Content

Basics

File Hierarchy Standard (FHS)

Path Content
/bin Binaries (User)
/boot Static boot loader files
/etc Host specific configs
/lib Shared libraries and kernel modules
/sbin Binaries (System/root)
/var Varying files (e.g. Logs)
/usr 3rd party software
/proc Pseudo file system
/sys Pseudo file system
/mnt Mountpoint for internal drives
/media Mountpoint for external drives
/home User homes
/run PID files of running processes

Commands

File System Commands

Command Param Description
cd - Navigate to last dir
~ Navigate to home
~username Navigate to home of specified user
pwd Print working dir
ls Print dir content
-l Format as list
-a Show hidden items (-A without . and ..)
-r Invert order
-R Recurse
-S Sort by size
-t Sort by date modified
mkdir -p Create dir with parents
cp -r Copy dir
rmdir -p Remove dir and empty parents
rm -rf Remove dir recursively, -f without confirmation
mv Move recursively
find -iname pattern Search dir/file case-insensitive
-mmin n Last modified n minutes ago
-mtime n Last modified n days ago
-regex pattern Path matches pattern
-size n[kMG] By file size (-n less than; +n greater than)
! searchparams Invert search

File Manipulation

Command Param Description
cat file Print content
tac file Print content inverted
sort file Print sorted
file -r -u Print sorted descending without dublicates
wc file Count Lines, Words, Chars (Bytes)
head -n10 file | tail -n5 Print lines 5-10
tail -f file Print new lines automatically
cut -f -4,7-10,12,15- file Print selected fields (tab delimited)
-c -4,7-10,12,15- file Print selected characters positions
-f 2,4 -d, --output-delimiter=$'\t' file Change delimiter (but use tab for output)
uniq file Hide consecutive identical lines
file -c Show consecutive identical line count
file -u Hide consecutive identical lines
file file Get file type

Archiving

Command Param Beschreibung
tar cfv archiv.tar file1 file2 Create archive / add or overwrite content
tfv archiv.tar Show content
xf archiv.tar [-C ~/extracted] Unpack (and decompress) archive (to ~/extracted)
cfvj archiv.tar.bz2 file bzip2 create compressed archive
cfvz archiv.tar.gz file create gzip compressed archive
cfa archiv.tar.[komp] file create compressed archive (auto type based on name)
bzip2 / gzip file1 file2 Compress files (individually)
-d file1 file2 Uncompress files

Other

Command Param Beschreibung
<command> --help Help of current command (not standardized)
-h
-?
man <command> Manual page of command
-k keyword Search command by keyword (oder apropos)
alias Show aliases
name='befehl' Create alias

Globs (Wildcards)

The dot . in front of hidden items is ignored by glob patterns!

Character Description
? Any single character
* Any characters
[ac-e] 1 character in enum
[!ac-e] 1 character not in enum

Regex

Bash itself does not know regex. Use programs like grep, sed, awk.

Steuerzeichen

Character Description
. Any single character
[ac-e] 1 character in enum
[^ac-e] 1 character not in enum
^ Start of string
$ End of string
\d Digit
\D Not a digit
\s Whitespace
\S Not a Whitespace
\< Start of word
\> End of word
pattern? Quantifier 0 or 1
pattern* Quantifier 0…n
pattern+ Quantifier 1…n
pattern{x} Quantifier exactly x
pattern{x,} Quantifier x…n
pattern{x,y} Quantifier x…y
pattern{,y} Quantifier 0…y

Grep

Command Param Description
grep pattern file Extended Regex
-E pattern file Extended Regex
-v pattern file Invert match
-w pattern file Word match
-i pattern file Ignore case

Stream redirection

Character Description
> file or 1> file STDOUT to file
< file Datei to STDIN
2> file STDERR to file
2>&1 STDERR to same target as STDOUT
> file 2>&1 STDOUT and STDERR to file

Disk and File System Management

General Disk Manipulation (non-LVM)

Creating physical partitions is not required! You can create PVs directly!

