The usage of Linux regular grep/egrep

* the main function of grep is to retrieve content based on keywords. Egrep is an extension of grep. Egrep includes all the functions of grep.

Grep usage: grep 'keyword' search object

Common parameters:

-c retrieve the number of lines containing keywords

-I is case-insensitive (keyword)

-n display line number

-v inverted (displays lines that do not contain keywords)

-r traverse all subdirectories (used to retrieve directories)

-An retrieves the lines containing keywords and the following n lines

-Bn retrieves the lines containing keywords and the n lines above

-Cn retrieves lines containing keywords as well as upper and lower n lines

-E equals egrep

Examples of special usage:

[root@localhost ~] # grep'[0-9]'/ etc/passwd

* retrieve rows containing numbers

[root@localhost ~] # grep-v'[0-9]'/ etc/passwd

* retrieve rows that do not contain numbers

[root@localhost ~] # grep'^ [0-9]'/ etc/passwd

* retrieve lines that start with a number

[root@localhost ~] # grep'[^ 0-9]'/ etc/passwd

* retrieve rows that do not contain numbers

[root@localhost ~] # grep'^ [^ 0-9]'/ etc/passwd

* retrieve lines that do not start with a number

[root@localhost ~] # grep 'r.t' / etc/passwd

*。 Means to match any character (number, letter, symbol, space)

[root@localhost ~] # grep 'oaks' / etc/passwd

* means to match 0 or more o (the character before the * sign)

[root@localhost ~] # grep'. *'/ etc/passwd

* means to match all characters (. Represents any character, and * represents 0 or more preceding characters)

[root@localhost ~] # grep'o\ {2\}'/ etc/passwd

* o {2} indicates that the row that appears twice in search o is equal to grep 'oo' / etc/passwd,\ escape character

[root@localhost ~] # egrep'o {2}'/ etc/passwd

* same as above, you don't need to add an escape character to use egrep.

[root@localhost ~] # egrep 'oaks' / etc/passwd

The * + sign indicates that it matches one or more characters preceding the + sign (o, oo, oo... Oo)

[root@localhost ~] # egrep 'oaks' / etc/passwd

*? Represents the character before 0 or 1 question mark

[root@localhost ~] # egrep 'root | test' / etc/passwd

* conduit character: user retrieves multiple keywords