The grep command. This tutorial explains the usage of the grep command for searching for regular expressions in files.
1. Grep
Grep is a command line tool to search for regular expressions.
Grep will print the matching line to the output and with the --color flag you can highlight the matching strings.
The usage of the Grep command is demonstrated by the following example.
# search for Strings
grep searchterm filename
grep searchterm /etc/*
# if you want to use regular expression uses quotes
grep "e*tools" /home/vogella/Use the -r "recursive" option to search a directory and all files included within it.
# search the "hello" string in all files and
# subdirectories of the current directory
grep -r "hello" .
# same as above but only list the file names
grep -rl "hello" .
# you can exclude directories from the search
# the following excludes all directories called ".git" from the seach
grep -r --exclude-dir='.git' "hello" .2. Important flags for using grep
The most important flags of the grep command are listed in the following table.
Flag |
Example |
Description |
-r |
grep -r text filepattern |
Searches also in subdirectories of the specified filepattern. |
-n |
grep -n text filepattern |
Prints the line number of output. |
-H |
grep -H text filepattern |
Includes the filename always in the output. By default the filename is only included if several files are searched. |
-c |
grep -c text filepattern |
Counts the occurrences of the search term in the specified files. |
-l |
grep -l text filepattern |
Lists the files which includes the search term. |
-L |
grep -L text filepattern |
Lists the files which do not include the search term. |
3. Chaining the grep
Grep is frequently used together with the find command.
For example the following will case-insensitive search for the pattern ".Legacy..xml" through the content of all files which ends with'`".java"` and lists them.
find . -name "*.java" -print0 | xargs -0 grep -i ".*Legacy.*xml"4. Links and Literature
4.1. vogella Java example code
If you need more assistance we offer Online Training and Onsite training as well as consulting