Command Description
fdisk -l List physical disks and partitions
fdisk /dev/sdb
n		 Create new partition
fdisk /dev/sdb
t
8e		 Change partition type to Linux LVM
mkfs.xfs /dev/myVG/myVol Format LV with XFS
mkfs.ext4 -f /dev/myVG/myVol Format LV with EXT4 (overwrite)
blkid /dev/myVG/myVol Show UUID and formatting of volume
mount Show what is mounted where
mount -t ext4 /dev/myVG/myVol /mountpoint Mount LV to /mountpoint
umount /dev/myVG/myVol Unmount LV from /mountpoint
umount /mountpoint Unmount LV from /mountpoint
mount -a Mount as configured in /etc/fstab
df Show disk usage
xfs_growfs /dev/myVG/myVol Resize xfs filesystem
resize2fs /dev/myVG/myVol Resize ext3/4 filesystem

LVM

Item Description
Physical Volume Representation of physical volume
Volume Group Group of physical volumes
Logical Volume Volumes placed on volume group

Physical Volume Commands

Command Description
pvs Show PVs
pvdisplay Show detailed PV information
pvcreate /dev/sdb1 Initialize physical volume for use with LVM
pvs -o+pv_used Show usage of PVs
pvmove /dev/sdb1 Move contents of PV to other PVs of VG
pvremove /dev/sdb1 Remove PV

Volume Group Commands

Command Description
vgs Show VGs
vgdisplay Show detailed VG information
vgcreate myVG /dev/sda1 Create new VG
vgextend myVG /dev/sdb1 Add PV to VG
vgreduce myVG /dev/sdb1 Remove PV from VG (pvmove first!)
vgremove myVG Remove VG

Logical Volume Commands

Command Description
lvs Show LVs
lvdisplay Show detailed LV information
lvcreate -n myVol -L 3G myVG Create LV
lvextend -L +10G /dev/myVG/myVol -r Expand LV by 10GB and resize filesystem
lvextend -L 10G /dev/myVG/myVol --resizefs Expand LV to 10GB and resize filesystem
lvextend /dev/myVG/myVol /dev/sdb1 Expand LV by PV’s size
lvextend -l +100%FREE /dev/myVG/myVol Expand LV to 100% available VG size
lvreduce -L -10G /dev/myVG/myVol Reduce LV by 10GB
lvremove /dev/myVG/myVol Remove logical volume

Automatic mounting (/etc/fstab)

Example:

file system mount point type option dump pass
/dev/sdb1 /myDir1 ext4 defaults 0 0
/dev/mapper/myVG-myVol /myDir2 xfs defaults 0 0
UUID=a924e3de-0414-4228-86bf-b65a4c8ff8ab /myDir3 xfs defaults 0 0
/dev/mapper/myVG-swap swap swap defaults 0 0
192.168.1.1:/shared /targetdir nfs async 0 0
//server/share /targetdir cifs credentials=/root/cifs/.username 0 0

Column dump configures file system for backup with dump (program).

Column pass configures order of file system check during boot. 0 means no check.

Text Readers & Editors

Less

Command Description
q Quit
R Refresh content
F Auto scroll
g number Go to line
m lowercaseLetter Mark line
' lowercaseLetter Go to mark
/pattern Search forward
?pattern Search backward
n Next search result
N Last search result
ESC u Remove highlighting from search

VI

Editing

To leave editing mode press ESC.

Command Description
i insert before cursor
a insert after cursor
A insert at end of line
o new line below
O new line above
u undo
. repeat last command
yy copy line
5yy copy 5 lines
p paste below
P paste above
x delete character
5x delete 5 characters
dd delete line
5dd delete 5 lines
:10,20d delete lines 10-20
d0 delete to line begin
d$ delete to line end

Navigation

Navigate as usual with arrow keys, home, end, pg up, pg dn.

