Critical system files in Linux

In Linux, almost all configuration parameters are stored in ordinary text files.  And there is a special location under which the configuration files are stored namely /etc.  The following table lists all major configuration files found in Linux and their purpose.

Critical system files in Linux

File/Directory	Permissions	Description
/var/log/	751	Directory containing all log files.
/var/log/messages	644	System messages.
/etc/crontab	600	System wide crontab file.
/etc/syslog.conf	640	Syslog daemon configuration file.
/etc/logrotate.conf	640	Controls rotation of system log files.
/var/log/wtmp	660	Who is logged in now. Use who to view.
/var/log/lastlog	640	Who has logged in before. Use last to view.
/etc/ftpusers	600	List of users who cannot FTP to the machine.
/etc/passwd	644	List of system’s user accounts.
/etc/shadow	600	Contains encrypted account passwords.
/etc/pam.d	750	PAM configuration files.
/etc/hosts.allow	600	Access control file.
/etc/hosts.deny	600	Access control file.
/boot/grub/grub.conf	600	Boot configuration file for GRUB bootloader.
/etc/securetty	600	TTY interfaces that allow root logins.
/etc/shutdown.allow	400	Users allowed to ctrl-alt-del
/etc/security	700	System access security policy files.
/etc/rc.d/init.d/	750	Program startup files on Red Hat systems.
/etc/init.d/	750	Program startup files on Debian systems.
/etc/sysconfig	751	System and network config files on Red Hat.
/etc/ssh	750	Secure shell configuration files.
/etc/sysctl.conf	400	Contains kernel tunable options.