Text processing tool: grep egrep fgrep

1. grep: global search Regular Expression and print out the line.

The text search tool performs a row-by-row search on the target file based on the user-specified text mode to display the rows that can be matched by the mode.

Format: grep [Option] 'pattern' file1,... where pattern is a regular expression.

-- Color = Auto: The matched rows are highlighted.

-V: reverse match. lines that cannot be matched by the pattern are displayed.

-O: only the matched content is displayed, not the whole line.

-I (ignore-case): case insensitive

-A n: displays the matching rows and the following n rows.

-B N: displays the matching rows and the following n rows.

-C: display the context of the matching row

Its regular expression:

1. Character matching:

.: Any single character

[]: Any single character in the specified range

[0-9]:

[A-Z]:

[A-Z]:

[^]: Any single character out of the specified range

2. Number of times matching: used to specify the number of times of characters before the matching

*: Any number of times

. *: Match any character of any length

\? : 0 or 1 time

\ {M \}: matching m times

\ {M, N \}: Match m to n times

\ {M, \}: match at least m times

\ {M, N \}: match up to n times

3. Position anchoring: used to specify the position where a character appears

^: Pin the beginning of a line

$: End of the anchor row

^ $: Blank line

Example:

Count the rows ending with Bash in/etc/rc. d/rc. sysinit.

Grep -- color = auto'bash $ '/etc/rc. d/rc. sysinit

Count the number of empty rows in/etc/rc. d/rc. sysinit.

Grep -- color = auto' ^ $ '/etc/rc. d/rc. sysinit | WC-l

\ <CHAR: the beginning of the anchor.

Example: rows starting with R in/etc/passwd

Grep -- color = auto' \ <[RR] [^ [: punct:] * [TT] '/etc/passwd

Char \>: Specifies the end of a word.

Example: \ <H. O \> matches a word that starts with H and ends with O and contains any two characters

4. \ (\): Group

Example: ^ \ (AB \) * XY $: matches a string that starts with AB and ends with XY. The middle part contains the task AB.

5. Reference:

\ 1: backward reference, referencing the first left brace in front and the content of the pattern in the corresponding right Brace

\ 2:

......

For example, \ (A. B \) xy \ 1, which can match abxyab a6bxya6b and a6bxya7b.

For example, display the rows starting with case s in the/proc/meminfo File

# Grep "^ [ss]"/proc/meminfo

# Grep-I "^ s"/proc/meminfo

For example, if the default shell is not bash

# Grep-V "bash $"/etc/passwd | cut-D:-F1

For example, retrieve the user whose default shell is Bash and whose ID is the largest

# Grep "bash $"/etc/passwd | sort-n-t:-K3 | tail-1 | cut-D:-F1

For example, the/etc/rc. d/rc. sysinit file starts with #, followed by at least one blank character, and then contains at least one non-blank line.

# Grep "^ # [[: Space:] \ {1, \} [^ [: Space:] \ {1, \}"/etc/rc. d/RC. sysinit

For example, find one or two digits in the/etc/passwd file.

# Grep -- color = auto "\ <[0-9] \ {1, 2 \} \>"/etc/passwd

Example: Find the line ending with "listen" in the result of the netstat-tan command execution.

Netstat-tan | grep -- color "Listen [[: Space:] * $"

Example: Find the line ending with "listen" or "established" in the result of the netstat-tan command execution.

Ii. egrep: Build mode using extended regular expression class: equivalent to grep-e

Its regular expression:

1. Character matching:

.: Any single character

[]: Any single character in the specified range

[0-9]:

[A-Z]:

[A-Z]:

[^]: Any single character out of the specified range

2. Number of times matching: used to specify the number of times of characters before the matching

*: Any number of times

. *: Match any character of any length

+: Match the first character at least once.

? : 0 or 1 time

{M}: matching m times

{M, n}: Match m to n times

{M ,}: match at least m times

{M, n}: a maximum of N matching times

3. Position anchoring: used to specify the position where a character appears

^: Pin the beginning of a line

$: End of the anchor row

^ $: Blank line

Example:

Count the rows ending with Bash in/etc/rc. d/rc. sysinit.

Grep -- color = auto'bash $ '/etc/rc. d/rc. sysinit

Count the number of empty rows in/etc/rc. d/rc. sysinit.

Grep -- color = auto' ^ $ '/etc/rc. d/rc. sysinit | WC-l

\ <CHAR: the beginning of the anchor.

Example: rows starting with R in/etc/passwd

Grep -- color = auto' \ <[RR] [^ [: punct:] * [TT] '/etc/passwd

Char \>: Specifies the end of a word.

Example: \ <H. O \> matches a word that starts with H and ends with O and contains any two characters

4. (): Group

Example: ^ \ (AB \) * XY $: matches a string that starts with AB and ends with XY. The middle part contains the task AB.

|: Or

5. Reference:

\ 1: backward reference, referencing the first left brace in front and the content of the pattern in the corresponding right Brace

\ 2:

......

For example, \ (A. B \) xy \ 1, which can match abxyab a6bxya6b and a6bxya7b.

Example: Find the line ending with "listen" or "established" in the result of the netstat-tan command execution.

Netstat-tan | egrep -- color "(Listen | established) [[: Space:] * $"

For example, locate the integer from 1 to 255 in the result of executing the ifconfig command.

# Ifconfig | grep-e -- color = auto "\ <([1-9] | [1-9] [0-9] | 1 [0-9] {2} | 2 [0-4] [0-9] | 25 [0-5]) |>"

Iii. fgrep: where F is fast and regular expressions are not parsed.

Text processing tool: grep egrep fgrep