At the Least, You Should Know:
How to install Apache.
How to configure Apache.
How to view Apache logs.
Must reads for Apache:
Official Apache HTTP Server Documentation
apt-get
apt-get is the command-line tool for handling packages, and may be considered the user’s “back-end” to other tools using the APT library. apt-get is used mainly on Debian and Ubuntu but is cross-platform and the actual APT library has many different front-ends built for it. Apt-get is a pretty simple tool to use, so theres not much to learn here, but you should still be familiar with how it works.
At the Least, You Should Know:
How to install a package.
How to remove a package.
Must reads for apt-get:
Apt-get Man Page
How to use apt-get
Bash - GNU Bourne-Again SHell
Bash is an sh-compatible command language interpreter that executes commands read from the standard input or from a file. Bash also incorporates useful features from the Korn and C shells (ksh and csh).
It’s important to know how to program in Bash. There are a million ways to save time and energy if you can whip up a little bash script that automates something on the command line for you.
At the Least, You Should Know:
How to write a basic Bash script.
How to configure Bash to execute jobs during login or logout.
Must reads for Bash:
Bash Man Page
Bash Guide for Beginners
Advanced Bash Scripting
Bash by Example
Bash Tutorial
chmod
chmod changes the permissions of each given file according to mode, which can be either a symbolic representation of changes to make, or an octal number representing the bit pattern for the new permissions.
Chmod is vital to any Linux user. If you don’t understand how to change permissions on files then you really shouldn’t be using Linux.
At the Least, You Should Know:
How to set file and folder permissions with chmod.
Must reads for chmod:
Chmod Man Page
Linux File Permissions
Crontab
crontab is the program used to install, deinstall or list the tables used to drive the cron(8) daemon in Vixie Cron. Each user can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly.
Crontab is important to know if you want to execute a sort of “scheduled task” command that cron manages.
At the Least, You Should Know:
How to schedule tasks with crontab.
How to edit each users crontab.
How to allow and disallow users to use crontab.
Crontab format.
Must reads for Crontab:
Crontab Man Page
Understanding Cron Jobs in 5 Minutes
Crontab: Scheduling Tasks
Find
GNU find searches the directory tree rooted at each given file name by evaluating the given expression from left to right, according to the rules of precedence (see section OPERATORS), until the outcome is known (the left hand side is false for and operations, true for or), at which point find moves on to the next file name.
Using find is crucial to saving time and energy on the Linux command line. You’ll find that the more you know about the awesome capabilities of find, the better off you are.
At the Least, You Should Know:
How to find files throughout the whole system.
How to find only directories.
How to find files owned by a certain user.
Must reads for Find:
Study of Find
CLI Magic: Searching with Find
10 Useful uses of the find command
Iptables
Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. Several different tables may be defined. Each table contains a number of built-in chains and may also contain user-defined chains.
With iptables you have the ability to create firewall rules on your Linux computer to allow or restrict access in and out through each network interface.
At the Least, You Should Know:
How to list iptables rules.
How to filter an IP from hitting your interface.
How to remove rules.
Must reads for iptables:
Iptables Man Page
Linux Firewalls using iptables
Designing a firewall using Iptables for the home user
Using iptables
MySQL
mysql is a simple SQL shell (with GNU readline capabilities). It supports interactive and non-interactive use. When used interactively, query results are presented in an ASCII-table format. When used non-interactively (for example, as a filter), the result is presented in tab-separated format. The output format can be changed using command options.
As a Linux user looking to land a Linux job, there is a strong possibility that you will be working in an environment that uses Linux-Apache-MySQL-PHP (LAMP). You should be familiar with administrating a mysql server.
At the Least, You Should Know:
How to install MySQL.
How to configure MySQL.
How to view MySQL error logs.
How To Add and Remove MySQL users.
Must reads for MySQL:
MySQL Man Page
Official MySQL Documentation
OpenSSH and SSH
Ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine. You’ll probably find yourself using SSH on a daily basis if you land a Linux job.
At the Least, You Should Know:
How to connect to a server with SSH.
How to set up key based authentication for SSH.
Perl
Perl is a language optimized for scanning arbitrary text files, extracting information from those text files, and printing reports based on that information. It’s also a good language for many system management tasks. The language is intended to be practical (easy to use, efficient, complete) rather than beautiful (tiny, elegant, minimal).
You’ll find administration tasks that you can create Perl scripts for will save you a ton of work in the long run if you know Perl.
At the Least, You Should Know:
How to execute a perl script.
