I've come across comments about not knowing the -c option in online forums. Didn't even know about the -color option. In my initial years of CLI usage as a VLSI engineer, I knew only some of the options listed in this chapter. # use -o to get each match on a separate line This option is more commonly used with regular expressions.
If the total number of matches is required, use the -o option to display only the matching portions (one per line), and then use wc to count them. See also stackoverflow: Fastest way to find lines of a text file from another larger text file - go through all the answers. # lines present in colors_1 but not in colors_2 # lines present in colors_2 but not in colors_1 $ printf 'blue\nblack\ndark green\nyellow\n' > colors_2 $ printf 'this\nis\ncool\n' | grep -c 'is' ip.txt. When multiple input files are passed, the count is displayed for each file separately. $ printf 'goal\nrate\neat\npit' | grep -vc 'g' # number of lines NOT matching the pattern Somehow piping grep output to the wc command is prevalent instead of simply using the -c option. Having to count the total number of matching lines comes up often. $ printf 'great\nneat\nuser' | grep -n 'eat' This is useful to quickly locate matching lines for further processing. The -n option will prefix line numbers to matching results, using a colon character as the separator. Look out for opposite pairs like -l -L, -h -H, negative logic in regular expressions and so on in the examples to follow. Text processing often involves negating a logic to arrive at a solution or to make it simpler. $ printf 'goal\nrate\neat\npit' | grep -v 'at' Use the -v option to get lines other than those matching the search term. $ printf 'Cat\ncOnCaT\ncut\n' | grep -i 'cat' In such cases, you can use the -i option to ignore case. Sometimes, you don't know if a log file contains case variable search terms, such as error, Error, or ERROR. If the search string doesn't have any regular expression metacharacters, GNU grep will try a literal search even if the -F option isn't used. # use the -F option to match strings literally For now, use the -F option to indicate that the patterns should be matched literally. But regular expressions is a topic for the next chapter. The search string (pattern) is treated as a Basic Regular Expression (BRE) by default. dev/stdin: ASCII text, with CRLF line terminators See stackoverflow: Why does my tool output overwrite itself and how do I fix it? for a detailed discussion and mitigation methods. If your input file has \r\n (carriage return and newline characters) as the line ending, convert the input file to Unix-style before processing.
# press Ctrl+d after the line containing 'histogram' Here's an example where grep reads user written stdin data and the filtered output is redirected to a file. $ printf 'apple\nbanana\nmango\nfig\n' | grep 'an'. $ printf 'apple\nbanana\nmango\nfig\n' | grep 'an' Grep will perform the search on stdin data if there are no file arguments or if - is used as a filename. Examples requiring shell interpretation will be discussed later. As a good practice, always use single quotes around the search string. To filter desired lines, invoke the grep command, pass the search string and then specify one or more filenames that have to be searched. Consider this sample input file: $ cat ip.txt
GREP SHOW FILENAME AND LINE HOW TO
This section will show you how to filter lines matching a given search string using grep. The newline character \n is the line separator by default. Basic string searchīy default, grep will print all the input lines that match the given search patterns. The example_files directory has all the files used in the examples. Literal (fixed string) matching refers to exact string comparison, so no special meaning is assigned for any of the search characters. Regular expressions will be covered later, so the examples in this chapter will only use literal strings as search patterns. This chapter will cover many of the options provided by GNU grep.
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