Usage - GNU Grep 2.18

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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. See Invoking, 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' *.c
```
lists the names of all 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, grep, and xargs. For example, the following command searches only C files:
```
          find /home/gigi -name '*.c' -print0 | xargs -0r grep -H 'hello'
```
This differs from the command:
```
          grep -H 'hello' *.c
```
which merely looks for ‘hello’ in all files in the current directory whose names end in ‘.c’. The ‘find ...’ command line above is more similar to the command:
```
          grep -rH --include='*.c' 'hello' /home/gigi
```
What if a pattern has a leading ‘-’?
```
          grep -e '--cut here--' *
```
searches for all lines matching ‘--cut here--’. Without -e, grep would attempt to parse ‘--cut here--’ as a list of options.
Suppose I want to search for a whole word, not a part of a word?
```
          grep -w 'hello' *
```
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\>' *
```
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' *
```
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.
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:]]*$’. To match no lines at all, use the command ‘grep -f /dev/null’.
How can I search in both standard input and in files?
Use the special file name ‘-’:
```
          cat /etc/passwd | grep 'alain' - /etc/motd
```
How to express palindromes in a regular expression?
It can be done by using back-references; for example, a palindrome of 4 characters can be written with a BRE:
```
          grep -w -e '$.$$.$.\2\1' file
```
It matches the word “radar” or “civic.”
Guglielmo Bondioni proposed a single RE that finds all palindromes up to 19 characters long using 9 subexpressions and 9 back-references:
```
          grep -E -e '^(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?).?\9\8\7\6\5\4\3\2\1$' file
```
Note this is done by using GNU ERE extensions; it might not be portable to other implementations of grep.
Why is this back-reference failing?
```
          echo 'ba' | grep -E '(a)\1|b\1'
```
This gives no output, because the first alternate ‘(a)\1’ does not match, as there is no ‘aa’ in the input, so the ‘\1’ in the second alternate has nothing to refer back to, meaning it will never match anything. (The second alternate in this example can only match if the first alternate has matched—making the second one superfluous.)
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. However, if your grep is compiled with Perl patterns enabled, the Perl ‘s’ modifier (which makes . match newlines) can be used:
```
          printf 'foo\nbar\n' | grep -P '(?s)foo.*?bar'
```
With the GNU grep option -z (see File and Directory Selection), the input is terminated by null bytes. Thus, you can match newlines in the input, but the output will be the whole file, so this is really only useful to determine if the pattern is present:
```
          printf 'foo\nbar\n' | grep -z -q 'foo[[:space:]]\+bar'
```
Failing either of those options, you need to 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.