reboot vs shutdown -r now

Generally shutdown -r now is safer.

In most of the unix distributions they perform exactly the same actions except for Mac OSX. On Mac OSX reboot performs an instant reboot. There is no unmounting and no syncing. So, to avoid headaches, play safe and do not use reboot.

Syntax of shutdown command:

shutdown [OPTION] [TIME] [MESSAGE]

To reboot the system right now safely write:

shutdown -r now

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Tail command with color

tail -f myfile.log | perl -pe 's/.*TEXT_TO_FIND.*/\e[1;31m$&\e[0m/g'

\e[1;31m will give you the red color. If you would like some yellow, use \e[1;33m, and for green use \e[1;32m. The \e[0m restores the normal text color.

SOURCE: http://unix.stackexchange.com/questions/8414/how-to-have-tail-f-show-colored-output
(Answer <37>)

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Test TCP and UDP with Netcap

Testing TCP connection

Create TCP listener:

$ nc -l 9999

Create TCP Client:

$ nc -vt 192.168.5.12 9999
Ncat: Version 6.40 ( http://nmap.org/ncat )
Ncat: Connected to 192.168.5.12:9999.

Testing UDP connection

Create UDP listener:

$ nc -ul 9999

Create UDP Client:

$ nc -vu 192.168.5.12 9999
Ncat: Version 6.40 ( http://nmap.org/ncat )
Ncat: Connected to 192.168.5.12:9999.

Test UDP Connection only:

$ nc -zvu 192.168.5.12 9999
Connection to 192.168.5.12 9999 port [udp/*] succeeded!

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Delete all files except one

For files:

find . ! -name 'file.txt' -type f -exec rm -f {} +

For directories:


find . ! -name 'directory' -type d -exec rm -rf {} +

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Start MySql

$ sudo /etc/init.d/mysqld start

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sysctl to change kernel system settings

To list all settings configurable in the /proc/sys/ directory:

/sbin/sysctl -a


To assign values to writable files in the /proc/sys/ directory:

sysctl -w [parameter] = [value]

Example:

sysctl -w kernel.sysrq="1"

This only works for testing purposes. Special settings within /proc/sys/ are lost when the 

machine is rebooted. To preserve custom settings, add them to the /etc/sysctl.conf file.

Each time the system boots, the init program runs the /etc/rc.d/rc.sysinit script.

This script contains a command to execute sysctl using /etc/sysctl.conf to determine the

values passed to the kernel.

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IPC Queues - ENOSPC (No space left on device) error

ENOSPC returns from msgget indicates that the system-wide maximum number of message queues has been reached.

Solution:

Change this limit editing:
/proc/sys/kernel/msgmni

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