The Process File System Information in Linux

The Process File System (/proc) give you access to information about the Linux kernel and all process currently running on your system.

Files and Directories in /proc

File Name                    Content

/proc/acpi                     Information about Advanced Configuration and 

                                     Power Interface(ACPI)

/proc/bus                      Bus-specific information for each bus type, such as PCI

/proc/cmdline              The command line used to start the Linux kernel

/proc/cpuinfo               Information about the CPU

/proc/devices               Available block and character devices

/proc/dma                    Information about DMA

/proc/driver/rtc            Information about Real Time Clock(RTC)

/proc/filesystems         List of supported file systems

/proc/ide                      Information about IDE devices

/proc/interrupts            Information about Interrupt Request(IRQ) numbers

/proc/ioports                Information about Input/Output(I/O) ports

/proc/kcore                  Image of the physical memory

/proc/kmsg                  Kernel messages

/proc/loadavg              Load average

/proc/locks                  Current kernel locks

/proc/meminfo            Information about physical memory and swap space usage

/proc/misc                   Miscellaneous information

/proc/modules             List of loaded driver modules

/proc/mounts               List of mounted file systems

/proc/net                      Directory of Information about networking

/proc/partitions            List of partitions known to Linux kernel

/proc/pci                      Information about PCI devices

/proc/scsi                     Information about SCSI devices

/proc/stat                     Overall statistics about the system

/proc/swaps                 Information about the swap space

/proc/sys                      Information about the system

/proc/uptime                Information about uptime

/proc/version               Kernel version number

The Most Popular Configuration Files in Linux System

List of the most popular configuration files and brief description

Configuration File                  Description

/boot

/boot/grub                             GRUB bootloader file
/boot/grub/menu.list             boot menu
/boot/System.map                 Linux kernel map
/boot/vmlinuz                        Linux kernel

/etc

/etc/apache2/httpd.conf        Apache web server configuration file
                                             (Debian)
/etc/apt/sources.list               lists the sources from which the APT obtains
                                              packages(Debian & Ubuntu)
/etc/at.allow                          users allowed to use the at command to 
                                              schedule jobs
/etc/at.deny                           users forbidden to use the at command
/etc/bashrc                            functions and aliases for bash shell
                                             (Fedora)

/etc/bash.bashrc                    functions and aliases for bash shell
                                             (Debian, SUSE & Ubuntu)
/etc/cups/cupsd.conf            The CPUS scheduler
/etc/fonts/local.conf             font configuration file
/etc/fstab                              Information about the various file system and mount
/etc/group                             Information about groups

/etc/grub.conf                       GRUB bootloader
                                             (Fedora& SUSE)
/etc/hosts                              IP address and their hostnames
/etc/hosts.allow                    Hosts allowed to access internet service
/etc/hosss.deny                     hosts forbidden to access internet services
/etc/httpd/conf/httpd.conf    Apache web server configuration file
                                             (Fedora)

/etc/init.d                              Directory with scripts to start and stop various servers
/etc/init.d/rcS                        Linux initialization script
                                              (Debian, SUSE & Ubuntu)
/etc/inittab                             The init process that starts all the other processes
/etc/issue                               Distribution name and the version number
/etc/lilo.conf                          Linux Loader(LILO) configuration file

/etc/login.defs                       Default information for creating user accounts
/etc/modules.conf                 Configuration file for loadable kernel module
                                             (Debian)
/etc/mtab                               Information about the currently mounted file systems
/etc/passwd                           Information about all user accounts
/etc/profile                            Systemwide environment and startup file for the bash sell

/etc/profile.d                         Directory containing file that the /etc/profile script executes
/etc/rc.d/rc.sysinit                 Linux initialization script
                                              (Fedora)
/etc/shadow                           Secure file with encrypted passwords for all user accounts
/etc/shells                              List of all the shells on the system that the user can use
/etc/skel                                 Directory that holds .bash_logout, .bash_profile, .bashrc files

/etc/sysconfig                        Linux configuration files
                                              (Fedora & SUSE)
/etc/sysctl.conf                      Configuration file with kernel parameters
/etc/termcap                          Database of terminal capabilities and options
                                             (Fedora & SUSE)
/etc/udev                               Directory containing configuration files for udev
/etc/X11                                Directory with configuration files for X Window 
                                              System

/etc/X11/xorg.conf               Configuration file for X.Org X11 the X Window System
                                             (Fedora, Ubuntu & SUSE)
/etc/xinetd.conf                    Configuration for the xinetd daemon that starts internet 
                                             service on demand
/etc/yum.conf                       Configuration for the Yum package updater and installer
                                             (Fedora)

/var

/var/log/apache2                  Web-server access and error logs
                                            (Debian)
/var/log/cron                        Log file with messages from the cron process that runs 
                                            scheduled jobs
/var/log/boot.msg                File with boot messages(SUSE)
/var/log/dmesg                    File with boot messages
                                            (Debian, Fedora & Ubuntu)
/var/log/httpd                      Web-server access and error logs(Fedora)
/var/log/message                 System log


