The following is a modification of the above example that uses the star wildcard (i.e., an asterisk), which represents any character or sequence of characters, to search all text files in the current directory (i.e., the directory in which the user is currently working): A wildcard is a character that can represent some specific class of characters or sequence of characters. Text searches with grep can be considerably broadened by combining them with wildcards and/or performing recursive searches. Thus, the above example could be modified to search for the phrase Linux is: This is accomplished by enclosing the sequence of strings that forms the pattern in quotation marks (either single or double). It can also search for sequences of strings, including phrases.
Grep is not limited to searching for just single strings. The inclusion of the file names in the output data can be suppressed by using the -h option. Thus, for example, the following would search the three files file1, file2 and file3 for any line that contains the string (i.e., sequence of characters) Lin:Įach result is displayed beginning on a separate line, and it is preceded by the name of the file in which it was found in the case of multiple files. Grep can search any number of files simultaneously. Thus, a line of text returned by grep can be as short as a single character or occupy many lines on the display screen. Newline characters are invisible characters that are represented in Unix-like operating systems by a backslash followed by the letter n and which are created when a user presses the ENTER key when using a text editor (such as gedit). When used with no options and no arguments (i.e., input files), grep searches standard input (which by default is text typed in at the keyboard) for the specified pattern and returns each line that contains a match to standard output (which by default is the display screen).Ī line of text is defined in this context not as what appears as a line of text on the display screen but rather as all text between two newline characters. The items in square brackets are optional. It is one of the most useful and powerful commands on Linux and other Unix-like operating systems. Grep is used to search text for patterns specified by the user. ➜ grep -n -w "dfff" test6.How to use the grep command, by The Linux Information Project (LINFO) LINFO In the second example, we used multiple grep commands and pipes to match lines containing both “dfff” and “apple” words in the file test6.txt. ➜ grep -n -w -e "dfff" -e "apple" test6.txt In the first example, we use the grep -e option to match the line containing the word “dfff” or “apple” in the file test6.txt. * Match file containing keyword1 or containing keyword2 … : OR * Match file containing keyword1 and containing keyword2 … : AND But matching multiple keywords has two meanings: Grep matches multiple keywords, which we often use on a daily basis. Sometimes, however, we also need to count the keyword to appear in the file, at the same time, according to the line number in reverse order. In the example above, we can count the number of lines or the total number of occurrences of a keyword in a file. In the following example, the grep directory contains files whose filenames contain the keyword “test”, and we use the ls command, pipe, and wc command to count the number of files whose filenames contain the keyword “test” in the directory. Grep count the number of files in the directory whose filename contains the specified keyword w, -word-regexp The expression is searched for as a word (as if surrounded by `]' see re_format(7)). o, -only-matching Prints only the matching part of the lines. In the following example, we use grep -w to count the number of times of the string “dfff” in the file ➜ grep -o -w "dfff" test6.txt | wc -l Options: Grep counts the number of times of the specified content in a file You can also use the grep command, pipe, and wc command to achieve the same effect as the grep-c option in the following example. Using grep -c options alone will count the number of lines that contain the matching word instead of the number of total matches. In the following example, we will use the grep command to count the number of lines in the file test6.txt that contain the string “dfff” ➜ grep -c "dfff" test6.txt Grep counts the number of lines in the file that contain the specified content
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