Command Description
5G go to line 5
H go to top of screen
M go to middle of screen
L go to end of screen
5w move over 7 words
5b move back 5 words

Other

Command Description
/foo search forward
?foo search backwards
n repeat search
:w save
:q close
:wq save and close
:q! close without saving
:!command run bash command
:r foo read file foo into this file

User and Group Management

UID

UID Type
<1000 system account
>1000 user account

User Database

User info without passwords is stored in /etc/passwd.

username PW UID GID Kommentar HOME SHELL
hfict x 1000 1000 /home/hfict /bin/bash

Group Database

Group info with secondary group members are stored in /etc/group.
Primary group members are identified by GID in user database.

groupname PW GID Users
wheel x 10 hfict,user2

Password Database

Hashed user passwords are stored in /etc/shadow.
Password encryption is configured in /etc/login.defs.

username PW Last PW change Minimum Maximum Warn Inactive Expire
hfict [hash] 17803 0 99999 7

PW:

Commands

Command Param Beschreibung
id username ID und Gruppen eines Benutzers anzeigen
who Angemeldete Benutzer anzeigen
last Letzte Logins anzeigen
lastb Letzte fehlgeschlagene Logins anzeigen
sudo -u user command Befehl mit Rechten von user ausführen (Default ist root)
-i oder su - Shell mit root Rechten
su Shell als root (non-login shell)
- Shell als root (login shell)
- user Shell als user
useradd -u 2101 -g primarygroup -c comment username Benutzer anlegen (ohne -g wird neue Gruppe erstellt)
usermod -G group1,group2 Sekundärgruppen definieren (überschreiben)
-aG group,group2 Sekundärgruppen hinzufügen
-l username Benutzernamen ändern
-L Lock Account
-U Unlock Account
-s shellpath Shell ändern
userdel -r username Benutzer inkl. Home und Mail-Spool löschen
passwd username Passwort ändern (interaktiv)
groupadd groupname Gruppe anlegen (Optional mit -g GID setzen)
groupdel groupname Gruppe löschen

File System Permissions

Permissions can be set on:

Only root can change User. User can change Group.

Basic permissions (Add binary flags to combine):

Char Binary Flag Permission
r 4 read
w 2 write
x 1 execute

Advanced permissions (place in front of basic permissions: chmod 1777 shared).:

Char Binary Flag Name Description
t / T 1 Sticky Bit Others can’t delete content (only applicable for directories)
s / S 2 SGID-Bit File: run with permissions of Group
Dir: New elements inherit Group
s / S 4 SUID-Bit File is run with permissions of User (only applicable for files)

Advanced permissions replace the x when using ls -l. Lower case if x is set, upper case if x is not set.

Read permission on a directory only allows to see the directory itself but not it’s contents. Use execute permission to show contents.

Commands

Command Param Beschreibung
chmod -R [uog] dirname Set permissions recursively using binary flags
+[suog] filename Add permissions using binary flags
-[suog] filename Remove permissions using binary flags
u+x filename Add execute permission for User
g+wx filename Add write and execute permissions for Group
o-r filename Remove read permission for Others
chown -R user:group filename Change owner (User & Group) recursively
user filename Change owner (User)
:group filename Change owner (Group)
chgroup group filename Change owner (Group)

SSH

Configuration is done in /etc/ssh/sshd_config.

Reload SSH service with systemctl reload sshd to apply changes!

DenyUsers, AllowUsers, DenyGroups, AllowGroups override each other and are applied in the order listed above.

Config Option Description
PermitRootLogin no Deny root to login via SSH
yes Allow root to login via SSH
without-password Allow only with private/public key auth
AllowUsers user1 user2 Allow only user1 and user2
DenyUsers user1 user2 Allow all users but user1 and user2
AllowGroups group1 group2 Allow only users from specified groups
DenyGroups group1 group2 Allow all users but those in specified groups

IPv6 (disable)

Change the grub config /etc/default/grub to disalbe IPv6.

