*nix tips

How to remove trailing ^M (carriage return) from within VI?

Administrator — Thu, 03 Dec 2009 08:30:04 +0000

:%s/^M//g

:set ff=unix

What is MTU? Where is it set?

Administrator — Sun, 21 May 2006 14:35:12 +0000

MTU: Maximum Transfer Unit. It is calculated in bytes. For example, commonly for ethernet devices the MTU is set to 1500.

MTU is set in the device configuration settings for Ethernet, Token ring etc.

==
In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet that a given layer of a communications protocol can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). The mtu may be required by standards (as is the case with Ethernet) or decided at connect time (as is usually the case with point-point serial links. A higher MTU brings higher bandwidth efficiency. However large packets can block up a slow interface for some time, increasing the lag on other packets. For example a 1500 byte packet, the largest allowed on an Ethernet, will block up a 14.4k modem for about one second.

Source: http://en.wikipedia.org/wiki/Maximum_transmission_unit
==

What is 2 raised to the power of 10 (2^10)?

Administrator — Wed, 17 May 2006 08:47:36 +0000

In computer sciece:

2^10 = 1,024

* the digital approximation of the kilo-, or 1,000 multiplier, which causes a change of prefix. For example: 1,024 bytes = 1 kilobyte (or kibibyte).
* This number has no special significance to computers, but is important to humans because we make use of powers of ten.

Source: http://en.wikipedia.org/wiki/Power_of_two

What is the command format to monitor only the SYN packet with tcpdump?

Administrator — Wed, 17 May 2006 08:06:30 +0000

Here it is:

tcpdump tcp[13] == 2

Here, tcp[13] contains the value of the 13th octet in the TCP header. And, to match only SYN packets, this value must be equal to 2 when interpreted as a 8-bit unsigned integer in network byte order.

Follow-up question: what would be the tcpdump format to see both SYN and ACK packets (but not SYN-ACK)?
Answer:

tcpdump ‘tcp[13] & 2 == 2′

To see only SYN and ACK packet (& not SYN-ACK packets) we will have to logically AND the value for the 13th octet. As it is entioned in the man pages of tcpdump:

In order to achieve our goal, we need to logically AND the binary value of octet 13 with some other value to pre‐
serve the SYN bit. We know that we want SYN to be set in any case, so we’ll logically AND the value in the 13th
octet with the binary value of a SYN:

00010010 SYN-ACK 00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
——– ——–
= 00000010 = 00000010

We see that this AND operation delivers the same result regardless whether ACK or another TCP control bit is set.
The decimal representation of the AND value as well as the result of this operation is 2 (binary 00000010), so we
know that for packets with SYN set the following relation must hold true:

( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )

This points us to the tcpdump filter expression
tcpdump -i xl0 ’tcp[13] & 2 == 2’

How to monitor ICMP packets that are not ping packets with tcpdump?

Administrator — Wed, 17 May 2006 06:14:51 +0000

Here is the command:

tcpdump ‘icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply’

How do you turn the bell (that annoying noise) off when you are in X windows console?

Administrator — Tue, 16 May 2006 08:41:36 +0000

To turn off bell:

xset -b b off

To turn it back on:

xset -b b

How would you find a bash string variables length?

Administrator — Tue, 16 May 2006 05:45:42 +0000

Two ways: using ‘expr’ or ${#variable}. Example:

zia@lappy:~$ string=supercalifragilisticexpialidocious
zia@lappy:~$ echo ${#string}
34
zia@lappy:~$ expr length $string
34
zia@lappy:~$ expr “$string” : ‘.*’
34
zia@lappy:~$ expr match “$string” ‘.*’
34

Source: http://www.tldp.org/LDP/abs/html/string-manipulation.html

What are the special characters of Bash?

