Dev {Tricks}

April 15, 2021 by dev

Restart an Apache Web Server

Using Init.d

/etc/init.d/apache2 restart

sudo /etc/init.d/apache2 restart

sudo service apache2 restart

For a graceful restart, replace restart with reload.

For CentOS and RHEL servers

service httpd restart

Using Systemctl

systemctl restart apache2.service

Using Apachectl

sudo apachectl -k restart

sudo apachectl -k graceful

April 15, 2021 by dev

How to change hostname on CentOS 8

Display the current CentOS 8 hostname

hostname

hostnamectl

CentOS 8 change hostname at

sudo vi /etc/hostname

Next Edit the /etc/hosts file

sudo vi /etc/hosts

Reboot system

sudo reboot

April 8, 2021 by dev

How to change the email sending limit in DirectAdmin?

SSH method

echo 500 > /etc/virtual/bdhostit.com/limit/helpdesk1

The above command will increase daily email sending limit to 500 of helpdesk1@bdhostit.com.
Change the bold text replacing your details.

April 7, 2021 by dev

Regular Expressions (regex) Cheat Sheet

Anchors

^	Start of string, or start of line in multi-line pattern
\A	Start of string
$	End of string, or end of line in multi-line pattern
\Z	End of string
\b	Word boundary
\B	Not word boundary
\<	Start of word
\>	End of word

Character Classes

\c	Control character
\s	White space
\S	Not white space
\d	Digit
\D	Not digit
\w	Word
\W	Not word
\x	Hexadecimal digit
\O	Octal digit

POSIX

[:upper:]	Upper case letters
[:lower:]	Lower case letters
[:alpha:]	All letters
[:alnum:]	Digits and letters
[:digit:]	Digits
[:xdigit:]	Hexadecimal digits
[:punct:]	Punctuation
[:blank:]	Space and tab
[:space:]	Blank characters
[:cntrl:]	Control characters
[:graph:]	Printed characters
[:print:]	Printed characters and spaces
[:word:]	Digits, letters and underscore

Assertions

?=	Lookahead assertion
?!	Negative lookahead
?<=	Lookbehind assertion
?!= or ?<!	Negative lookbehind
?>	Once-only Subexpression
?()	Condition [if then]
?()\|	Condition [if then else]
?#	Comment

Quantifiers

*	0 or more	{3}	Exactly 3
+	1 or more	{3,}	3 or more
?	0 or 1	{3,5}	3, 4 or 5

Add a ? to a quantifier to make it ungreedy.

Escape Sequences

\	Escape following character
\Q	Begin literal sequence
\E	End literal sequence

“Escaping” is a way of treating characters which have a special meaning in regular expressions literally, rather than as special characters.

Common Metacharacters

^	[	.	$
{	*	(	\
+	)	\|	?
<	>

The escape character is usually \

Special Characters

\n	New line
\r	Carriage return
\t	Tab
\v	Vertical tab
\f	Form feed
\xxx	Octal character xxx
\xhh	Hex character hh

Groups and Ranges

.	Any character except new line (\n)
(a\|b)	a or b
(…)	Group
(?:…)	Passive (non-capturing) group
[abc]	Range (a or b or c)
[^abc]	Not (a or b or c)
[a-q]	Lower case letter from a to q
[A-Q]	Upper case letter from A to Q
[0-7]	Digit from 0 to 7
\x	Group/subpattern number “x”

Ranges are inclusive.

Pattern Modifiers

g	Global match
i *	Case-insensitive
m *	Multiple lines
s *	Treat string as single line
x *	Allow comments and whitespace in pattern
e *	Evaluate replacement
U *	Ungreedy pattern

* PCRE modifier

String Replacement

$n	nth non-passive group
$2	“xyz” in /^(abc(xyz))$/
$1	“xyz” in /^(?:abc)(xyz)$/
$`	Before matched string
$’	After matched string
$+	Last matched string
$&	Entire matched string

Some regex implementations use \ instead of $.