![]() ![]() For example, grep uses a dollar sign as a special character matching the end of a line - so if you actually want to search for a dollar sign, you have to precede it by a backslash (and include the whole search string in single quotes).īut fgrep lets you just type in that dollar sign.Īnd you can also use fgrep to match for a dot or a caret without having to precede it with a backslash - which does make things more readable. Again it warns that fgrep “is deprecated” but “provided to allow historical applications that rely on them to run unmodified.” I’ve heard it referred to as “ fast grep,” because it basically throws away all of grep’s regular expression-matching, and just concentrates on quickly looking only for matching strings. Grep also spawned yet-another standalone tool, fgrep, which grep’s man page explains is the same as grep -F. You can also create a group of characters - which will only be considered a match if the entire group is present. Again, grep requires that you precede the pipe character with a backslash - but with egrep you can simply include it in your commands. The “pipe” character lets you separate a pair of characters, and will match if a line contains either character. You can also do the same quantity-matching with grep, but you have to precede the characters with a backslash.Īnd there’s one more powerful way to soup up your pattern matching: alternation. ![]() That lets you get more specific about how many times you want a character to appear in your matches. It’s like grep with a superpower - it searches through every subdirectory.Īnd then there’s egrep , which is the same as grep -E, though the grep man page warns that egrep “is deprecated,” but “is provided to allow historical applications that rely on them to run unmodified.” Searches with egrep match not only the usual metacharacters ( .* ^ $) but also the Posix-defined set of (E)xtended regular expressions. For example, there’s the recursive rgrep, which is the same as grep -r. ![]() History has it that Ken Thompson coded up the grep tool overnight to help a colleague search through the entire text of the Federalist Papers without having to load the whole thing into memory first.īut in the same way that the ed command g/ re/p became a stand-alone tool named “grep,” some of grep’s most useful flags eventually spun off into more tools. This is pretty helpful if we want to check the entry details.Yes, it was such a useful command that eventually became its own stand-alone tool. Also, if a file contains multiple “ ” records, its filename will be printed multiple times, such as myApp/logs/myApp.log in the example above. MyApp/logs/archives/app.log.: 10:47:00 Network is brokenĪs we can see, grep outputs all matched lines together with the filenames. MyApp/logs/archives/app.log.: 10:00:00 application cannot start! Cause: No DNS configurated MyApp/logs/myApp.log: 10:10:00 Cannot access the database. MyApp/logs/myApp.log: 10:08:00 Cannot access the database. MyApp/logs/myApp.log: 10:07:00 Cannot access the database. So, let’s first search all level log records among all the log files and archived logs.įirst, as our target files are located in different directories, we’ll use the -r option to do a recursive search.įurther, as ‘ ‘ have special meaning in regex, we should either escape them or use the -F option to tell grep to do a fix-string search: $ grep -rF '' myApp/logs Usually, we need to pay attention to the log entries with the ERROR level. 11:08:00 new admin user is created: Īs we can see, as regular log files, those files contain log entries with different log levels. 11:01:00 Login failed 10 times in 1 min. 11:00:00 42 new users regsitered in the last 5 minutes ![]() So next, let’s quickly peek into the content of those files: $ head logs/**/*.* The tree output above shows that the log files and archives are stored in different directories. Let’s say we have an application called myApp. As usual, we’ll address the problem through an example. ![]()
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