How to troubleshoot perl errors.
Must reads for Perl:
Perl Man Page
Official Perl Documentation
Simple Perl Scripts
Postfix
Postfix is a mail transfer agent (MTA) used on Linux as an alternative to Qmail and Sendmail. It handles the routing and delivery of email. Chances are you’ll run into some sort of postfix work in your Linux career, so why not learn what you can?
At the Least, You Should Know:
How to configure the postfix main.cf and master.cf files.
How to troubleshoot postfix errors and issues.
Must reads for Postfix:
Postfix Man Page
Official Postfix Documentation
Postfix Wiki
Postfix config How To
PHP
PHP (recursive acronym for “PHP: Hypertext Preprocessor”) is a widely-used Open Source general-purpose scripting language that is especially suited for Web development and can be embedded into HTML. Learning a bit about PHP as well as Apache and MySQL is essential to your Linux career and landing that Linux job.
At the Least, You Should Know:
How to install and configure PHP on Linux.
How to debug and troubleshoot a PHP error.
Must reads for PHP:
Official PHP Documentation
How To Write PHP Scripts
Qmail
Qmail is a secure, reliable, efficient, simple message transfer agent. If your Linux job isn’t running Postfix or Sendmail, you’re probably going to need to know Qmail.
At the Least, You Should Know:
How to Install and Configure Qmail.
How to find and interpret the Qmail log files.
How to debug and troubleshoot Qmail errors.
Must reads for Qmail:
Qmail Man Page
The Qmail Documentation Project
Qmail HowTo
Life With Qmail
RPM
rpm is a powerful Package Manager, which can be used to build, install, query, verify, update, and erase individual software packages. If you’re trying to land a Linux job with a large corporation, theres a good chance they run Red Hat, which uses RPM as it’s default package manager.
At the Least, You Should Know:
How to install RPM packages.
How to remove RPM packages.
How to check for RPM dependencies.
Must reads for RPM:
RPM Man Page
Official RPM Documentation
rsync
rsync uses the rsync remote-update protocol to greatly speed up file transfers when the destination file is being updated. Rsync basically only copies the diffs of files that have been changed.
Rsync is great for backing up files to another Linux host. If you plan on landing a Linux job you’ll definitely need to know rsync.
At the Least, You Should Know:
The syntax of using rsync to copy files to another host machine.
Must reads for rsync:
rsync Documentation
rsync Tips and Tricks
Using rsync and SSH
Samba
The Samba software suite is a collection of programs that implements the Server Message Block (commonly abbreviated as SMB) protocol for UNIX systems. This protocol is sometimes also referred to as the Common Internet File System (CIFS). For a more thorough description, see http://www.ubiqx.org/cifs/. Samba also implements the NetBIOS protocol in nmbd.
Most large companies that run a Linux and Windows environment use Samba to share files across the network. You should be familiar with Samba as much as possible.
At the Least, You Should Know:
How to install and configure Samba.
How to set up Samba users.
How to set up Samba shares.
Must reads for Samba:
Samba Man Page
Official Samba Documentation
Samba Setup Guide for Linux
SCP
scp copies files between hosts on a network. It uses ssh for data transfer, and uses the same authentication and provides the same security as ssh. scp will ask for passwords or passphrases if they are needed for authentication.
SCP is important to know if you need to copy files between Linux hosts over a public or private network with security in mind.
At the Least, You Should Know:
The syntax for copying a file from one host to another host.
Must reads for SCP:
SCP Man Page
SCP Tricks
Sendmail
Sendmail is another Mail Transfer Agent, similar to Qmail and Postfix, that sends a message to one or more recipients, routing the message over whatever networks are necessary. Sendmail does internetwork forwarding as necessary to deliver the message to the correct place. A majoriy of servers run Sendmail as their MTA so it’s important to know.
At the Least, You Should Know:
How to Install Sendmail.
How to configure Sendmail to send and receive Email.
How to view Sendmail logs.
Must reads for Sendmail:
Sendmail Man Page
Official Sendmail Documentation
Tar
Tar is an archiving program designed to store and extract files from an archive file known as a tarfile. You most defenitley need to know tar if you plan on landing any Linux job.
At the Least, You Should Know:
How to extract a tarball and tar/gzip file.
How to create a tar file.
Must reads for Tar:
Tar Man Page
GNU tar Documentation
VI/Vim
Vim is a text editor that is upwards compatible to Vi. It can be used to edit all kinds of plain text. It is especially useful for editing programs.
