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grep or global regular expression printing is a command line utility to search input files for a search string and return the matching lines; it searches the information that is piped to it or the files in the current directory. Surprisingly, this simple tool is by far one of the most useful Linux command line tools out there; provides several additional and useful features that make search queries less time consuming and more effective. We can achieve this using a wide range of command line options (flag keywords) and regular expression patterns.
The search process of this program is very efficient even when dealing with a large number of directories/files, grep does not store any lines; copies a line to a buffer, checks the search string, then prints the line if a match is found. Although the goal of this tool is to search for a given string, one of its features is to exclude lines matching the given search string from the output while iterating through the directory tree. In this article we will teach you how to exclude patterns, files, directories with grep.
How to exclude patterns, files and directories with Grep
Exclude words and patterns
Use the -v (or –invert-match) option to display only lines that do not match a search pattern.
To print the lines that do not contain the nologin string, run the following command:
- grep -wv nologin /etc/passwd
Production
- root:x:0:0:root:/root:/bin/bashgit:x:994:994:git daemon user:/:/usr/bin/git-shellvega:x:1000:1000:vega:/home/ vega:/bin/bash
You must enclose the search string in single or double quotes if it contains spaces.
Use the -e option to define two or more search patterns:
- grep -wv -e nologin -e bash /etc/passwd
The -e option can be used as many times as necessary.
Another technique to exclude multiple search patterns is to use the OR | to match the patterns.
Perl-compatible, extended, and basic regular expression syntaxes are supported by GNU grep. grep reads the pattern as a regular regular expression by default, which means that metacharacters like | they lose their special meaning and must be replaced with their backslash variants.
The operator | must not be escaped if you use the -E extended regular expression option, as shown below:
- grep -Ewv ‘nologin|bash’ /etc/passwd
You can choose from a variety of possible matches, such as literal strings or sets of expressions. Lines where string sets appear at the beginning of a line are removed in the following example:
- grep -v “^games” file.txt
The output of a command can be filtered using grep and pipe, and only the lines that match a specific pattern are printed to the terminal.
For example, you can filter the output of the ps command to print all running processes on your system except those running as root:
Exclude directories and files
You may want to exclude specific directories from the search result when using the -r or -R parameters to run a recursive search.
Use the –exclude-dir option to exclude a directory from the search. The excluded directory path is relative to the search directory path.
Here’s how to search for the string vega in all files in the /etc directory, skipping the /etc/pki directory:
- grep -R –exclude-dir=pki vega /etc
To exclude multiple directories, use square brackets to enclose them and commas to divide them without spaces.
For example, to discover files on your Linux system that include the string gnu, you would use the following command, excluding the proc, boot, and sys directories:
- grep -r –exclude-dir={proc,boot,sys} gnu /
You can exclude files whose base name matches the GLOB provided in the –exclude option when performing wildcard matching.
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