Книга: Fedora™ Unleashed, 2008 edition

Searches for a String in Input with grep

Searches for a String in Input with grep

The grep command, like find, is an incredibly powerful search tool in the right hands. Unlike find, though, grep processes any text, whether in files, or just in standard input.

The basic use of grep is this:

$ grep "some text" *

That query searches all files in the current directory (but not subdirectories) for the string some text and prints matching lines along with the name of the file. To enable recursive searching in subdirectories, use the -r parameter, like this:

$ grep -r "some text" *

Each time a string is matched within a file, the filename and the match are printed. If a file contains multiple matches, each of the matches is printed. You can alter this behavior with the -l parameter (lowercase L), which forces grep to print the name of each file that contains at least one match, without printing the matching text. If a file contains more than one match, it is still printed only once. Alternatively, the -c parameter prints each filename that was searched and includes the number of matches at the end, even if there were no matches.

You have a lot of control when specifying the pattern to search for. You can, as previously, specify a simple string like some text, or you can invert that search by specifying the -v parameter. For example, this returns all the lines of the file myfile.txt that do not contain the word hello:

$ grep -v "hello" myfile.txt

You can also use regular expressions for your search term. For example, you can search myfile.txt for all references to cat, sat, or mat with this command:

$ grep "[cms]at" myfile.txt

Adding the -i parameter to that removes case sensitivity, matching Cat, CAT, MaT, and so on:

$ grep -i [cms]at myfile.txt

The output can also be controlled to some extent with the -n and --color parameters. The first tells grep to print the line number for each match, which is where it appears in the source file. The --color parameter tells grep to color the search terms in the output, which helps them stand out when among all the other text on the line. You choose which color you want, using the GREP_COLOR environment variable: export GREP_COLOR=36 gives you cyan, and export GREP_COLOR=32 gives you lime green.

This next example uses these two parameters to number and color all matches to the previous command:

$ grep -in --color [cms]at myfile.txt

Later you will see how important grep is for piping with other commands.

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