It’s important to know vim because it is the one text editor that is almost always available by default on most Linux distributions. You can’t say the same for the simpler editors like pico and nano. Learn vim!
At the Least, You Should Know:
How to open a file and make a change.
How to save a file.
How to exit vim.
Must reads for Vim:
Vi/Vim Man Page
Official Vim Documentation
The Vim Commands Cheat Sheet
Vim Graphical Cheat Sheet based Tutorial
vsftpd
vsftpd is the Very Secure File Transfer Protocol Daemon. Most Linux servers that run an FTP daemon use vsftpd.
At the Least, You Should Know:
How to install vsftpd.
How to configure vsftpd.
How to view vsftpd logs and troubleshoot issues.
Must Reads for vsftpd:
Vsftpd.conf Man Page
A vsftpd Guide
I’ve covered all the applications I believe you should be familiar with if you’re trying to land a Linux job. You may not use all of them in an everyday Linux job, but these applications are the most widely used with Linux administration and Linux engineering jobs.
Do you have any other applications you feel should be on this list? Let us know by leaving a comment below.
Posted by Serious Blogger at 12:20 PM 0 comments Links to this post
Linux Commands -Hardware Informations
Running kernel and system information
# uname -a # Get the kernel version (and BSD version)
# lsb_release -a # Full release info of any LSB distribution
# cat /etc/SuSE-release # Get SuSE version
# cat /etc/debian_version # Get Debian version
Use /etc/DISTR-release with DISTR= lsb (Ubuntu), redhat, gentoo, mandrake, sun (Solaris), and so on. See also /etc/issue.
# uptime # Show how long the system has been running + load
# hostname # system's host name
# hostname -i # Display the IP address of the host. (Linux only)
# man hier # Description of the file system hierarchy
# last reboot # Show system reboot history
Hardware Informations
Kernel detected hardware
# dmesg # Detected hardware and boot messages
# lsdev # information about installed hardware
# dd if=/dev/mem bs=1k skip=768 count=256 2>/dev/null | strings -n 8 # Read BIOS
Linux
# cat /proc/cpuinfo # CPU model
# cat /proc/meminfo # Hardware memory
# grep MemTotal /proc/meminfo # Display the physical memory
# watch -n1 'cat /proc/interrupts' # Watch changeable interrupts continuously
# free -m # Used and free memory (-m for MB)
# cat /proc/devices # Configured devices
# lspci -tv # Show PCI devices
# lsusb -tv # Show USB devices
# lshal # Show a list of all devices with their properties
# dmidecode # Show DMI/SMBIOS: hw info from the BIOS
FreeBSD
# sysctl hw.model # CPU model
# sysctl hw # Gives a lot of hardware information
# sysctl vm # Memory usage
# dmesg | grep "real mem" # Hardware memory
# sysctl -a | grep mem # Kernel memory settings and info
# sysctl dev # Configured devices
# pciconf -l -cv # Show PCI devices
# usbdevs -v # Show USB devices
# atacontrol list # Show ATA devices
# camcontrol devlist -v # Show SCSI devices
Load, statistics and messages
The following commands are useful to find out what is going on on the system.
# top # display and update the top cpu processes
# mpstat 1 # display processors related statistics
# vmstat 2 # display virtual memory statistics
# iostat 2 # display I/O statistics (2 s intervals)
# systat -vmstat 1 # BSD summary of system statistics (1 s intervals)
# systat -tcp 1 # BSD tcp connections (try also -ip)
# systat -netstat 1 # BSD active network connections
# systat -ifstat 1 # BSD network traffic through active interfaces
# systat -iostat 1 # BSD CPU and and disk throughput
# tail -n 500 /var/log/messages # Last 500 kernel/syslog messages
# tail /var/log/warn # System warnings messages see syslog.conf
Users
# id # Show the active user id with login and group
# last # Show last logins on the system
# who # Show who is logged on the system
# groupadd admin # Add group "admin" and user colin (Linux/Solaris)
# useradd -c "Colin Barschel" -g admin -m colin
# usermod -a -G
# groupmod -A
# userdel colin # Delete user colin (Linux/Solaris)
# adduser joe # FreeBSD add user joe (interactive)
# rmuser joe # FreeBSD delete user joe (interactive)
# pw groupadd admin # Use pw on FreeBSD
# pw groupmod admin -m newmember # Add a new member to a group
# pw useradd colin -c "Colin Barschel" -g admin -m -s /bin/tcsh
# pw userdel colin; pw groupdel admin
Encrypted passwords are stored in /etc/shadow for Linux and Solaris and /etc/master.passwd on FreeBSD. If the master.passwd is modified manually (say to delete a password), run # pwd_mkdb -p master.passwd to rebuild the database.
