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Grep wc11/25/2023 ![]() ![]() When the Linux shell sees the meta character, it does the expansion and gives all the files as input to grep. The grep output will also include the file name in front of the line that matched the specific pattern as shown below. For this example, let us copy the demo_file to demo_file1. This is also a basic usage of grep command. Checking for the given string in multiple files. The basic usage of grep command is to search for a specific string in the specified file as shown below. Search for the given string in a single file This Line Has All Its First Character Of The Word With Upper Case.Īnd this is the last line. This line is the 1st lower case line in this file. THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE. In this article let us review 15 practical examples of Linux grep command that will be very useful to both newbies and experts.įirst create the following demo_file that will be used in the examples below to demonstrate grep command. Earlier we discussed 15 practical examples for Linux find command, Linux command line history and mysqladmin command. This is part of the on-going 15 Examples series, where 15 detailed examples will be provided for a specific command or functionality. The -m and -w options to grep are not standard, but often ( -w) or sometimes ( -m) implemented.You should get a grip on the Linux grep command. With -c, grep then outputs 3 if at least three lines matches the expression, so we test with -eq 3 to see if this was the case. Where -m 3 makes grep stop after three matches. With GNU grep (and some others), the whole test could made further effective (faster): The -c option to grep makes it report the number of lines matched, making the use of wc -l unnecessary. The grep and wc test could be shortened into just This is something that we don't really want here, especially since we don't know the value of $IFS (if it contains digits, for whatever reason, this may affect the result of the test). Without the double quotes, the shell would split the result of the command substitution on the characters in $IFS (space, tab, newline, by default), and would then apply filename globbing to the resulting strings. You may want to check the manuals for the grep, wc and test utilities, and possibly to read up on pipes and command substitutions. Without or test, the command is nonsense. This returns an exit status that could be use by an if statement (as a "boolean", like you say), for example, but it's hard to say much more without seeing more of the code. test "$(grep -w "xyz" prog.R | wc -l)" -ge 3 the $(.) bit.Īssuming that the command substitution ( $(.)) sits inside or as arguments to the test utility, the -ge 3 then is a test to see whether the number of lines counted by wc -l is greater than or equal to three. ![]() This will be the output of the command substitution as a whole, i.e. The wc -l command reads the output from grep via the | pipe, and counts the number of lines that the grep command produces. The -w option makes it only match complete words, so it won't match e.g. The grep command extracts all lines in the file called prog.R that contains the word xyz. ![]()
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