SysadminSG RHCSA Study Guide

This is the RHCSA Study Guide for the System Administration Study Group.

The study guide is intended to be printed by those who wish to study common tasks performed by many system administrators. This study guide is based upon the Red Hat Certified System Administrator Exam Objectives. Other useful study components will be added here. Community contributions are always welcome.

Understand and Use Essential Tools

Access a shell prompt and issue commands with correct syntax

Access to shell can be done by logging in at boot, or if a graphical desktop environment (X) is used, access can be gained by opening gnome-terminal, konsole, or Terminal. Alternatively, using Alt-Ctrl+fX (F1-F6) will gain access to a virtual teletype terminal. The default shell environment is bash, though sh, zsh, and csh are also available.

Use input-output redirection (>, >>, |, 2>)

The Linux command prompt allows the user access to a great variety of useful tools. Furthermore, the shells support advanced ways in which the information generated by the tools can be used further. By using pipes and redirects, the user can take the output from a program (for example: cat) and send that information to a file or another program. The following are some basic tools:

> (example: cat filename1 filename2 > filename.txt) This puts all the output of the program into a file. If the filename specified does not exist, it will be created. If the filename does exist, it will be completely overwritten, providing the user has permissions to do so.

>> (example: cat filename 1 filename2 >> filename.txt) This basically does the same thing as > except that instead of overwritting files, it amends the file by adding to any existing content. It appends the output to the end of the specified file.

| (example: dmesg | tail | fpaste) Instead of sending the output to a file, the user can send the output to another program. In this example, the user effectively runs dmesg, the output is then sent to tail to truncate it to the last 5 lines, which is then sent to the fpaste program which will then send the output to a pastebin.

2> (example: foo 2>file.txt ) To understand this, you must understand that there are essentially two outputs for programs. Standard output is basically what you see on the screen . Standard error contains errors that occur when the program runs that do not get sent to standard output (represented by "2"). In this example, standard error alone is getting sent to a file. This may be useful for debugging purposes.

2>&1, &> (example: ls > dirlist.txt 2>&1 or: ls &>dirlist.txt ) This is also functionally the same as &> and >&, though 2>&1 is more compatible. This is similar to the above example of 2> except that instead of piping standard error alone, this will send the output of both standard output and standard error to a file. This is useful for debugging and logging purposes. This will create the specified output file if it doesn't exist and completely overwrite it if it does exist, providing the user has such permissions.

&>>, >> 2>&1 (example ls &>>dirlist.txt or: ls >> dirlist.txt 2>&1 ) This also will send the output to a file, but in the case the file exists, it will append the output of both standard output and standard error to the end of the file rather than overwriting it.

Use grep and regular expressions to analyze text

The grep utility can be used to search for text in a file or files. For example, to look for your own username in the /etc/passwd file, use the following:

grep $USER /etc/passwd

You can also use grep to exclude lines from a file. To do this, the -v flag is used.

grep -v root /etc/passwd

Also, -i makes searches case insensitive.

grep -i avc /var/log/audit/audit.log

When run as just grep, grep supports basic regular expressions. When run as egrep, grep runs as though the -E flag was used, which means to support extended regular expressions. If run as fgrep, grep acts as though the -F flag was used, which turns off regular expressions and searches only simple strings.

egrep '\<[0-9]\>' /etc/passwd
fgrep ':1:' /etc/passwd

Access remote systems using ssh and VNC

Remote systems can be accessed by a variety of different methods, including ssh, vnc, freenx, telnet, and rdp. ssh and vnc are the most commonly used methods for accessing Linux systems remotely.

SSH - The ssh command can be used to access a remote server that has sshd running. The most basic syntax is ssh <hostname> or ssh <IP>. A user can be specified by ssh user@host. If a graphical interface is available at the remote machine, -X or -Y may be used to run graphical programs (recommended only for use over high bandwidth or LAN connections). ssh keys may be used to further increase the security. For more information on the features and syntax, please read the ssh manual (man ssh).

VNC - Virtual Network Computing (vnc) uses a different protocol than ssh and is primarily meant for graphical remote access. Because this protocol works by sending compressed images, it is recommended more for high bandwidth or LAN connections. VNC should be run within a graphical environment. The two clients which are primarily used are the Tiger VNC Viewer (vncviewer) and vino.

In both cases, you should ensure that your firewall allows the necessary client connections for ssh (22) and vnc (5900-590X).

Log in and switch users in multi-user runlevels

By default, the default multiple user environments in Fedora (and RHEL) are levels 3, 4, and 5 (the default run level is set in /etc/inittab ). Once logged in, the shell environment (bash, by default) is loaded.

To switch users, the su (basic syntax: su user) command may be used. When changing users, to ensure that the user profile is updated to the target user's profile, the - character should follow su (example: su - user or su -l user). su or su - without a user specified will default to root.

To end a user session and return to the previously logged in user session, simply run the 'exit' command.

