linux grep command

1. Function
The grep command in Linux system is a powerful text search tool. It can use regular expressions to search text and print out the matching lines. The full name of grep is Global Regular Expression Print, which means the global regular expression version. Its usage permissions are for all users.

2. Format
grep [options]

3. Main parameters
[options] Main parameters:
-c: Only output the count of matching lines.
-I: Insensitive to uppercase and lowercase (only applicable to single characters).
-h: Do not display file names when querying multiple files.
-l: When querying multiple files, only the file names containing matching characters will be output.
-n: Display matching lines and line numbers.
-s: Do not display error messages that do not exist or have no matching text.
-v: Display all lines that do not contain matching text.
pattern regular expression main parameters:
: Ignore the original meaning of special characters in the regular expression.
^: Matches the starting line of the regular expression.
$: Matches the end line of the regular expression.
<: Start from the line matching the regular expression.
>: Go to the end of the line matching the regular expression.
[ ]: A single character, such as [A], that is, A meets the requirements.
[-]: Range, such as [A-Z], that is, A, B, C to Z all meet the requirements.
. : All single characters.
*: There are characters, and the length can be 0.

4. Simple example of using grep command
$ grep ‘test’ d*
Display all lines containing test in files starting with d.
$ grep ‘test’ aa bb cc
Displays the lines matching test in the aa, bb, cc files.
$ grep ‘[a-z]{5}’ aa
Displays all lines containing strings each of which has at least 5 consecutive lowercase characters.
$ grep 'w(es)t.*1′ aa
If west is matched, es is stored in memory and marked as 1, and then searches for any number of characters (.*) followed by Another es(1), if found, displays the line. If you use egrep or grep -E, there is no need to escape with the "" sign, just write it directly as 'w(es)t.*1'.

5.grep command usage complex example
Suppose you are searching for a file with the string 'magic' in the '/usr/src/Linux/Doc' directory:
$ grep magic /usr/src/Linux/Doc/*
sysrq.txt:* How do I enable the magic SysRQ key?
sysrq.txt:* How do I use the magic SysRQ key?
The file 'sysrp.txt' contains this string and discusses the function of SysRQ.
By default, 'grep' only searches the current directory. If there are many subdirectories under this directory, 'grep' will list it like this:
grep: sound: Is a directory
This may make the output of 'grep' difficult to read. There are two solutions here:
Explicitly ask to search the subdirectory: grep -r
or ignore the subdirectory: grep -d skip
If there is a lot of output, you can pipe it to 'less' to read:
$ grep magic /usr/src/Linux/Documentation/* | less
This way, you can read more conveniently.

One thing to note is that you must provide a file filtering method (use * to search all files). If you forget, 'grep' will wait until the program is interrupted. If you encounter this, press and try again.

There are some interesting command line parameters below:
grep -i pattern files: Search case-insensitively. The default is case-sensitive,
grep -l pattern files: only match file names are listed,
grep -L pattern files: list unmatched file names,
grep -w pattern files: only match whole words, not Part of the string (such as matching 'magic', not 'magical'),
grep -C number pattern files: matching context displays [number] lines respectively,
grep pattern1 | pattern2 files: displays lines matching pattern1 or pattern2 ,
grep pattern1 files | grep pattern2: Display lines that match both pattern1 and pattern2.

grep -n pattern files to display line number information

grep -c pattern files to find the total number of lines

There are also some special symbols for searching:
< and > mark the beginning and end of words respectively .
For example:
grep man * will match 'Batman', 'manic', 'man', etc.,
grep 'grep '< man>' only matches 'man', not other strings such as 'Batman' or 'manic'.
‘^’: means that the matched string is at the beginning of the line,
‘$’: means that the matched string is at the end of the line,

Grep command usage list

1. Parameters:
-I: ignore case
-c: print the number of matching lines
-l: find matching items from multiple files
-v: find lines that do not contain matching items
-n: print Lines and line identifiers containing matching items

2, RE (regular expression)
Ignore the original meaning of special characters in regular expressions
^ Match the beginning line of the regular expression
$ Match the end line of the regular expression
< ; Starting from the line that matches the regular expression
> To the end of the line that matches the regular expression
[ ] single character; such as [A], that is, A meets the requirements
[ - ] range; such as [A-Z], that is, A, B, C All the requirements are met until Z
. All single characters
* All characters, the length can be 0

3, for example
# ps -ef | grep in.telnetd
root 19955 181 0 13:43:53 ? 0:00 in.telnetd

# more size.txt size file content
b124230
b034325
a081016
m7187998
m7282064
a022021
a061048
m9324 822
b103303
a013386
b044525
m8987131
B081016
M45678
B103303
BADc2345

# more size.txt | grep '[a-b]' range; such as [A-Z], that is, A, B, C to Z all meet the requirements
b124230
b034325
a081016
a022021
a061048
b103303
a013386
b044525
# more size.txt | grep '[a-b]'* 3
a013386
b044525
m8987131
B081016
M45678
B103303
BADc2345

