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.
Here is a more complex example, showing the location and contents of
any line containing ‘f’ and ending in ‘.c’, within all files in the
current directory whose names start with non-‘.’, contain ‘g’, and end
in ‘.h’. The ‘-n’ option outputs line numbers, the ‘--’ argument treats
any later arguments as file names not options even if ‘*g*.h’ expands to
a file name that starts with ‘-’, and the empty file ‘/dev/null’ causes
file names to be output even if only one file name happens to be of the
form ‘*g*.h’.
grep -n -- 'f.*\.c$' *g*.h /dev/null
Note that the regular expression syntax used in the pattern differs from
the globbing syntax that the shell uses to match file names.
⇒Invoking, for more details about how to invoke ‘grep’.
Here are some common questions and answers about ‘grep’ usage.
1. How can I list just the names of matching files?
grep -l 'main' test-*.c
lists names of ‘test-*.c’ files in the current directory whose
contents mention ‘main’.
2. How do I search directories recursively?
grep -r 'hello' /home/gigi
searches for ‘hello’ in all files under the ‘/home/gigi’ directory.
For more control over which files are searched, use ‘find’ and
‘grep’. For example, the following command searches only C files:
find /home/gigi -name '*.c' ! -type d \
-exec grep -H 'hello' '{}' +
This differs from the command:
grep -H 'hello' /home/gigi/*.c
which merely looks for ‘hello’ in non-hidden C files in
‘/home/gigi’ whose names end in ‘.c’. The ‘find’ command line
above is more similar to the command:
grep -r --include='*.c' 'hello' /home/gigi
3. What if a pattern or file has a leading ‘-’? For example:
grep "$pattern" *
can behave unexpectedly if the value of ‘pattern’ begins with ‘-’,
or if the ‘*’ expands to a file name with leading ‘-’. To avoid
the problem, you can use ‘-e’ for patterns and leading ‘./’ for
files:
grep -e "$pattern" ./*
searches for all lines matching the pattern in all the working
directory's files whose names do not begin with ‘.’. Without the
‘-e’, ‘grep’ might treat the pattern as an option if it begins with
‘-’. Without the ‘./’, there might be similar problems with file
names beginning with ‘-’.
Alternatively, you can use ‘--’ before the pattern and file names:
grep -- "$pattern" *
This also fixes the problem, except that if there is a file named
‘-’, ‘grep’ misinterprets the ‘-’ as standard input.
4. Suppose I want to search for a whole word, not a part of a word?
grep -w 'hello' test*.log
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\>' test*.log
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' test*.log
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:]]*$’. To match no lines at all, use an
extended regular expression like ‘a^’ or ‘$a’. To match every
line, a portable script should use a pattern like ‘^’ instead of
the empty pattern, as POSIX does not specify the behavior of the
empty pattern.
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. Why can't I combine the shell's ‘set -e’ with ‘grep’?
The ‘grep’ command follows the convention of programs like ‘cmp’
and ‘diff’ where an exit status of 1 is not an error. The shell
command ‘set -e’ causes the shell to exit if any subcommand exits
with nonzero status, and this will cause the shell to exit merely
because ‘grep’ selected no lines, which is ordinarily not what you
want.
There is a related problem with Bash's ‘set -e -o pipefail’. Since
‘grep’ does not always read all its input, a command outputting to
a pipe read by ‘grep’ can fail when ‘grep’ exits before reading all
its input, and the command's failure can cause Bash to exit.
14. Why is this back-reference failing?
echo 'ba' | grep -E '(a)\1|b\1'
This outputs an error message, because the second ‘\1’ has nothing
to refer back to, meaning it will never match anything.
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’, ‘-E’, and ‘-F’ 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
The ‘-E’ option stands for Extended ‘grep’. The ‘-F’ option stands
for Fixed ‘grep’;
17. What happened to ‘egrep’ and ‘fgrep’?
7th Edition Unix had commands ‘egrep’ and ‘fgrep’ that were the
counterparts of the modern ‘grep -E’ and ‘grep -F’. Although
breaking up ‘grep’ into three programs was perhaps useful on the
small computers of the 1970s, ‘egrep’ and ‘fgrep’ were deemed
obsolescent by POSIX in 1992, removed from POSIX in 2001,
deprecated by GNU Grep 2.5.3 in 2007, and changed to issue
obsolescence warnings by GNU Grep 3.8 in 2022; eventually, they are
planned to be removed entirely.
If you prefer the old names, you can use your own substitutes, such
as a shell script named ‘egrep’ with the following contents:
#!/bin/sh
exec grep -E "$@"
Info Catalog
grep: Regular Expressions
grep: Top
grep: Performance