  1. Add ipv6.disable=1 to the GRUB_CMDLINE_LINUX property.
    e.g. GRUB_CMDLINE_LINUX="ipv6.disalbe=1 crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap biosdevname=0 net.ifnames=0 rhgb quiet"
  2. Recreate grub config with grub2-mkconfig -o /boot/grub2/grub.cfg
  3. Reboot the system with init 6 or shutdown -r now

Cronjobs

Crontab

Cronjobs are configured in crontab files. Do not edit these files directly. Use crontab -e instead. This runs all required actions to activate a cronjob after saving the edited crontab. The locations are as follows:

The format of the files is (user specific crontabs do not have the column user-name):

Example of job definition:
.---------------- minute (0 - 59 | */5 [every 5 minutes])
|  .------------- hour (0 - 23)
|  |  .---------- day of month (1 - 31)
|  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
|  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
|  |  |  |  |
*  *  *  *  * user-name  command to be executed
Command Description
rpm -q cronie Check if package is installed
systemctl status crond.service Check if service is running
crontab -l List current users crontab
crontab -e Edit current users crontab
crontab -e -u username Edit specific users crontab
crontab -r Remove current users crontab

Script folders

Scripts in one of the following directories will be executed at the intervall specified by the directory’s name:

Allow / Deny usage

Add user names one per line to the following files:

If none of the files exists, all users are allowed.

Logs and Results

Execution of cronjobs is logged in /var/log/cron.
Results are sent to the users mail /var/spool/mail/username.

Package Management

RPM

Command Description
rpm -i rpmfile|rpmurl Install package
rpm -e packagename Uninstall package
rpm -q packagename Check if package is installed
rpm -ql packagename List files in a package
rpm -qa List all installed packages
rpm -qf /path/to/file Get package that installed the file
rpm -qf $(which <exe>) Get package that installed the executable
rpm -V packagename Validate installed package

YUM

YUM is configured in /etc/yum.conf

Repos are configured in /etc/yum.repos.d/

Log is in /var/log/yum.log

Command Description
yum install packagename [-y] Install package (-y no confirm message)
yum remove packagename Uninstall package
yum update Update all installed packages
yum update packagename Update specific package
yum update pattern* Update packages using wildcard
yum info packagename Get detailed info about package
yum list packagename List installed and available packages
yum search searchstring search for a package (name & summary)
yum search all searchstring search for a package (all infos)
yum deplist packagename List dependencies of a package
yum reinstall packagename Reinstall (corrupted) package
yumdownloader --resolve packagename Download rpm package with dependencies

Services (systemd)

Command Description
systemctl list-dependencies Show Services
systemctl start servicename Start service
systemctl enable servicename Start service with system
systemctl status servicename Show service status

NFS

Installation (Client & Server)

yum install nfs-utils

Server
Configuration is done in /etc/exports.

systemctl start nfs-server.service  # Start service
systemctl enable nfs-server.service # Start service at system start
vi /etc/exports                     # Change config
exportfs -fa                        # Reload config (server)

In /etc/exports create a new line for every directory you want to share in the following format:

<dir> <destination1>(<options>) [<destination2>(<options>)]

Examples:

/               master(rw) trusty(rw,no_root_squash)
/projects       proj*.local.domain(rw)
/usr            *.local.domain(ro)
/home/joe       pc001(rw,all_squash,anonuid=150,anongid=100)
/pub            *(ro,insecure,all_squash)
/foo            2001:db8:9:e54::/64(rw) 192.0.2.0/24(rw)
/build          buildhost[0-9].local.domain(rw)

Client

showmount -e 192.168.1.1                    # Show NFS shares  
mkdir /targetdir                            # Create mountpoint
mount -t nfs 192.168.1.1:/shared /targetdir # Mount manually   
umount /targetdir                           # Unmount manually 
vi /etc/fstab                               # Mount on system start
mount -a                                    # Test fstab
init 0                                      # Auto mount

Example of /etc/fstab:

192.168.1.1:/shared  /targetdir nfs  async  0 0

Samba

Server

Configuration is done in /etc/samba/smb.conf.