# more size.txt | grep ' b' single character; such as [A], that is, A meets the requirements
b124230
b034325
b103303
b044525
# more size.txt | grep '[bB]'
b124230
b034325
b103303
b044525
B081016
B103303
BADc2345

# grep 'root' /etc/group
root::0:root
bin::2:root,bin,daemon
sys::3:root,bin,sys,adm
adm::4:root,adm, daemon
uucp::5:root,uucp
mail::6:root
tty::7:root,tty,adm
lp::8:root,lp,adm
nuucp::9:root,nuucp
daemon ::12:root,daemon

# grep '^root' /etc/group matches the starting line of the regular expression
root::0:root

# grep 'uucp' /etc/group
uucp::5: root,uucp
nuucp::9:root,nuucp

# grep 'uucp::5:root,uucp

# grep 'root$' /etc/group matches regular expression The ending line of
root::0:root
mail::6:root

# more size.txt | grep -i 'b1..*3' -i : ignore case

b124230
b103303
B103303

# more size.txt | grep -iv 'b1... 2
a013386
b044525
m8987131
B081016
M45678
BADc2345

# more size.txt | grep -in 'b1..*3'
1:b124230
9:b103303
15:B103303

# grep '$' /etc/init.d/nfs.server | wc -l
128
# grep '$' /etc/init.d/nfs.server | wc -l ignores the original meaning of special characters in regular expressions

15
# grep '$' /etc/init .d/nfs.server
case "$1" in
>/tmp/sharetab.$$
[ "x$fstype" != xnfs ] &&
echo "$patht$rest$fstypet$optst$desc"
> ;>/tmp/sharetab.$$
/usr/bin/touch -r /etc/dfs/sharetab /tmp/sharetab.$$
/usr/bin/mv -f /tmp/sharetab.$$ /etc /dfs/sharetab
if [ -f /etc/dfs/dfstab ] && /usr/bin/egrep -v '^[ ]*(#|$)'
if [ $startnfsd -eq 0 -a -f /etc /rmmount.conf ] &&
if [ $startnfsd -ne 0 ]; then
elif [ ! -n "$_INIT_RUN_LEVEL" ]; then
while [ $wtime -gt 0 ]; do
wtime=`expr $wtime - 1 `
if [ $wtime -eq 0 ]; then
echo "Usage: $0 { start | stop }"

# more size.txt

the test file
their are files
The end

# grep 'the' size.txt
the test file
their are files

# grep 'the test file
their are files

# grep 'the>' size.txt
the test file

# grep '' size.txt
the test file

# grep '<[Tt]he>' size.txt
the test file

============== ================================================== ===

1, Introduction
A multi-purpose text search tool using regular expressions. This php?name=%C3%FC%C1%EE" onclick="tagshow(event)" class="t_tag"> The command was originally a php?name=%C3%FC%C1%EE" onclick="tagshow(event)" class="t_tag">Command/Filter:
g/re/p - - global - regular expression - print.
Basic format
grep pattern [file...]
(1)grep search string [filename]
(2)grep regular expression [filename]
Search for all pattern occurrences in the file position, pattern can be either the string to be searched or a regular expression.
Note: It is best to use double quotes/ when entering the string to be searched, and when using regular expressions for pattern matching, pay attention to using Single quotes

2, grep options
-c only outputs the count of matching lines
-i is case-insensitive (for single characters)
-n displays matching line numbers
-v does not display lines that do not contain matching text There are lines
-s does not display error messages
-E uses extended regular expressions
For more options, please see: man grep

3, commonly used grep examples

(1) Multiple file query
grep "sort" *. doc #See file name matching

(2) Line matching: Output the count of matching lines
grep -c "48" data.doc #Output the number of lines containing 48 characters in the document

(3) Display matching lines and lines Number
grep -n "48" data.doc #Display all lines and line numbers that match 48

(4) Display non-matching lines
grep -vn "48" data.doc #Output all lines that do not contain 48

(4) Display non-matching lines
grep -vn "48" data.doc #Output all lines that do not contain 48

(5) Case sensitive
grep -i "ab" data.doc #Output all lines containing ab Or the line of the string of Ab

4, application of regular expressions

(1) Application of regular expressions (note: it is best to enclose regular expressions in single quotes)
grep '[239].' data .doc      #Output all lines that start with 2, 3 or 9 and are two numbers

(2) mismatch test
  grep '^[^48]' data.doc  # Unmatched lines starting with 48 Line

(3) uses extended pattern matching
grep -E '219|216' data.doc

(4) ...
This requires continuous application and summary in practice, and proficiency in regular expressions.
5 9]
[[:alnum:]] [0-9a-zA-Z]
[[:space:]] Space or tab
[[:alpha:]] [a-zA-Z]

(1) Use
grep '5[[:upper:]][[:upper:]]' data.doc #Query the lines starting with 5 and ending with two capital letters