To temporarily prevent logins system wide (for all users but root) use nologin. The message in nologin will be displayed (might not work with ssh pre-shared keys).
# echo "Sorry no login now" > /etc/nologin # (Linux)
# echo "Sorry no login now" > /var/run/nologin # (FreeBSD)
Limits
Some application require higher limits on open files and sockets (like a proxy
web server, database). The default limits are usually too low.
Linux
Per shell/script
The shell limits are governed by ulimit. The status is checked
with ulimit -a. For example to change the open files limit from
1024 to 10240 do:
# ulimit -n 10240 # This is only valid within the shell
The ulimit command can be used in a script to change the limits for the script only.
Per user/process
Login users and applications can be configured in /etc/security/limits.conf. For example:
# cat /etc/security/limits.conf
* hard nproc 250 # Limit user processes
asterisk hard nofile 409600 # Limit application open files
System wide
Kernel limits are set with sysctl. Permanent limits are set in /etc/sysctl.conf.
# sysctl -a # View all system limits
# sysctl fs.file-max # View max open files limit
# sysctl fs.file-max=102400 # Change max open files limit
# echo "1024 50000" > /proc/sys/net/ipv4/ip_local_port_range # port range
# cat /etc/sysctl.conf
fs.file-max=102400 # Permanent entry in sysctl.conf
# cat /proc/sys/fs/file-nr # How many file descriptors are in use
FreeBSD
Per shell/script
Use the command limits in csh or tcsh or as in Linux, use ulimit in an sh or bash shell.
Per user/process
The default limits on login are set in /etc/login.conf. An unlimited value is still limited by the system maximal value.
System wide
Kernel limits are also set with sysctl. Permanent limits are set in /etc/sysctl.conf or /boot/loader.conf. The syntax is the same as Linux but the keys are different.
# sysctl -a # View all system limits
# sysctl kern.maxfiles=XXXX # maximum number of file descriptors
kern.ipc.nmbclusters=32768 # Permanent entry in /etc/sysctl.conf
kern.maxfiles=65536 # Typical values for Squid
kern.maxfilesperproc=32768
kern.ipc.somaxconn=8192 # TCP queue. Better for apache/sendmail
# sysctl kern.openfiles # How many file descriptors are in use
# sysctl kern.ipc.numopensockets # How many open sockets are in use
# sysctl -w net.inet.ip.portrange.last=50000 # Default is 1024-5000
# netstat -m # network memory buffers statistics
See The FreeBSD handbook Chapter 11http://www.freebsd.org/handbook/configtuning-kernel-limits.html for details.
Solaris
The following values in /etc/system will increase the maximum file descriptors per proc:
set rlim_fd_max = 4096 # Hard limit on file descriptors for a single proc
set rlim_fd_cur = 1024 # Soft limit on file descriptors for a single proc
Runlevels
Linux
Once booted, the kernel starts init which then starts rc which starts all scripts belonging to a runlevel. The scripts are stored in /etc/init.d and are linked into /etc/rc.d/rcN.d with N the runlevel number.
The default runlevel is configured in /etc/inittab. It is usually 3 or 5:
# grep default: /etc/inittab
id:3:initdefault:
The actual runlevel (the list is shown below) can be changed with init. For example to go from 3 to 5:
# init 5 # Enters runlevel 5
* 0 Shutdown and halt
* 1 Single-User mode (also S)
* 2 Multi-user without network
* 3 Multi-user with network
* 5 Multi-user with X
* 6 Reboot
Use chkconfig to configure the programs that will be started at boot in a runlevel.
# chkconfig --list # List all init scripts
# chkconfig --list sshd # Report the status of sshd
# chkconfig sshd --level 35 on # Configure sshd for levels 3 and 5
# chkconfig sshd off # Disable sshd for all runlevels
Debian and Debian based distributions like Ubuntu or Knoppix use the command update-rc.d to manage the runlevels scripts. Default is to start in 2,3,4 and 5 and shutdown in 0,1 and 6.
