First, grep –help lists most of its options, which is the go-to command for most grep questions.

Like most CLI tools, options of grep can be combined. For example, -io is same as -i -o, -A3 is same as -A 3. Also, the options can be anywhere in the command.

$ grep hello a.txt -i --color

Stop after first match

$ grep -m 1 search-word file

-m, –max-count=NUM stop after NUM matches

Only print the 1000th match.

$ grep -m1000 search-word file | tail -n1

$ grep -l search-word *.txt

-l, –files-with-matches print only names of FILEs containing matches

It’s useful when you grep lots of files and only care about names of matched files.

Find unmatched files

-L, –files-without-match print only names of FILEs containing no match

-L is the opposite of -l option. It outputs the files which don’t contain the word to search.

$ grep -L search-word *.txt

Show line number of matched lines

$ grep -n search-word file

-n, –line-number print line number with output lines

Don’t output filename when grep multiple files

When grep multiple files, by default filename is included in the output. Like,

$ grep hello *.txt
a.txt:hello
b.txt:hello

Use -h to not output filenames.

$ grep -h hello *.txt
hello
hello

-h, –no-filename suppress the file name prefix on output

Search in “binary” files

Sometimes, a text file may contains a few non-printable characters, which makes grep consider it as a “binary” file. grep doesn’t print matched lines for a “binary” file.

$ printf "hello\000" > test.txt
$ grep hello test.txt 
Binary file test.txt matches

Use -a to let grep know the file should be seen as a “text” file.

$ grep -a hello test.txt 
hello

-a, –text equivalent to –binary-files=text

Search in directories

-r, –recursive like –directories=recurse

-R, –dereference-recursive likewise, but follow all symlinks

Without specifying a directory, grep searches in current working directory by default.

$ grep -R hello
b.md:hello
a.txt:hello

Specify directories.

$ grep -R hello tmp/ tmp2/
tmp/b.md:hello
tmp/a.txt:hello
tmp2/b.md:hello
tmp2/a.txt:hello

–include=FILE_PATTERN search only files that match FILE_PATTERN

Use --include to tell grep the pattern of the filenames you’re interested in.

$ grep -R hello --include="*.md"
b.md:hello

-i, –ignore-case ignore case distinctions

$ grep -i Hello a.txt 
hello
HELLO

The pattern to search begins with - (hyphen)

$ grep -- -hello a.txt
-hello

To know what -L option does.

$ grep --help | grep -- -L
  -L, --files-without-match  print only names of FILEs containing no match

Use pattern file

-f FILE, –file=FILE Obtain patterns from FILE, one per line. If this option is used multiple times or is combined with the -e (–regexp) option, search for all patterns given. The empty file contains zero patterns, and therefore matches nothing.

$ cat test.txt
111
222
333

$ cat patterns.txt
111
333

$ grep -f patterns.txt test.txt
111
333

NOTE: Do not put an empty line, i.e. a line with \n only, in the pattern file. Otherwise, the pattern file would match every line, since every line contains \n as its last character. It’s easy to make a mistake to put empty lines in the end of the pattern file.

Use -c, or --count to print only count of matching lines. For example, below command line is to find out the count of <OrderLine> tag in files of current directory.

$ grep "<OrderLine>" -c -R .

It outputs like below.

./order-1.xml:3
./order-2.xml:9
./order-3.xml:1

To sort the output, use command like below.

$ grep "<OrderLine>" -c -R . | sort -t : -k 2

./order-3.xml:1
./order-1.xml:3
./order-2.xml:9