Timer units can only be defined within a service unit’s file.

The command executed by the timer is specified in the timer unit’s [Cmd] section.

A dedicated system service, systemd-cron, handles the execution of timer units.

Timer units only exist in the system scope and are not available for users.

Each systemd timer unit controls a specific systemd service unit.

export LANGUAGE=“pt”

export LC_MESSAGES=“pt”

export UI_MESSAGES=“pt”

export MESSAGE=“pt”

export ALL_MESSAGES=“pt”

UID have precedence over GIDs, therefore the user is available while the group doesn’t.

The user as well as the group are not available to avoid ambiguity due to the ID conflict.

UIDs and GIDs are independent of each other, therefore the user as well as the group are still available.

The user is the only member of the group, even if the group configuration contains other members.

GIDs have precedence over UIDs, therefore the group is available while the user isn’t.

getargs

lsenv

ls

env

lsshell

lpd

lpr

lp

lsq

lpq

The timestamp of the user’s last login

The user’s private SSH keys

The hashed password of the user

The numerical user ID (UID)

The path to the user’s home directory

cat

convert

dd

iconv

utf2utf

It converts bitmap images from one format to another such as PNG to JPEG.

It verifies that the root directory tree complies to all conventions from the Filesystem Hierarchy Standard (FHS).

It displays additional meta information from icon files ending in .ico.

It changes the mode of an inode in the ext4 file system.

It converts files from one character encoding to another.

groupdel

groupmod

groups

groupedit

The number of days since January 1, 1970 after which the user's account will no longer be accessible.

The number of days since January 1, 1970 after which the password can change.

The number of days since January 1, 1970 since the password was last changed.

The maximum number of days during which a password is valid.

The number of days of inactivity after a password has expired before the account is locked.

The user bob is removed from the system's user database.

The user bob's home directory is removed.

The locate database is updated to drop files owned by bob.

All files owned by bob are remove from all mounted filesystems.

In case bob was the last member of a group, that group is deleted.

*

-

s

x

Postfix

Procmail

Sendmail

Exim

SMTPd

It fetches new emails from a remote server using POP3 or IMAP.

It is a multi-user mailing list manager.

It is a proprietary tool contained only in the qmail MTA.

It queries the mail queue of the local MTA.

It is a command-line based tool for reading and writing emails.

rotatelog

striplog

syslogd --rotate

logrotate

logger

Because the conversion of Unix timestamps to local time relies on the time zone configuration.

Because the time zone is saved as part of the modification times of files and cannot be changed after a file is created.

Because the environment variables LANG and LC_MESSAGES are, by default, set according to the time zone.

Because NTP chooses servers nearby based on the configured time zone.

/etc/alias

~/.alias

/etc/mail/forwarders

~/.forward

~/.vacation

It is the primary management command for the NTP time server.

It updates the local system's date (i.e. day, month and year) but not the time (i.e. hours, minutes, seconds).

It queries one or more NTP time servers and adjusts the system time accordingly.

It sends the local system time to one or many remote NTP time servers for redistribution.

It can be used by any user to set the user clock independently of the system clock.

cups-devices.conf

snmp.conf

printcap.conf

printers.conf

cupsd.conf

export $VARIABLE

export VARIABLE

set $VARIABLE

set VARIABLE

env VARIABLE

mydate="$(date)"

mydate="exec date"

mydate="$((date))"

mydate="date"

mydate="${date}"

for

loop

until

while

/etc/bashrc

/etc/profile

~/.bash_profile

/etc/.bashrc

CASE

FROM

WHERE

IF

env

set

env -a

echo $ENV

LIMIT

FROM

WHERE

IF

A B

A B C

A C

B C

C B A

A continuous stream of numbers increasing in increments of 10 until stopped.

The numbers 1 through 10 with one number per line.

The numbers 0 through 9 with one number per line.

The number 10 to standard output.

umask

usermod

ulimit

passwd

chage

sshd

ssh-agent

ssh-keygen

ssh-add

/etc/ssh/sshd_config

/etc/ssh/ssh

/etc/ssh/server

/etc/ssh/ssh_config

/etc/ssh/ssh_server

It must be sent by any SSH client in addition to a user key in order to identify the client's host.

It provides the server's identity information to connecting SSH clients.

It is the root key by which all user SSH keys must be signed.

It authenticates any user that logs into a remote machine from the key's host.

It is used by system services like cron, syslog or a backup job to automatically connect to remote hosts.

dig

netstat

ping

route

traceroute

With IPv6, the TCP port numbers of most services have changed.

IPv6 no longer supports broadcast addresses.

IPv4 addresses can be used without any change with IPv6.

IPv6 no longer supports multicast addresses.

For IPv6, UDP and TCP have been replaced by the Rapid Transmission Protocol RTP.

Port 23 is the well known port for the telnet service which is a plain text protocol that should no longer be used.

Port 23 is the well known port for the SSH service which provides secure logins.

Port 23 is the well known port for the rlogin service which is SSL secured by default.

Port 23 is the well known port for the system login services which are encrypted when the user runs the starttls command in his login shell.

resolv

dns

remote

files

hosts

dnsq

dig

hostname

dnslookup

zoneinfo

Set a network interface active or inactive.

Specify the kernel module to be used with a network interface.

Allow regular users to change the network configuration of a network interface.

Change the netmask used on a network interface.

Specify which network services are available on a network interface.

When KDM starts

When a user's X session exits

When KDM crashes

When X is restarted

When X crashes

It reads text displayed on the screen to blind or visually impaired people.

It reads the parameters of the attached monitors and creates an appropriate X11 configuration.

It displays lines and markers to help people use speed reading techniques.

It manages and displays files that contain e-books.

xcd

xcdepth

xwininfo

xcolordepth

cat /etc/X11