# update-rc.d sshd defaults # Activate sshd with the default runlevels
# update-rc.d sshd start 20 2 3 4 5 . stop 20 0 1 6 . # With explicit arguments
# update-rc.d -f sshd remove # Disable sshd for all runlevels
# shutdown -h now (or # poweroff) # Shutdown and halt the system
FreeBSD
The BSD boot approach is different from the SysV, there are no runlevels. The final boot state (single user, with or without X) is configured in /etc/ttys. All OS scripts are located in /etc/rc.d/ and in /usr/local/etc/rc.d/ for third-party applications. The activation of the service is configured in /etc/rc.conf and /etc/rc.conf.local. The default behavior is configured in /etc/defaults/rc.conf. The scripts responds at least to start|stop|status.
# /etc/rc.d/sshd status
sshd is running as pid 552.
# shutdown now # Go into single-user mode
# exit # Go back to multi-user mode
# shutdown -p now # Shutdown and halt the system
# shutdown -r now # Reboot
The process init can also be used to reach one of the following states level. For example # init 6 for reboot.
* 0 Halt and turn the power off (signal USR2)
* 1 Go to single-user mode (signal TERM)
* 6 Reboot the machine (signal INT)
* c Block further logins (signal TSTP)
* q Rescan the ttys(5) file (signal HUP)
Reset root password
Linux method 1
At the boot loader (lilo or grub), enter the following boot option:
init=/bin/sh
The kernel will mount the root partition and init will start the bourne shell
instead of rc and then a runlevel. Use the command passwd at the prompt to change the password and then reboot. Forget the single user mode as you need the password for that.
If, after booting, the root partition is mounted read only, remount it rw:
# mount -o remount,rw /
# passwd # or delete the root password (/etc/shadow)
# sync; mount -o remount,ro / # sync before to remount read only
# reboot
FreeBSD method 1
On FreeBSD, boot in single user mode, remount / rw and use passwd. You can select the single user mode on the boot menu (option 4) which is displayed for 10 seconds at startup. The single user mode will give you a root shell on the / partition.
# mount -u /; mount -a # will mount / rw
# passwd
# reboot
Unixes and FreeBSD and Linux method 2
Other Unixes might not let you go away with the simple init trick. The solution is to mount the root partition from an other OS (like a rescue CD) and change the password on the disk.
* Boot a live CD or installation CD into a rescue mode which will give you a shell.
* Find the root partition with fdisk e.g. fdisk /dev/sda
* Mount it and use chroot:
# mount -o rw /dev/ad4s3a /mnt
# chroot /mnt # chroot into /mnt
# passwd
# reboot
Kernel modules
Linux
# lsmod # List all modules loaded in the kernel
# modprobe isdn # To load a module (here isdn)
FreeBSD
# kldstat # List all modules loaded in the kernel
# kldload crypto # To load a module (here crypto)
Compile Kernel
Linux
# cd /usr/src/linux
# make mrproper # Clean everything, including config files
# make oldconfig # Reuse the old .config if existent
# make menuconfig # or xconfig (Qt) or gconfig (GTK)
# make # Create a compressed kernel image
# make modules # Compile the modules
# make modules_install # Install the modules
# make install # Install the kernel
# reboot
FreeBSD
Optionally update the source tree (in /usr/src) with csup (as of FreeBSD 6.2 or later):
# csup
I use the following supfile:
*default host=cvsup5.FreeBSD.org # www.freebsd.org/handbook/cvsup.html#CVSUP-MIRRORS
*default prefix=/usr
*default base=/var/db
*default release=cvs delete tag=RELENG_7
src-all
To modify and rebuild the kernel, copy the generic configuration file to a new name and edit it as needed (you can also edit the file GENERIC directly). To restart the build after an interruption, add the option NO_CLEAN=YES to the make command to avoid cleaning the objects already build.
# cd /usr/src/sys/i386/conf/
# cp GENERIC MYKERNEL
# cd /usr/src
# make buildkernel KERNCONF=MYKERNEL
# make installkernel KERNCONF=MYKERNEL
To rebuild the full OS:
# make buildworld # Build the full OS but not the kernel
# make buildkernel # Use KERNCONF as above if appropriate
# make installkernel
# reboot
# mergemaster -p # Compares only files known to be essential
# make installworld
# mergemaster -i -U # Update all configuration and other files
# reboot
For small changes in the source you can use NO_CLEAN=yes to avoid rebuilding the whole tree. But use with care.
# make buildworld NO_CLEAN=yes # Don't delete the old objects
# make buildkernel KERNCONF=MYKERNEL NO_CLEAN=yes
Posted by Serious Blogger at 12:05 PM 0 comments Links to this post
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