APT    - Advanced Packaging Tool
CPUS - Common Unix Printing System

Basic TCP/IP Network Configuration Files

Summary of basic TCP/IP configuration files

File                                        Description

/etc/hosts                           IP address and hostnames for local network

/etc/networks                    Names and IP address of networks

/etc/host.conf                    Translate host name in to IP address

/etc/resolv.conf                 IP address of name servers

/etc/hosts.allow                 Allow which system can access internet

/etc/hosts.deny                  Deny internet access to systems

/etc/nsswitch.conf            Translate hostnames into ip address

Common vi Commands

List of commonly used vi commands

Command              Does the following function

Insert Text

a                              Inserts text after the cursor

A                             Inserts text at the end of the current line

I                               Inserts text at the beginning of the current line

i                               Inserts text before the cursor

Delete Text

D                             Deletes up to the end of the current line

dd                           Deletes the current line

dG                          Deletes from the current line to the end of the file

dw                          Deletes the current word where the cursor presently resides

x                              Deletes the character on which the cursor rests

Change Text

C                             Changes up to the end of the current line

cc                            Changes the current line

J                              Joins the current line with the next one

rx                            Replaces the character under the cursor with x
                                (where x is any character)

Search Text       

/string                  Searches forward for a string

?string                  Searches backward for a string

Scroll Text

Ctrl+D                   Scrolls forward by half a screen

Ctrl+U                   Scrolls backward by half a creen

Ctrl+L                    Redraws/refresh the screen

Cut and Paste Text

yy                           Yanks(copies)current line to an unnamed buffer

P                             Puts the yanked line above the current line

p                             Puts the yanked line below the current line

Move Cursor     

h                              Moves one character to the left

j                               Moves one line down

k                              Moves one line up

L                              Moves to the end of the screen

l                               Moves one character to the right

w                             Moves to the beginning of the following word

b                              Moves to the beginning of the previous word

Colon commands

:!command           Executes a shell command

:q                            Quits the editor

:q!                           Quits without saving changes

:r filename             Reads the file and inserts it after the current line

:w filename           Writes a buffer to the file

:wq                         Saves changes and exits

More

u                             Undoes the last command

Esc                          Ends input mode and enters visual command mode

U                             Undoes resent changes to the current line

Learn more:- Vim Cheat Sheet

Top Level Directories in the Linux File System

Top Level Directories in the Linux

Directory        Contains

/                       Base of the files system.
/bin                  Executable programs that are part of the Linux Operating System.
/boot                Linux Kernel and other files that the LILO and GRUB needed.
/dev                 Special files that represent devices attached to the system.
/etc                  System configuration files and the initialization scripts.
/home              Home directories of all users.
/lib                   Library files stored in /sbin and /bin needed to start Linux.
/lost+found     Lost files. Every disk partition has a lost+found directory.
/media             Mounting file systems on removable media.
/mnt                 Temporarily mounted file systems.
/opt                  Storage for large application software packages.
/proc                Information about the process running in the Linux system.
/root                 Home directory for the root user.
/sbin                 Executable commands for administration tasks.
/srv                  Data for services offered by this system.
/sys                  Information about the devices as seen by the Linux kernel.
/tmp                 Temporary directory that any user can use.
/usr                  Subdirectories for many important programs.
/var                  Various system files.

Important /usr Subdirectories

 Subdirectory  Description

/usr/bin            Executable files including utility programs

/usr/games       Some old Linux games.
/usr/include      Header files for the C, C++,  X11 and Linux kernel.
/usr/lib             Libraries for C, C++, other database libraries and graphical toolkit.
/usr/local          Local files.
/usr/sbin           Administrative commands, such as for email and networking.
/usr/share         Shared data.
/usr/share/man Online manual.
/usr/src             Source code for the Linux kernel.

 Important /var subdirectories

Subdirectory  Description.

/var/cache        Storage area for cached data for applications.
/var/lib             Information relating to the current state of applications.
/var/lock          Locked files ensures that a resource is used by only one application.
/var/log            Log files organized into subdirectories.
/var/mail          User mailbox files.
/var/opt            Variable data for packages stored in the /opt directory
/var/run            Data describing the system since the time it was booted.
/var/spool         Data that's waiting for some kind of processing.
/var/tmp           Temporary files preserved between system reboots.
/var/yp             Network Information Services (NIS) database files.

Programmers want to know about Linux

These features make Linux a productive software development environment.

GNU C compiler (gcc)
GNU C++ compiler (g++)
GNU compiler for java (gcj)
GNU make utility
GNU debugger (gdb)
GNU profiling utility (gprof)
Subversion, Concurrent Versions System (CVS) and Revision Control System (RCS)
GNU emacs editor
Perl
Tool Command Language and its graphical toolkit (tcl/tk)
Python
Dynamically linked, shared Libraries

Well known GNU Packages

GNU was designed to be a replacement for Unix operating systems of the 1980s and used the POSIX standards as a guide. The following list is a small set of GNU packages.