Local accounts must be registered with Samba using smbpasswd.
Enter the same password as for the local account and add unix password sync = yes to /etc/samba/smb.conf. Then it should be synced automatically when you change it with passwd.

yum install samba samba-client samba-common # Samba installation
systemctl start smb                         # Start service
systemctl enable smb                        # Start service on system start
vi /etc/samba/smb.conf                      # Change config
testparm                                    # Check if config is ok
systemctl restart smb                       # Reload config
smbpasswd -a username                       # Register user with samba

Example of /etc/samba/smb.conf:

[global]
  workgroup = WORKGROUP             # workgroup of smb server
  netbios name = centos             # name of smb server
  security = user                   # security mode (user / ads / domain)
  unix password sync = yes          # sync unix password with smb password
  invalid user = root bin daemon    # deny access globally (config in share section overrides this)

[Transfer]                        # name of share
  path = /transfer                  # path of directory to be shared
  comment = File transfer           # description
  read only = no                    # access permissions
  guest ok = no                     # allow guest access (no password)
  browsable = yes                   # visible or hidden?
  valid users = username @groupname # allow user/group to access the share

The special [homes] section automatically shares the users home directory under the following conditions:

If the path is not defined in the section, the home path of the user in /etc/passwd is used.
The browsable options has a different meaning in this section and specifies wheter the personal share should be listed. The usual browsable option is inherited from the [global] section.
You can use variables to change the path for example:

Example:

[homes]
  browsable = no
  writable = yes
  path = %H/smb

Client

yum install samba-client cifs-utils                       # Install client and tools
smbclient -L //server                                     # Show SMB shares
mkdir /targetdir                                          # Create mountpoint
mount.cifs -o username=jenkins //server/james /mountpoint # Mount manually
umount.cifs /mountpoint                                   # Unmount manually
vi /etc/fstab                                             # Mount on system start
vi /root/cifs/.username                                   # Create credential file
mount -a                                                  # Test fstab
init 0                                                    # Auto mount

Example for /etc/fstab:

//server/share  /targetdir cifs  credentials=/root/cifs/.username  0 0

Example for /root/cifs/.username:

username=username
password=password

FTP

Configuration is done in /etc/vsftpd/vsftpd.conf

Example:

anonymous_enable=NO
local_enable=YES
write_enable=YES
local_umask=022
dirmessage_enable=YES
xferlog_enable=YES
connect_from_port_20=YES
xferlog_std_format=YES
listen=NO
listen_ipv6=YES
pam_service_name=vsftpd
tcp_wrappers=YES
use_localtime=YES
dirlist_enable=YES
pasv_enable=YES
pasv_min_port=41361
pasv_max_port=65534
pasv_address=192.168.1.10

yum install vsftpd ftp    # Install server and client
systemctl start vsftpd    # Start service
systemctl enable vsftpd   # Start service on system start
vi /etc/vsftp/vsftpd.conf # Change config
systemctl restart vsftpd  # Load modified config

MySQL Server

yum install mariadb-server # Install server
systemctl start mariadb    # Start service
systemctl enable mariadb   # Start service on system start
mysql_secure_installation  # Run security wizard
mysql -u root -p           # Connect to MySQL

Other

Stuff probably relevant for the exam.

Exam result file: /sbin/result

useradd username [-G wheel]             # add user and allow sudo
usermod -aG wheel username              # allow sudo for existing user
sudo -i                                 # enter interactive root session
yum install man-pages                   # install man pages
timedatectl set-timezone Europe/Zurich  # change time zone
timedatectl set-local-rtc 0             # set RTC to use UTC
yum check-update                        # check for updates
yum update                              # install all available updates
localectl                               # show locale configuration
localectl list-keymaps | grep ch        # show available keymaps
localectl set-keymap ch                 # set keymap to ch (doesn't affect ssh session)
localectl set-x11-keymap ch             # set keymap to ch (doesn't affect ssh session)