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UsingGrepUsing GREP $ grep root /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin
$ grep -n root /etc/passwd
1:root:x:0:0:root:/root:/bin/bash
12:operator:x:11:0:operator:/root:/sbin/nologin
-n, --line-number
Prefix each line of output with the line number within its input
file.
$ grep -v bash /etc/passwd | grep -v nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
news:x:9:13:news:/etc/news
v, --invert-match
Invert the sense of matching, to select non-matching lines.
$ grep -c bash /etc/passwd
2
$ grep -c nologin /etc/passwd
29
-c, --count
Suppress normal output; instead print a count of matching lines
for each input file. With the -v, --invert-match option (see
below), count non-matching lines.
$ grep -i ps ~/.bash*
Binary file /home/trinity/.bashbashscript.swp matches
/home/trinity/.bash_history:pstree -p
/home/trinity/.bash_history:pstree
$ grep -i ps ~/.bash* | grep -v history
Binary file /home/shibu/.bashbashscript.swp matches
-i, --ignore-case
Ignore case distinctions in both the PATTERN and the input
files.
We now exclusively want to display lines starting with the string "root":
$ grep -rwl 'ar' /home/trinity/
/home/trinity/.gnome-desktop/starthere.desktop
/home/trinity/test
-R, -r, --recursive
Read all files under each directory, recursively; this is equiv-
alent to the -d recurse option.
-w, --word-regexp
Select only those lines containing matches that form whole
words.
-x, --line-regexp
Select only those matches that exactly match the whole line.
-l, --files-with-matches
Suppress normal output; instead print the name of each input
file from which output would normally have been printed. The
scanning will stop on the first match.
$ grep ^root /etc/passwd
root:x:0:0:root:/root:/bin/bash
The caret ^ and the dollar sign $ are metacharacters that respectively
match the empty string at the beginning and end of a line. The symbols
\< and \> respectively match the empty string at the beginning and end
of a word. The symbol \b matches the empty string at the edge of a
word, and \B matches the empty string provided it’s not at the edge of
a word.
$ grep :$ /etc/passwd
news:x:9:13:news:/etc/news:
$ grep -w / /etc/fstab
LABEL=/ / ext3 defaults 1 1
$ grep / /etc/fstab
LABEL=/ / ext3 defaults 1 1
LABEL=/boot /boot ext3 defaults 1 2
none /dev/pts devpts gid=5,mode=620 0 0
none /proc proc defaults 0 0
none /dev/shm tmpfs defaults 0 0
/dev/hda4 swap swap defaults 0 0
/dev/fd0 /mnt/floppy auto noauto,owner,kudzu 0 0
/dev/cdrom /mnt/cdrom udf,iso9660 noauto,owner,kudzu,ro 0 0
Here is an example shell 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. *Note Regular Expressions::. The `-i' option causes `grep' to ignore case, causing it to match the line `Hello, world!', which it would not otherwise match. *Note Invoking::, for more details about how to invoke `grep'. Here are some common questions and answers about `grep' usage.
1. 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'.
2. How do I search directories recursively?
grep -r 'hello' /home/gigi
searches for `hello' in all files under the directory
`/home/gigi'. For more control of which files are searched, use
`find', `grep' and `xargs'. For example, the following command
searches only C files:
find /home/gigi -name '*.c' -print | xargs grep 'hello' /dev/null
This differs from the command:
grep -r 'hello' *.c
lists the names of all C files in the current directory whose contents mention `main'.
2. How do I search directories recursively?
grep -r 'hello' /home/gigi
searches for `hello' in all files under the directory
`/home/gigi'. For more control of which files are searched, use
`find', `grep' and `xargs'. For example, the following command
searches only C files:
find /home/gigi -name '*.c' -print | xargs grep 'hello' /dev/null
This differs from the command:
grep -r 'hello' *.c
which merely looks for `hello' in all files in the current
directory whose names end in `.c'. Here the `-r' is probably
unnecessary, as recursion occurs only in the unlikely event that
one of `.c' files is a directory.
3. 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.
4. 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'.
5. How do I output context around the matching lines?
grep -C 2 'hello' *
prints two lines of context around each matching line.
6. 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:DNGUTF58.IMe.:98:11:Eli Smith:/home/do/eli:/bin/bash
7. 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 some platforms
`ps' limit the ouput to the width of the screen, grep does not
have any limit on the length of a line except the available memory.
8. 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.
9. Why doesn't `grep -lv' print nonmatching 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.
10. 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'.
11. How can I search in both standard input and in files?
Use the special file name `-':
cat /etc/passwd | grep 'alain' - /etc/motd 12. How to express palindromes in a regular expression?
It can be done by using the back referecences, for example a
palindrome of 4 chararcters can be written in BRE.
grep -w -e '\(.\)\(.\).\2\1' file
It matches the word "radar" or "civic".
Guglielmo Bondioni proposed a single RE that finds all the
palindromes up to 19 characters long.
egrep -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 on other greps.
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