Usage follow:
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.
The ‘-i’ option causes grep to ignore
case,
causing it to match the line ‘Hello, world!’,
which it would not otherwise match.
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
‘/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 -rH ’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.
The ‘find ...’ command line above is more
similar to the command:
grep -rH --include=’*.c’ ’hello’
/home/gigi
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:x:2098:1000:Eli
Smith:/home/eli:/bin/bash
Alternatively, use ‘-H’, which is a gnu
extension:
grep -H ’eli’ /etc/passwd
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
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.
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 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.
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 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.
13. 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.)
14. 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.
