Grep Command Tutorial For Unix

 

Output Line Prefix Control

-b, --byte-offset

grep -b pattern filename

Displays the byte offset of each matching text instead of the line number. The first byte in the file is byte 0, and invisible line-terminating characters (the newline in Unix) are counted. Because entire lines are printed by default, the number displayed is the byte offset of the start of the line. This is particularly useful for binary file analysis, constructing (or reverse-engineering) patches, or other tasks where line numbers are meaningless.

grep -b -o pattern filename

A -o option prints the offset along with the matched pat- tern itself and not the whole matched line containing the pattern. This causes grep to print the byte offset of the start of the matched string instead of the matched line.

-H, --with-filename

grep -H pattern filename

 

Includes the name of the file before each line printed, and is the default when more than one file is input to the search. This is useful when searching only one file and you want the filename to be contained in the output. Note that this uses the relative (not absolute) paths and filenames.

-h, --no-filename

grep -h pattern *

The opposite of -H. When more than one file is involved, it suppresses printing the filename before each output. It is the default when only one file or standard input is in- volved. This is useful for suppressing filenames when searching entire directories.

--label=LABEL

gzip -cd file.gz | grep --label=LABEL pattern

 

When the input is taken from standard input (for instance, when the output of another file is redirected into grep), the –label option will prefix the line with LABEL. In this example, the gzip command displays the contents of the uncompressed file inside file.gz and then passes that to

grep.

-n, --line-number

grep -n pattern filename

 

Includes the line number of each line displayed, where the first line of the file is 1. This can be useful in code debug- ging, allowing you to go into the file and specify a partic- ular line number to start editing.

-T, --initial-tab

grep -T pattern filename

 

Inserts a tab before each matching line, putting the tab between the information generated by grep and the match- ing lines. This option is useful for clarifying the layout. For instance, it can separate line numbers, byte offsets, labels, etc., from the matching text.

-u, --unix-byte-offsets

grep -u -b pattern filename

 

This option only works under the MS-DOS and Microsoft Windows platforms and needs to be invoked with -b. This option will compute the byte-offset as if it were running under a Unix system and strip out carriage return characters.

-Z, --null

grep -Z pattern filename

 

Prints an ASCII NUL (a zero byte) after each filename. This is useful when processing filenames that may contain special characters (such as carriage returns).

 

Context Line Control

-A NUM, --after-context=NUM

grep -A 3 Copyright filename

Offers a context for matching lines by printing the NUM lines that follow each match. A group separator (–) is placed between each set of matches. In this case, it will print the next three lines after the matching line. This is useful when searching through source code, for instance. The example here will print three lines after any line that contains “Copyright”, which is typically at the top of source code files.

-B NUM, --before-context=NUM

grep -B 3 Copyright filename

Same concept as the -A NUM option, except that it prints the lines before the match instead of after it. In this case, it will print the three lines before the matching line. This is useful when searching through source code, for in- stance. The example here will print three lines before any line that contains “Copyright”, which is typically at the top of source code files.

-C NUM, -NUM, --context=NUM

grep -C 3 Copyright filename

The -C NUM option operates as if the user entered both the  -A NUM and -B NUM options. It will display NUM lines before and after the match. A group separator (–) is placed be- tween each set of matches. In this case, three lines above and below the matching line will be printed. Again, this is useful when searching through source code, for instance. The example here will print three lines before and after any line that contains “Copyright”, which is typically at the top of source code files.

 

File and Directory Selection

-a, –text

grep -a pattern filename

Equivalent to the –binary-files=text option, allowing a binary file to be processed as if it were a text file.

--binary-files=TYPE

grep --binary-files=TYPE pattern filename

 

TYPE can be either binary, without-match, or text. When grep first examines a file, it determines whether the file is a “binary” file (a file primarily composed of non-human- readable text) and changes its output accordingly. By default, a match in a binary file causes grep to display sim- ply the message “Binary file somefile.bin matches.” The default behavior can also be specified with the

--binary-files=binary option.

