Detailed explanation of grep command and related examples

1. Match characters

. Match any single character

[ ] Match any character within the specified range

[^] Match any character within the specified range

[:alpha:] Alphabetic characters

[:lower :] Lowercase alphabetic characters

[:upper:] Uppercase alphabetic characters

[:digit:] Numbers

[:alnum:] Alphanumeric characters

[:space:] White space characters (printing is prohibited), such as carriage return characters, line feeds, vertical tabs and form feeds

[:punct:] punctuation characters

[:cntrl:] control characters (printing prohibited)

[:print:] printable characters

when used Generally, two square brackets are used, which will be used in the following examples.

2. Matching times

* Match the previous character any time

.* Match any character of any length (note the greedy mode, such as grep "r.*t" /etc/passwd )

x{m,n } Specifies that the preceding character appears at least m times and at most N times.

x{m,} Specifies that the previous character appears at least m times

x{0,n} Specifies that the previous character appears at most N times

x{m} Exactly matches m times

? Matches the preceding character 0 Or 1 time

Three. Anchor character

1.^ Anchor at the beginning of the line grep "^r..t" /etc/passwd

2.$ Anchor at the end of the line grep "h$" /etc/passwd

3.^$ Anchor blank line grep "^$" /etc/passwd

4.< (b) Anchor word beginning grep "

5.> (b) Anchor word beginning grep "r..t>" /etc/passwd

Example (easy to confuse):

contains at least one whitespace character grep "[[:space:]]{1,}" /etc /passwd

contains at least one non-whitespace character grep "[^[:space:]]{1,}" /etc/passwd

does not have a whitespace character grep -v "[^[:space:]]{1, }" /etc/passwd

6.() Group characters grep "(l..e).*1r"

Example:

grep --color "l([13]):1:.*: 1" /etc/inittab

Four. Options

-v Invert the result

-i Ignore the case of letters

-o Only display the matched string (other contents of the line are not displayed)

-E Support extended regular expressions

-A n Display n lines below the matched line

-B n Display n lines above the matched line

-C n Display n lines above and below the matched line

Exercise:

1. Find relevant information about user1 in the system. (Create user11, myuser1 in advance) (Error-prone)

grep "user1" /etc/passwd All lines containing user1

grep "" /etc/passwd It seems OK, but adding the following users will not OK

useradd -c "user1's uncle" /etc/passwd -c is a comment

grep "^" /etc/passwd successfully matched

2. Find the system that starts with user followed by a number User related information.

grep "^user[0-9]{1,}>" /etc/passwd

3. Analyze the characteristics of the following two lines of text in the /etc/inittab file, and write a pattern similar to the two lines that can be accurately found ,

requires that the numbers in each row must be the same.

l1:1:wait:/etc/rc.d/rc 1

l3:3:wait:/etc/rc.d/rc 3

grep "l([13]):1:.*: .* 1" inittab

Extension: Match all the above characteristics: grep "l([0-9]):1:.*:.* 1" inittab

If it exceeds 10, you need to add a minimum match: grep "l( [0-9]{1,}):1:.*:.* 1" inittab

4. Display the lines starting with case-insensitive s in the /proc/meminfo file

grep "^[sS] " /proc/meminfo

5. Display the lines ending with nologin in /etc/passwd

grep "nologin$" /etc/passwd

6. Display the lines starting with # in /etc/inittab and followed by one or Lines with multiple whitespace characters, followed by any non-whitespace characters

grep "^#[[:space:]]{1,}[^[:space:]]" /etc/inittab

7. Display /etc/inittab contains lines with a number between two colons

grep ":[0-9]:" /etc/inittab

8. Display the /boot/grub/grub.conf file with one or more lines with blank characters

grep ":[0-9]:" /etc/inittab

9. Displays lines in the /etc/inittab file that start with a number and end with a number that has the same starting number.

grep "^([0-9]).*1$" /etc/inittab

10. Display non-blank lines in the /etc/inittab file

grep -v "^$" /etc/inittab

11. Get the relevant IP address of the current network interface (excluding 127.0.0.1)

ifconfig |grep "inet addr" |grep -v "127.0.0.1"| cut -d: -f2|cut -d" " - f1

ifconfig |grep -A 1 "eth" |grep -o "addr:[0-9.]{1,}"|cut -d: -f2

5. Extended regular expressions

are different from regular expressions Where:

() is replaced with ()

{} is replaced with {}

+ Number of matches, match the character before it one or more times

| or

Example:

appears in the ifconfig result The number is an integer between 1-255

ifconfig|grep --color -E "<([1-9]|[1-9][0-9]|1[1-9][0-9 ]|2[0-5][0-5])>"