Here is the command:

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

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

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

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.

use named pipes in bash, i.e.:
# diff <(process one) <(process two)

“$*” : All the positional parameters (as a single word) [Must be quoted, otherwise it defaults to “$@”]
“$@” : All the positional parameters (as separate strings)

The $@ and $* parameters differ only when between double quotes.

Source: http://www.tldp.org/LDP/abs/html/internalvariables.html#APPREF

In the $? variable.

For example:

zia@lappy:~$ find . -name something-that-cant-be-found
zia@lappy:~$ echo $?
1

That would be:

root@lappy:~# !!

Works in every OS if you are in bash.

Lets say we have a file where the items in it are space separated. And we have around 5000 items in it. A simple sample looks like this:

item1 item2 item3 item4 item5 item6 item7 item8 item9 item10 item11 item12 item13 item14 item15 item16 item17 item18 item19 item20 item21 item22 item23 item24 item25 item26 item27 item28 item29 item30

Now, what we want to achieve is show only 10 items in a line or show 10 columns at a time. Unix “findutils” has a very nifty utility called “xargs” to achieve this in a blink.

Here it goes:

cat /path/to/infile | xargs -n10 echo

Yes, its that easy.

The output will look something like:

item1 item2 item3 item4 item5 item6 item7 item8 item9 item10
item11 item12 item13 item14 item15 item16 item17 item18 item19 item20
item21 item22 item23 item24 item25 item26 item27 item28 item29 item30

If you have some items/fields in a file in this format:

item1
item2
item3
item4

and you want to convert it to

item1 item2 item3 item4

then you can use

tr '\n' ' ' </path/to/infile >/path/to/outfile

Example:

awk -F: '{print $1'} /etc/passwd | tr '\n' ' '

Have to do some quick math or calculate something complicated and don’t have the calculator handy? Try this:

echo '(1 + sqrt(5))/2' | bc -l

or

echo '(65536 / 1024)' | bc -l

or

echo '(2 ^ 16)' | bc -l

Voila!

Ref: http://www.iol.ie/~padraiga/cmdline.html

‘ps’ is one of the most important commands in the *nix world. But, what if someone wants to find out the processes running in a system without using the command ‘ps’. Why? There could be many reasons, one of them being “what if one wants to find out hidden processes in the system supposing the system was compromised…”.

So here it is:

cd /proc
for n in [0-9]* ; do echo -n "pid: "$n" "; cat $n/cmdline; echo; done

Ref: found in a post of Robert Mognet.

It is possible to redirect the output of a list of commands to a single file with a dash of style.

Here it is:

{ comand1; command2; command3 ; } > outputfile

Replace “command1″, “command2″ and “command3″ with your commands.

To append to a file instead of overwriting use “>>” instead of “>”.