes useful. It saves the effort of running the search command for each file individually.

Explanation:

• The code structure for searching for a pattern within multiple files is similar to the previous use case.
• "search_pattern": The pattern or string you want to search for.
• path/to/file1 path/to/file2 path/to/file3: The paths to the files in which you want to search for the pattern.

Example output: If we run the command egrep "error" file1.txt file2.txt file3.txt, it will search for the pattern “error” in the files file1.txt, file2.txt, and file3.txt. If there are any lines containing the word “error” in any of these files, it will display them as the output.

Use case 3: Search ‘stdin’ for a pattern

Code:

cat path/to/file | egrep search_pattern

Motivation: This use case is beneficial when you want to process the output of a command or use a text file as input and search for a specific pattern within it.

Explanation:

• cat path/to/file: The ‘cat’ command is used to display the contents of the file specified in the path.
• |: The pipe operator is used to take the output from the previous command and pass it as input to the next command.
• egrep: The ’egrep’ command is used here to search for the pattern within the input received from the previous command.
• search_pattern: The pattern or string you want to search for.

Example output: If we run the command cat file.txt | egrep "error", it will display the contents of the file.txt and search for the pattern “error” within it. If there are any lines containing the word “error” in the file, it will display them as the output.

Use case 4: Print file name and line number for each match

Code:

egrep --with-filename --line-number "search_pattern" path/to/file

Motivation: By using this use case, you can get the filename and line number along with the matching lines. It can be useful when you want to identify the specific location of the pattern within a file or when you need to extract relevant information.

Explanation:

• --with-filename: This option is used to display the filename along with the matching lines.
• --line-number: This option is used to display the line number of each matching line.
• "search_pattern": The pattern or string you want to search for.
• path/to/file: The path to the file in which you want to search for the pattern.

Example output: If we run the command egrep --with-filename --line-number "error" file.txt, it will search for the pattern “error” in the file.txt file. If there are any lines containing the word “error,” it will display them along with the filename and line number as the output.

Use case 5: Search for a pattern in all files recursively in a directory, ignoring binary files

Code:

egrep --recursive --binary-files=without-match "search_pattern" path/to/directory

Motivation: This use case is useful when you want to search for a pattern across multiple files recursively within a directory, excluding binary files. It can be helpful for tasks like code auditing, finding specific error messages, or extracting specific information from a large collection of text files.

Explanation:

• --recursive: This option is used to search for a pattern in all files within the specified directory and its subdirectories recursively.
• --binary-files=without-match: This option is used to exclude binary files from the search process.
• "search_pattern": The pattern or string you want to search for.
• path/to/directory: The path to the directory in which you want to search for the pattern.

Example output: If we run the command egrep --recursive --binary-files=without-match "error" /var/log/, it will search for the pattern “error” in all files within the /var/log/ directory and its subdirectories. If there are any lines containing the word “error” in the text files, it will display them as the output.

Use case 6: Search for lines that do not match a pattern

Code:

egrep --invert-match "search_pattern" path/to/file

Motivation: This use case is useful when you want to search for lines that do not contain a specific pattern. It helps to filter out irrelevant information and focus only on lines that do not match a particular criterion.

Explanation:

• --invert-match: This option is used to invert the matching process. Instead of returning lines that match the pattern, it returns lines that do not match the pattern.
• "search_pattern": The pattern or string you want to search for.
• path/to/file: The path to the file in which you want to search for the pattern.

Example output: If we run the command egrep --invert-match "error" file.txt, it will search for lines that do not contain the pattern “error” in the file.txt file and display them as the output.