Basic Linux Commands -Part 3

 cal :        Display a calendar

Syntax:

                        cal [-mjy] [[month] year]

Options:

           -m         Display monday as the first day of the week.

            -j          Display julian dates (days one-based,numbered from Jan 1

           -y          Display a calendar for the current year.

 chgrp :   Change group ownership

Syntax :

                 chgrp [Options]... {Group | --reference=File} File...

Key :

          -c

          --changes

                    Verbosely describe the action for each File whose group actually changes.

          -f

         --silent

         --quiet

                   Do not print error messages about files whose group cannot be changed.

         -f

         --no-dereference

                   Act on symbolic links themselves instead of what they point to.

         --reference=FILE

                  Use the group of the reference FILE instead of an explicit GROUP.

         -v

        --verbose

                 Verbosely describe the action or non-action taken for every File.

       -R

       --recursive

                Recursively change the group ownership of directories and their contents.

BOOT Process Of Linux Server

The following are the 6 high level stages of a typical Linux boot process.

1. BIOS

• BIOS stands for Basic Input/Output System
• Performs some system integrity checks
• Searches, loads, and executes the boot loader program.
• It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.
• Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.
• So, in simple terms BIOS loads and executes the MBR boot loader.

2. MBR

• MBR stands for Master Boot Record.
• It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda
• MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes.
• It contains information about GRUB (or LILO in old systems).
• So, in simple terms MBR loads and executes the GRUB boot loader.

3. GRUB

• GRUB stands for Grand Unified Bootloader.
• If you have multiple kernel images installed on your system, you can choose which one to be executed.
• GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.
• GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).
• Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample grub.conf of CentOS.

#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/boot/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-194.el5PAE)
          root (hd0,0)
          kernel /boot/vmlinuz-2.6.18-194.el5PAE ro root=LABEL=/
          initrd /boot/initrd-2.6.18-194.el5PAE.img

• As you notice from the above info, it contains kernel and initrd image.
• So, in simple terms GRUB just loads and executes Kernel and initrd images.

5. Init

• Looks at the /etc/inittab file to decide the Linux run level.
• Following are the available run levels
  • 0 – halt
  • 1 – Single user mode
  • 2 – Multiuser, without NFS
  • 3 – Full multiuser mode
  • 4 – unused
  • 5 – X11
  • 6 – reboot
• Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.
• Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level
• If you want to get into trouble, you can set the default run level to 0 or 6. Since you know what 0 and 6 means, probably you might not do that.
• Typically you would set the default run level to either 3 or 5.

6. Runlevel programs

• When the Linux system is booting up, you might see various services getting started. For example, it might say “starting sendmail …. OK”. Those are the runlevel programs, executed from the run level directory as defined by your run level.
• Depending on your default init level setting, the system will execute the programs from one of the following directories.
  • Run level 0 – /etc/rc.d/rc0.d/
  • Run level 1 – /etc/rc.d/rc1.d/
  • Run level 2 – /etc/rc.d/rc2.d/
  • Run level 3 – /etc/rc.d/rc3.d/
  • Run level 4 – /etc/rc.d/rc4.d/
  • Run level 5 – /etc/rc.d/rc5.d/
  • Run level 6 – /etc/rc.d/rc6.d/
• Please note that there are also symbolic links available for these directory under /etc directly. So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.
• Under the /etc/rc.d/rc*.d/ directories, you would see programs that start with S and K.
• Programs starts with S are used during startup. S for startup.
• Programs starts with K are used during shutdown. K for kill.
• There are numbers right next to S and K in the program names. Those are the sequence number in which the programs should be started or killed.
• For example, S12syslog is to start the syslog deamon, which has the sequence number of 12. S80sendmail is to start the sendmail daemon, which has the sequence number of 80. So, syslog program will be started before sendmail.

Thats all about the Linux boot Process ..

Commonly Used Ports -Part 3

<<Previous Part 2

Port    TCP       UDP             DESCRIPTION

21   TCP   UDP    FTP control (command)
22   TCP   UDP    Secure Shell 
23   TCP   UDP    Telnet protocol—unencrypted text communications
24   TCP   UDP    Priv-mail : any private mail system.
25   TCP   UDP    Simple Mail Transfer Protocol 
26   TCP   UDP    Unassigned
27   TCP   UDP    NSW User System FE
29   TCP   UDP    MSG ICP
33   TCP   UDP    Display Support Protocol
35   TCP   UDP    Any private printer server protocol
37   TCP   UDP    TIME protocol
39   TCP   UDP    Resource Location Protocol
40   TCP   UDP    Unassigned
42   TCP   UDP    ARPA Host Name Server Protocol
42   TCP   UDP    Windows Internet Name Service
43   TCP   UDP    WHOIS protocol
47   TCP   UDP    NI FTP
49   TCP   UDP    TACACS Login Host protocol
50   TCP   UDP    Remote Mail Checking Protocol
51   TCP   UDP    IMP Logical Address Maintenance
52   TCP   UDP    XNS (Xerox Network Systems) Time Protocol
53   TCP   UDP    Domain Name System (DNS)
54   TCP   UDP    XNS (Xerox Network Systems) Clearinghouse
55   TCP   UDP    ISI Graphics Language (ISI-GL)
56   TCP   UDP    XNS (Xerox Network Systems) Authentication
56   TCP   UDP    Route Access Protocol (RAP)
57   TCP   UDP    any private terminal access
58   TCP   UDP    XNS (Xerox Network Systems) Mail
64   TCP   UDP    CI (Travelport) (formerly Covia) Comms Integrator
67   TCP   UDP    Bootstrap Protocol (BOOTP) Server; also used by DHCP
68   TCP   UDP    Bootstrap Protocol (BOOTP) Client; also used by DHCP
69   TCP   UDP    Trivial File Transfer Protocol (TFTP)
70   TCP   UDP    Gopher protocol
71   TCP   UDP    NETRJS protocol
72   TCP   UDP    NETRJS protocol
73   TCP   UDP    NETRJS protocol
74   TCP   UDP    NETRJS protocol
77   TCP   UDP    Any private Remote Job Entry
79   TCP   UDP    Finger protocol
80   TCP   UDP    Hypertext Transfer Protocol (HTTP)

Commonly Used Ports -Part 2

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Port    TCP      UDP       DESCRIPTION

0             UDP        Reserved
1     TCP   UDP        TCP Port Service Multiplexer (TCPMUX)
2     TCP   UDP       CompressNET[5] Management Utility
3     TCP   UDP       CompressNET[5] Compression Process
4     TCP   UDP       Unassigned
5     TCP   UDP       Remote Job Entry
6     TCP   UDP       Unassigned
7     TCP   UDP       Echo Protocol
8     TCP   UDP       Unassigned
9     TCP   UDP       Discard Protocol 
9             UDP       Wake-on-LAN
10    TCP   UDP       Unassigned
11    TCP   UDP       Active Users (systat[disambiguation needed] service)
12    TCP   UDP       Unassigned
13    TCP   UDP       Daytime Protocol (RFC 867)
14    TCP   UDP       Unassigned
15    TCP   UDP       Previously netstat service
16    TCP   UDP       Unassigned
17    TCP   UDP       Quote of the Day
18    TCP   UDP       Message Send Protocol
19    TCP   UDP       Character Generator Protocol (CHARGEN)
20    TCP   UDP       FTP data transfer