When the command is executed, the shell expands the asterisk to the name of any file it finds (in the current directory) which ends in ". Instead of specifying product-listing.html, we can use an asterisk (" *") and the. If we have multiple files to search, we can search them all using a wildcard in our FILE name. Searching multiple files using a wildcard Using the -i option, grep finds a match on line 23 as well. What if "our products" appears at the beginning of a sentence, or appears in all uppercase? We can specify the -i option to perform a case-insensitive match: Performing case-insensitive grep searches Our matching line is prefixed with " 18:" which tells us this corresponds to line 18 in our file. If we specify the -n option, grep will prefix each matching line with the line number: It will be even more useful if we know where the matching line appears in our file. Viewing line numbers of successful matches If we use the -color option, our successful matches will be highlighted for us: For more information, see: Regular expression quick reference. Other characters have special meanings, however - some punctuation marks, for example. In the above example, all the characters we used (letters and a space) are interpreted literally in regular expressions, so only the exact phrase will be matched. This page was generated by GitHub Pages.The PATTERN is interpreted by grep as a regular expression. Training-modules is maintained by hbctraining. These are open access materials distributed under the terms of the Creative Commons Attribution license (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This lesson has been developed by members of the teaching team at the Harvard Chan Bioinformatics Core (HBC). What we mean by this is that if you were to have a variable named at that holds AT:ġ) Use grep to find all matches in catch.txt that start with “B” and have a “T” anywhere in the string after the “B”.Ģ) Use grep to find all matches in catch.txt that don’t start with “C” and don’t end with “H”ģ) Use grep to find all matches in catch.txt that have atleast two “A”s in them Additional Resources Notably, bash variables within single-quotations are NOT expanded. However, within bash, single-quotation marks ( ') are intepreted literally, meaning that the expression within the quotation marks will be interpreted by bash EXACTLY the way it is written. The big advantage of using quotation marks, single or double, when using grep is that it allows you to use search expressions with whitespace in them. So grep doesn’t ever “see” quotation marks, but rather quotation marks are interpreted by bash first and then the result is passed to grep. ![]() As a result, if your search term doesn’t have whitespace it doesn’t matter if you put quotations, but if it does, then it won’t behave the way you’d like it to behave. If you are using grep to search and have whitespace (space or tabs) in your search, grep will treat the expression before the whitespace as the search term and the expression after the whitespace(s) as a file(s). Let’s briefly discuss the differences: No quotation However, if you would like to use grep to do certain types of searches, it is better or safer to wrap your search term in quotations, and likely double quotations. When using grep it is usually not required to put your search term in quotes. If you want to make it a habit to always use the -E option when using regular expressions in grep it is a bit more safe. We won’t use too many of these types of regular expressions and we will point them out when we need them. There is a -E option when using grep that allows the user to use what is considered “extended regular expressons”. There are two principles that we should discuss more, the -E option and the use of quotation marks. These differences are not exhaustive, but they will be helpful in exploring how regular expressions are implemented in grep. In here, you can see that we have a variety of case differences and misspellings.
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