Usage (GNU Grep 3.7)

4 Usage

Here is an example command that invokes GNU grep:

grep -i 'hello.*world' menu.h main.c

This lists all lines in the files menu.h and main.c that contain the string ‘hello’ followed by the string ‘world’; this is because ‘.*’ matches zero or more characters within a line. See Regular Expressions. The -i option causes grep to ignore case, causing it to match the line ‘Hello, world!’, which it would not otherwise match.

Here is a more complex example, showing the location and contents of any line containing ‘f’ and ending in ‘.c’, within all files in the current directory whose names start with non-‘.’, contain ‘g’, and end in ‘.h’. The -n option outputs line numbers, the -- argument treats any later arguments as file names not options even if *g*.h expands to a file name that starts with ‘-’, and the empty file /dev/null causes file names to be output even if only one file name happens to be of the form ‘*g*.h’.

grep -n -- 'f.*\.c$' *g*.h /dev/null

Note that the regular expression syntax used in the pattern differs from the globbing syntax that the shell uses to match file names.

See Invoking grep, for more details about how to invoke grep.

Here are some common questions and answers about grep usage.

How can I list just the names of matching files?
```
grep -l 'main' test-*.c
```
lists names of ‘test-*.c’ files in the current directory whose contents mention ‘main’.
How do I search directories recursively?
```
grep -r 'hello' /home/gigi
```
searches for ‘hello’ in all files under the /home/gigi directory. For more control over which files are searched, use find and grep. For example, the following command searches only C files:
```
find /home/gigi -name '*.c' ! -type d \
  -exec grep -H 'hello' '{}' +
```
This differs from the command:
```
grep -H 'hello' /home/gigi/*.c
```
which merely looks for ‘hello’ in non-hidden C files in /home/gigi whose names end in ‘.c’. The find command line above is more similar to the command:
```
grep -r --include='*.c' 'hello' /home/gigi
```
What if a pattern or file has a leading ‘-’?
```
grep -- '--cut here--' *
```
searches for all lines matching ‘--cut here--’. Without --, grep would attempt to parse ‘--cut here--’ as a list of options, and there would be similar problems with any file names beginning with ‘-’.

Alternatively, you can prevent misinterpretation of leading ‘-’ by using -e for patterns and leading ‘./’ for files:
```
grep -e '--cut here--' ./*
```
Suppose I want to search for a whole word, not a part of a word?
```
grep -w 'hello' test*.log
```
searches only for instances of ‘hello’ that are entire words; it does not match ‘Othello’. For more control, use ‘\<’ and ‘\>’ to match the start and end of words. For example:
```
grep 'hello\>' test*.log
```
searches only for words ending in ‘hello’, so it matches the word ‘Othello’.
How do I output context around the matching lines?
```
grep -C 2 'hello' test*.log
```
prints two lines of context around each matching line.

How do I force grep to print the name of the file?

Append /dev/null:

grep 'eli' /etc/passwd /dev/null

gets you:

/etc/passwd:eli:x:2098:1000:Eli Smith:/home/eli:/bin/bash

Alternatively, use -H, which is a GNU extension:

grep -H 'eli' /etc/passwd

Why do people use strange regular expressions on ps output?
```
ps -ef | grep '[c]ron'
```
If the pattern had been written without the square brackets, it would have matched not only the ps output line for cron, but also the ps output line for grep. Note that on some platforms, ps limits the output to the width of the screen; grep does not have any limit on the length of a line except the available memory.
Why does grep report “Binary file matches”?
If grep listed all matching “lines” from a binary file, it would probably generate output that is not useful, and it might even muck up your display. So GNU grep suppresses output from files that appear to be binary files. To force GNU grep to output lines even from files that appear to be binary, use the -a or ‘--binary-files=text’ option. To eliminate the “Binary file matches” messages, use the -I or ‘--binary-files=without-match’ option, or the -s or --no-messages option.
Why doesn’t ‘grep -lv’ print non-matching file names?
‘grep -lv’ lists the names of all files containing one or more lines that do not match. To list the names of all files that contain no matching lines, use the -L or --files-without-match option.
I can do “OR” with ‘|’, but what about “AND”?
```
grep 'paul' /etc/motd | grep 'franc,ois'
```
finds all lines that contain both ‘paul’ and ‘franc,ois’.
Why does the empty pattern match every input line?
The grep command searches for lines that contain strings that match a pattern. Every line contains the empty string, so an empty pattern causes grep to find a match on each line. It is not the only such pattern: ‘^’, ‘$’, and many other patterns cause grep to match every line.

To match empty lines, use the pattern ‘^$’. To match blank lines, use the pattern ‘^[[:blank:]]*$’. To match no lines at all, use the command ‘grep -f /dev/null’.
How can I search in both standard input and in files?
Use the special file name ‘-’:
```
cat /etc/passwd | grep 'alain' - /etc/motd
```
Why is this back-reference failing?
```
echo 'ba' | grep -E '(a)\1|b\1'
```
This outputs an error message, because the second ‘\1’ has nothing to refer back to, meaning it will never match anything.
How can I match across lines?
Standard grep cannot do this, as it is fundamentally line-based. Therefore, merely using the [:space:] character class does not match newlines in the way you might expect.

With the GNU grep option -z (--null-data), each input and output “line” is null-terminated; see Other Options. Thus, you can match newlines in the input, but typically if there is a match the entire input is output, so this usage is often combined with output-suppressing options like -q, e.g.:
```
printf 'foo\nbar\n' | grep -z -q 'foo[[:space:]]\+bar'
```
If this does not suffice, you can transform the input before giving it to grep, or turn to awk, sed, perl, or many other utilities that are designed to operate across lines.
What do grep, fgrep, and egrep stand for?
The name grep comes from the way line editing was done on Unix. For example, ed uses the following syntax to print a list of matching lines on the screen:
```
global/regular expression/print
g/re/p
```
fgrep stands for Fixed grep; egrep stands for Extended grep.