When TYPE is without-match, grep does not search the bi- nary file and proceeds as if it had no matches (equivalent to the -l option). When TYPE is text, the binary file is pro- cessed like text (equivalent to the -a option). When TYPE is without-match, grep will simply skip those files and not search through them. Sometimes –binary-files=text outputs binary garbage and the terminal may interpret some of that garbage as commands, which in turn can render the terminal unreadable until reset. To recover from this, use the commands tput init and tput reset.

-D ACTION, --devices=ACTION

grep -D read 123-45-6789 /dev/hda1

 

If the input file is a special file, such as a FIFO or a socket, this flag tells grep how to proceed. By default, grep will process these files as if they were normal files on a system. If ACTION is set to skip, grep will silently ignore them. The example will search an entire disk partition for the fake Social Security number shown. When ACTION is set to read, grep will read through the device as if it were a normal file.

-d ACTION, --directories=ACTION

grep -d ACTION pattern path

 

This flag tells grep how to process directories submitted as input files. When ACTION is read, this reads the directory as if it were a file. recurse searches the files within that directory (same as the -R option), and skip skips the di- rectory without searching it.

--exclude=GLOB

grep --exclude=PATTERN path

 

Refines the list of input files by telling grep to ignore files whose names match the specified pattern. PATTERN can be an entire filename or can contain the typical “file- globbing” wildcards the shell uses when matching files (*, ? and []). For instance, –exclude=*.exe will skip all files ending in .exe.

--exclude-from=FILE

grep --exclude-from=FILE path

Similar to the –exclude option, except that it takes a list of patterns from a specified filename, which lists each pat- tern on a separate line. grep will ignore all files that match any lines in the list of patterns given.

--exclude-dir=DIR

grep --exclude-dir=DIR pattern path

 

Any directories in the path matching the pattern DIR will be excluded from recursive searches. In this case, the ac- tual directory name (relative name or absolute path name) has to be included to be ignored. This option also must be used with the -r option or the -d recurse option in order to be relevant.

-l

grep -l pattern filename

Same as the –binary-files=without-match option. When grep finds a binary file, it will assume there is no match in the file.

--include=GLOB

grep --include=*.log pattern filename

 

Limits searches to input files whose names match the given pattern (in this case, files ending in .log). This option

is particularly useful when searching directories using the

-R option. Files not matching the given pattern will be ig- nored. An entire filename can be specified, or can contain the typical “file-globbing” wildcards the shell uses when matching files (*, ? and []).

-R, -r, --recursive

grep -R pattern path

grep -r pattern path

 

Searches all files underneath each directory submitted as an input file to grep.

 

Other Options

--line-buffered

grep --line-buffered pattern filename

 

Uses line buffering for the output. Line buffering output usually leads to a decrease in performance. The default behavior of grep is to use unbuffered output. This is generally a matter of preference.

--mmap

grep --mmap pattern filename

Uses the mmap() function instead of the read() function to process data. This can lead to a performance improvement but may cause errors if there is an I/O problem or the file shrinks while being searched.

-U, --binary

grep -U pattern filename

 

An MS-DOS/Windows-specific option that causes grep to treat all files as binary. Normally, grep would strip out carriage returns before doing pattern matching; this op- tion overrides that behavior. This does, however, require you to be more thoughtful when writing patterns. For in- stance, if content in a file contains the pattern but has a

newline character in the middle, a search for that pattern will not find the content.

-V, --version

Simply outputs the version information about grep and then exits.

-z, --null-data

grep -z pattern

 

Input lines are treated as though each one ends with a zero byte, or the ASCII NUL character, instead of a newline. Similar to the -Z or –null options, except this option works with input, not output.

 

One final limitation of basic grep: the “extended” regular ex- pressions metacharacters—?, +, {, }, |, (, and )—do not work with basic grep. The functions provided by those characters exist if you preface them with an escape. More on that in the next section.

 

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