grep: Usage
4 Usage
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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’ (‘--null-data’), each input and
output “line” is null-terminated; 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.
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: Regular Expressions
grep: Top
grep: Performance