Software Package            Description

autoconf                         Automatically configure source code packages
automake                       makes *.in files
bash                               shell
bc                                   interactive calculator
binutils                           binary files utilities
coreutils                         A combined package of Fileutils, Shellutils and Textutils

cpio                                copy to and from, a disc or file system
diff                                 compare files
ed                                   line oriented text editor
emacs                             extensible text editor
findutils                          find, locate and xargs utilities
finger                             find information about users

gawk                              awk programming
gcc                                 for C,C++, Objective-C and other
gdb                                debugger for C, C++ and FORTRAN
gdbm                              replacement for dbm and ndbm libraries
gettext                           writing multilingual programs
GIMP                              GNU image manipulation program

GNOME                           GUI
Gnumeric                        graphical spreadsheet
grep                                includes grep, egrep and fgrep
groff                               document formatting
gtk+                                GUI toolkit for X Window system
gzip                                compressing and decompressing utility

less                                 similar to more
libpng                             library for image files
make                              determine which pieces of a program need to be recompiled
patch                              generate the modified version
rcs                                  revision control system
sed                                 stream oriented version of ed text editor

tar                                  tape archiving program
textinfo                          for online info docs
time                               actual time that a process uses

All GNU packages
https://www.gnu.org/software/software.en.html

Reset Root Password On CentOS 7

Change the root password on CentOS 7 Servers

At the boot menu, press e to edit the existing kernel (Core).

scroll down to the list until you see the line ro ,change ro to rw and start into a bash shell. It should look like rw init=/sysroot/bin/sh.

Press Ctrl + X  to start into single user mode.

In the single user mode run the command,
chroot /sysroot

Run the commands to change the root password,
passwd root

After creating the password, run the commands below to update SELinux parameters
touch /.autorelabel

Exit and reboot your system. You should be able to sign on and use the system with the new password.

Monitoring Memory with Swap

A low amount of free memory is an indication that the system has many thing to  do; this may lead to disk swapping. A high use of swap space make a system go slow because disk access is many times slower than access.

cat /usr/local/src/memory.sh
#!/bin/bash
C=$(/usr/bin/free -m| head -2 | tail -n +2 | awk {'print $4'})
C=$(/usr/bin/free -m| head -4 | tail -n +4 | awk {'print $3'})
E=$(date)
printf "%s %s %s %s %s %s\n" "Mem: " "$C" "Swap: " "$D" "At" "$E"

crontab -l
*/5 * * * * /usr/local/src/memory.sh >> /var/log/memory.data

tail -5 /var/log/memory.data
Mem: 76080 Swap: 5274188 At Sat Oct  8 12:44:48 PDT 2016
Mem: 72128 Swap: 5274264 At Sat Oct  8 12:45:01 PDT 2016
Mem: 67780 Swap: 5274428 At Sat Oct  8 12:50:01 PDT 2016
Mem: 68720 Swap: 5274492 At Sat Oct  8 12:55:01 PDT 2016
Mem: 68472 Swap: 5274944 At Sat Oct  8 13:00:01 PDT 2016

Monitoring the TCP ESTABLISHED Connections

The following script tcpConnect.sh uses nestat to record the number of connection as well as the date and the time of the measurement.

cat /usr/local/src/tcpConnet.sh
#!/bin/bash
C=$(/bin/netstat -nt | tail -n +3 | grep ESTABLISHED | wc -l)
D=$(date)
printf "%s %s %s\n" "$C" "||" "$D"

crontab -l
*/5 * * * * /usr/local/src/tcpConnet.sh >> /var/log/tcpConnet.data

tail -5 /var/log/tcpConnet.data
25 || Sat Oct  8 11:10:01 PDT 2016
26 || Sat Oct  8 11:15:01 PDT 2016
23 ||  Sat Oct  8 11:20:01 PDT 2016
23 || Sat Oct  8 11:25:01 PDT 2016
23 || Sat Oct  8 11:30:01 PDT 2016

Monitoring Load Average

The easiest way to monitor the load average is by using the output o the uptime command and a small awk script that will run as a cron job to store your data into a text file.

The awk script is the following :

cat /usr/local/src/uptime.sh
#!/bin/bash
uptime | awk  {'print $9 $10 $11'} | awk -F, {'print $1 " " $2 " " $3'}

sh /usr/local/src/uptime.sh
0.68 0.86 1.48

crontab -l
*/5 * * * * /usr/local/src/uptime.sh >> /var/log/uptime.data

head -5 /var/log/uptime.data
0.61 1.40 1.96
0.47 1.33 1.92
0.49 0.94 1.68
0.64 0.80 1.41
0.45 0.61 1.16

       
The Linux system we used has only one CPU, so any load average greater than 1.00 indicates a performance issue.

The first number shows the load average during the last minute, the second during the last five minutes and the third during the last 15 minute,. The three values can show you if the load is increasing, decreasing or steady.