Alternatively, it is possible to run specific commands as another user. This can be doe with su -c. sudo is another option for this and may be configured to run specific commands as superuser (example sudo command). Use the visudo command to configure sudo.

Specific commands may also be set up to be run under the permissions of another user or group. This is done by setuid or setgid.

Create and edit text files

Text files may be easily created and edited from the Linux command line as well as graphically.

To simply create a file with no content, the touch command may be used for this. To create and edit files, a text editor such as vi (vim), emacs, or nano. Nano is the most friendly for new users and is pretty self explanatory, but vim and emacs are more popular among admins. To learn more about vim, try vimtutor. To explore emacs, open emacs and type Ctrl-H and then 'T' or visit the Emacs guided tour page.

Graphically, there are an array of editing tools available. Most desktop environments contain basic text editors, such as gedit and kwrite up to advanced word processors such as libreoffice-writer, abiword, and koffice.

As explained above, redirects may also be used to create and add data to files (example: echo 'I just created this file!' > mynewfile.txt ). Please see the > and >> redirects above for more information on how to do this.

Lastly, basic text files (excluding .doc and .odf) created in Linux may not be formatted correctly for viewing in Windows and vice versa. Before opening a basic text (.txt) created in Windows Notepad, you may need to run dos2unix on the file, and conversely, you might need to run unix2dos on a file before opening it in Notepad, though Wordpad in Windows is capable of reading basic text files created in Linux.

Delete, copy and move files and directories

Once files have been created, it is then possible to rename, move, copy, and delete these files. These files are organized into folders called directories.

mkdir - Directories can be created using mkdir. If you intend to make a into one that also hasn't been created yet, use the -p option. For instance, if you want to create the path ./directory1/directory2 where directory1 may or may not yet exist, mkdir -p directory1/directory2 will create both if directory1 doesn't exist.

rmdir - To delete an empty directory, rmdir may be used. If the directory is not empty, see rm below.

mv - Both renaming and moving files and folders may be doe by the mv command. To simply change the name, the basic syntax is mv oldname newname. To move the file to another place (and also optionally change the name), you must specify a new. For example: mv file ./directory/

cp - Files may be copied from one directory to another (or in the same with a new name) using cp (example: cp file ./directory ). The difference between this action and mv is that it leaves the original file intact. Using cp -R, the user may also copy entire directories and their contents to a new location.

rm - To delete files, use rm. Be careful when using this because rm cannot be undone without considerable effort. There is no undel equivalent in Linux. To remove entire directories, including those containing files, rm -r may be used to recursively delete them.

User/Group permissions can affect the ability to do any of these things.

Operate Running Systems

Boot, reboot, and shut down a system normally

Boot systems into different runlevels manually

Use single-user mode to gain access to a system

Identify CPU/memory intensive processes, adjust process priority with renice, and kill processes

Locate and interpret system log files

Access a virtual machine's console

Start and stop virtual machines

Start, stop and check the status of network services

Configure Local Storage

List, create, delete and set partition type for primary, extended, and logical partitions

Create and remove physical volumes, assign physical volumes to volume groups, create and delete logical volumes

Create and configure LUKS-encrypted partitions and logical volumes to prompt for password and mount a decrypted file system at boot

Configure systems to mount file systems at boot by Universally Unique ID (UUID) or label

Add new partitions, logical volumes and swap to a system non-destructively

Create and Configure File Systems

Create, mount, unmount and use ext2, ext3 and ext4 file systems

Mount, unmount and use LUKS-encrypted file systems

Mount and unmount CIFS and NFS network file systems

Configure systems to mount ext4, LUKS-encrypted and network file systems automatically

Extend existing unencrypted ext4-formatted logical volumes

Create and configure set-GID directories for collaboration

Create and manage Access Control Lists (ACLs)

Diagnose and correct file permission problems

Deploy, Configure and Maintain Systems

Configure networking and hostname resolution statically or dynamically

Schedule tasks using cron

Configure systems to boot into a specific runlevel automatically

Install Red Hat Enterprise Linux automatically using Kickstart

Configure a physical machine to host virtual guests

Install Red Hat Enterprise Linux systems as virtual guests

Configure systems to launch virtual machines at boot

Configure network services to start automatically at boot

Configure a system to run a default configuration HTTP server

Configure a system to run a default configuration FTP server

Install and update software packages from a remote repository, or from the local filesystem

Update the kernel package appropriately to ensure a bootable system

Modify the system bootloader

Configure a system to run a default configuration NTP server and synchronize time using other NTP peers

Manage Users and Groups

Create, delete, and modify local user accounts

Change passwords and adjust password aging for local user accounts

Create, delete and modify local groups and group memberships

Configure a system to use an existing LDAP directory service for user and group information

Manage Security

Configure firewall settings using system-config-firewall or iptables

Set enforcing and permissive modes for SELinux

List and identify SELinux file and process context

Restore default file contexts

Use boolean settings to modify system SELinux settings

Diagnose and address routine SELinux policy violations