Linux provides a flexible multi-operating system boot loader named grub (GRand Unified Bootloader) that can be used to boot different operating systems. Like Windows bootloaders, it can also sit on the Master Boot Record (MBR) of the boot device and transfer control to specific operating system kernels depending on either a user type-in or a default. Grub comes in two flavors, 1.0 and 2.0.
Previously, there was lilo, which is still seen occasionally (especially when booting from cd/dvd).
Here's a grub configuration file on a Linux Mint 17.1 machine:
# Uncomment to enable BadRAM filtering, modify to suit your needs # This works with Linux (no patch required) and with any kernel that obtains # the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...) #GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef" # Uncomment to disable graphical terminal (grub-pc only) #GRUB_TERMINAL=console # The resolution used on graphical terminal # note that you can use only modes which your graphic card supports via VBE # you can see them in real GRUB with the command `vbeinfo' #GRUB_GFXMODE=640x480
# inittab is no longer used when using systemd. # # ADDING CONFIGURATION HERE WILL HAVE NO EFFECT ON YOUR SYSTEM. # # Ctrl-Alt-Delete is handled by /etc/systemd/system/ctrl-alt-del.target # # systemd uses 'targets' instead of runlevels. By default, there are two main targets: # # multi-user.target: analogous to runlevel 3 # graphical.target: analogous to runlevel 5 # # To set a default target, run: # # ln -s /lib/systemd/system/.target /etc/systemd/system/default.target #
The idea for init is that soft-links that start with a capital ``S'' are executed at startup; soft-links with a leading capital ``K'' are executed at shutdown (you can see this behavior in /sbin/rc2 on Solaris and /etc/rc.d/rc on RedHat).
The Linux init package usually includes a utility named runlevel that you can run to determine the current machine's run level.
Notice the location of the actual init shell scripts can vary (/etc, /etc/rc.d, etc.), even between different versions of UNIX. For instance, Solaris: /etc/rc?.d (/etc/rc2.d is typical), CentOS Linux: /etc/rc.d
On some systems, you can use chkconfig to control the maze of links.
# chkconfig --list # show what is on and what is off for runlevels anacron 0:off 1:off 2:on 3:on 4:on 5:on 6:off atd 0:off 1:off 2:off 3:on 4:on 5:on 6:off atieventsd 0:off 1:off 2:off 3:off 4:off 5:on 6:off auditd 0:off 1:off 2:on 3:on 4:on 5:on 6:off autofs 0:off 1:off 2:off 3:on 4:on 5:on 6:off # chkconfig --level 2345 sendmail on # have sendmail start 2345 # chkconfig --del sendmail # remove sendmail from chkconfig management # chkconfig --add sendmail # add sendmail to chkconfig management
root@(none):/home/CNT5605# pstree
init─┬─agetty
├─apache2─┬─apache2
│ └─2*[apache2───26*[{apache2}]]
├─atd
├─cron
├─dhclient
├─exim4
├─getty
├─ntpd
├─rpc.idmapd
├─rpc.statd
├─rpcbind
├─rsyslogd───3*[{rsyslogd}]
├─sshd─┬─sshd───bash───pstree
│ ├─3*[sshd───sshd───COP4342_bash]
│ └─sshd───sshd
└─udevd───2*[udevd]
Beware that older versions of UNIX/Linux used symbolic links between a common script directory and the particular runlevel directory. Treat any modifications to the startup shell scripts as you would any other program -- edit, test (reboot), document, debug until it works.
In a major site, you may find that weekly reboots at an off time (such as early on Sunday morning) are automatically done to discover any bad interactions among system modifications that might have been made recently.