(grep.info.gz) Usage
Info Catalog
(grep.info.gz) Regular Expressions
(grep.info.gz) Top
(grep.info.gz) Reporting Bugs
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. Regular
Expressions. The `-i' option causes `grep' to ignore case, causing
it to match the line `Hello, world!', which it would not otherwise
match. 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 `/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
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. 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'.
12. How can I search in both standard input and in files?
Use the special file name `-':
cat /etc/passwd | grep 'alain' - /etc/motd
13. 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'.
14. 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.)
15. 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' ( File and Directory
Selection), the input is terminated by null bytes. 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.
16. 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'.
Info Catalog
(grep.info.gz) Regular Expressions
(grep.info.gz) Top
(grep.info.gz) Reporting Bugs
automatically generated by
info2html