Administrator — Mon, 15 May 2006 13:25:15 +0000

#	Comments. (with exception of “#!”).
;	Command separator (semicolon).
;;	Terminator in a ‘case’ option (double semicolon)
.	“dot” command (period). Equivalent to “source;” or “dot” as a componant of a filename or “dot” character match in regular expression.
“	partial quoting (double quote)
‘	full quoting (single quote)
,	comma operator. The comma operator links together a series of arithmetic operations.
\	escape (backslash). A quoting mechanism for single characters.
/	Filename path separator (forward slash).
`	command substitution. The `command` construct makes available the output of command for assignment to a variable.
:	null command (colon). This is the shell equivalent of a “NOP” (no op, a do-nothing operation).
!	reverse (or negate) the sense of a test or exit status (bang).
*	wild card (asterisk) or arithmetic operator (denotes multiplication)
?	test operator. Within certain expressions, the ? indicates a test for a condition or wild card. The ? character serves as a single-character “wild card” for filename expansion.
$	Variable substitution (contents of a variable) or end-of-line. In a regular expression, a “$” addresses the end of a line of text.
${}	Parameter substitution.
$*, $@	positional parameters.
$?	exit status variable. The $? variable holds the exit status of a command, a function, or of the script itself.
$$	process ID variable. The $$ variable holds the process ID of the script in which it appears.
()	command group.
{}	{xxx,yyy,zzz,…}, this is brace expansion & {}, this is block of code (curly brackets).
{} \;	pathname. Mostly used in find constructs. This is not a shell builtin.
[ ]	Test expression between [ ].
[[ ]]	test. Test expression between [[ ]] (shell keyword).
[ ]	array element (n the context of an array) or range of characters (As part of a regular expression)
(( ))	integer expansion. Expand and evaluate integer expression between (( )).
> &> >& >> <	redirection.
<<	redirection used in a here document.
<<<	redirection used in a here string.
<<<	redirection used in a here string.
>, <	ASCII comparison.
\<, \>	word boundary in a regular expression.
\|	pipe. Passes the output of previous command to the input of the next one, or to the shell.
<\|	force redirection (even if the noclobber option is set).
\|\|	OR logical operator.
&	Run job in background.
&&	AND logical operator.
-	option, prefix. Option flag for a command or filter. Prefix for an operator. or redirection from/to stdin or stdout (if postfixed) or previous working directory. A cd - command changes to the previous working directory. or Minus. Minus sign in an arithmetic operation.
=	Equals. Assignment operator.
+	Plus. Addition arithmetic operator. or Option. Option flag for a command or filter.
%	modulo. Modulo (remainder of a division) arithmetic operation.
~	home directory (tilde).
~+	current working directory.
~-	previous working directory.
=~	regular expression match.
^	beginning-of-line. In a regular expression, a “^” addresses the beginning of a line of text.

Source: http://www.tldp.org/LDP/abs/html/special-chars.html

How do you view the routing table?

Administrator — Mon, 15 May 2006 09:21:00 +0000

Mostly in linux:

route -rn

Almost all *nix:

netsat -rn

What is the difference between a single-quote, a quote, and a back-tick in the shell?

Administrator — Mon, 15 May 2006 09:12:48 +0000

Single quote: variables ($), backticks (“) & backslash (\) are not treated specially in single quote.
Example:

zia@lappy:~$ echo ‘$(ls -al t*)’
$(ls -al t*)
zia@lappy:~$ echo ‘`ls -lah t*`’
`ls -lah t*`
zia@lappy:~$ echo ‘`ls -lah t*` \”‘
`ls -lah t*` \”

Synopsis: with single-quote, the special characters (i.e. $, “, \ etc.) are not treated specially, they are treated literally.

Double quote: variables ($), backticks (“) & backslash (\) are treated specially or interpreted other than its literal meaning.
For example:

zia@lappy:~$ echo “$(ls -al t*)”
-rwxr-xr-x 1 zia zia 449 2006-05-03 17:07 t
-rw-r–r– 1 root root 3365 2006-03-24 12:31 target.xml
zia@lappy:~$ echo “`ls -lah t*`”
-rwxr-xr-x 1 zia zia 449 2006-05-03 17:07 t
-rw-r–r– 1 root root 3.3K 2006-03-24 12:31 target.xml
zia@lappy:~$ echo “`ls -lah t*` \”"
-rwxr-xr-x 1 zia zia 449 2006-05-03 17:07 t
-rw-r–r– 1 root root 3.3K 2006-03-24 12:31 target.xml “

Synopsis: with double quote (”"), the special characters do exactly what they are supposed to do.