Important Files & Their Purpose
Kernel Files & System Info
/boot/vmlinuz - The Linux Kernel file
/etc/exports - Information of the file system available on network
/etc/modules.conf - Configuration files for system modules
/etc/mtab - Currently mounted blocks information
/usr/bin - Normal user executable commands
/usr/bin/X11 - Binaries of X windows System
/usr/include - Contains include files used by ‘c‘ program
/usr/share - Shared directories of man files, info files
/usr/lib - Library files which are required during program compilation
/usr/sbin - Commands for Super User, for System Administration
/proc/cpuinfo - CPU Information
/proc/filesystems - File-system Information being used currently
/proc/interrupts - Information about the current interrupts being utilised currently
/proc/ioports - Contains all the Input/Output ports and addresses used by devices on the server.
/proc/meminfo - Memory Usages Information
/proc/modules - Currently using kernel module
/proc/mounts - Mounted File-system Information
/proc/stat - Detailed Statistics of the current System
/proc/swaps - Swap File Information
/proc/version - Linux kernel version #
Startup & INIT
/etc/fstab - Information of Disk Drive and their mount point
/etc/grub.conf - grub bootloader configuration file
/etc/lilo.conf - lilo bootloader configuration file. (Not used in newer linux versions)
/etc/inittab - INIT process and their interaction at various run level (newer Linux versions switched to systemd process)
/etc/init.d - Directory that contains startup scripts during OS init
/etc/rc.d - Directory that contains Run-level scripts for various applications & processes
User/Group & Login
/etc/issue - Allows to edit the pre-login message.
/etc/motd - motd stands for Message Of The Day, The Message users gets upon login
/etc/profile - Bash shell defaults
/etc/passwd - Contains list of system users
/etc/shadow - Contains encrypted passwords for sys users
/etc/group - Information of Security Group
/etc/profile.d - Application scripts directorys, executed after user login
/etc/securetty - Terminal List, where root login is possible
/etc/X11 - Configuration files of X-window System.
Devices
/dev/hda - Device file for the first IDE HDD (Hard Disk Drive)
/dev/hdc - Device file for the IDE Cdrom, commonly
/dev/null - A pseudo device, that don’t exist. Sometime garbage output is redirected to /dev/null, so that it gets lost, forever
Network Config
etc/hosts - Information of Ip addresses and corresponding host names
/etc/hosts.allow - List of hosts allowed to access services on the local machine/etc/host.deny - List of hosts denied to access services on the local machine
/etc/resolv.conf - Domain Name Servers (DNS) being used by System
Shell Config
/etc/bashrc - Contains system defaults and aliases used by bash shell
/etc/profile - Bash shell defaults
Scheduling
/etc/crontab - A shell script to run specified commands on a predefined time Interval
Logging
/var/log/lastlog - log of last boot process.
/var/log/messages - log of messages produced by syslog daemon at boot
/var/log/wtmp - list login time and duration of each user on the system currently