Cvičení: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14.
Toto a následující cvičení se zaměřuje na správcovské úlohy. Obsahuje absolutní minimum potřebné pro správu vašeho Linuxového stroje. Ale obsahuje i pár bonusů, které se mohou hodit do shellových skriptů.
Většina věcí zde popsaných vyžaduje vlastní Linuxovou instalaci, protože potřebují práva superuživatele. Nebudou fungovat na sdílených strojích v IMPAKTu.
Nezapomeňte, že Čtení před cvičením je povinné a je z něj kvíz, který musíte vyplnit před cvičením.
sudo
To try sudo, you can try running fdisk -l. Generally, fdisk is a tool
for partitioning disks. With -l, it reads information about all disks on
your system and displays information about partitions on them.
Without sudo, it will likely show only the following message:
fdisk: cannot open /dev/sda: Permission denied
Running it with sudo displays the actual information.
sudo fdisk -l
Disk /dev/sda: 480 GiB, 515396075520 bytes, 1006632960 sectors
Disk model: QEMU HARDDISK
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xdc505942
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 2099199 2097152 1G 83 Linux
/dev/sda2 2099200 18620415 16521216 7.9G 82 Linux swap / Solaris
/dev/sda3 18620416 1006632959 988012544 471.1G 83 Linux
Note that sudo typically asks for a password (though that can be
configured). It is the password of the current user, not root’s. (If you
want to authenticate using the root’s password, you can use su instead.)
dnf (tj. správce balíčků pro Fedoru)
Note: Fedora used to have yum as the package manager and it can be found
in many tutorials on the Internet (even in quite recent ones).
It is considered obsolete and you should better avoid it.
If you are used to yum from older versions of Fedora or from other RPM-based
distributions, you will find dnf very similar and in many situations faster
than yum.
The package manager for Fedora is called DNF.
Note: if you decided to use a different distribution, you will need to update the commands to match your system. Generally, the operations would be rather similar but we cannot provide a tutorial for every package manager here.
You can use the search command to get a list of packages which match the
given name. Note that searching is not a privileged operation, hence it
does not require sudo.
dnf search arduino
dnf search atool
Note that searching for a very generic term can yield hundreds of results.
The output is in the following format:
atool.noarch : A perl script for managing file archives of various types
ratools.x86_64
The .noarch and .x86_64 strings describe the nature of the package.
noarch usually refers to a data package or package using interpreted
languages, while .x86_64 denotes a package with binaries for the x86-64
architecture (e.g., written in C or Rust and then compiled to machine code).
To install a software package, run dnf with the install subcommand,
giving it the name of the package to install. Here, sudo is needed as we
are modifying the system.
sudo dnf install atool
Some applications are not a part of any software repository, but you can still download them in a format understandable by your package manager. That is a better situation than installing the files manually, because your package manager knows about the files (although it cannot upgrade it automatically). One such example is the Zoom client which has to be installed like this:
sudo dnf install "https://zoom.us/client/latest/zoom_x86_64.rpm"
To upgrade the whole system, simply run sudo dnf upgrade. DNF will ask
for confirmation and then upgrade all available packages.
Note that unlike on other systems, you can always choose when to upgrade. The system will never reboot the machine for you or display a message about needed restart, unless you explicitly ask for it.
If you want to install a whole group of packages, you can use dnf grouplist to view their list and sudo dnf install @GROUP_NAME to install
it.
The commands above contain the basics for maintaining your Fedora installation with respect to package management. The following links provide more information. The official Wiki page is a good source of information if you already know the system a bit.
For beginners, this guide about DNF and this tutorial are probably a better starting point.
Práce s procesy
Execute ps -ef --forest to view all process on your machine. Because of
your graphical interface, the list will be probably quite long.
Practically, a small server offering web pages, calendar and SSH access can have about 80 processes, for a desktop running Xfce with browser and few other applications, the number will rise to almost 300 (this really depends a lot on the configuration but it is a ballpark estimate). About 50–60 of these are actually internal kernel threads. In other words, a web/calendar server needs about 20 “real” processes, a desktop about 200 of them :-).
You can view the same information with htop. You can also easily configure
it to display information about your system like amount of free memory or
CPU usage.
It can look like this (this one from a 32G/24CPU machine):
Date & Time: 2022-03-03 10:07:26 Uptime: 50 days, 20:55:31
1[|| 4.5%] 7[||| 5.0%] 13[|||| 8.9%] 19[| 3.2%]
2[| 1.3%] 8[| 1.9%] 14[|| 3.8%] 20[|| 4.5%]
3[|| 5.1%] 9[| 0.6%] 15[|||||| 17.8%] 21[||| 5.7%]
4[ 0.0%] 10[ 0.0%] 16[|| 4.4%] 22[|| 4.5%]
5[|| 4.5%] 11[|| 5.1%] 17[|| 4.5%] 23[||| 4.5%]
6[ 0.0%] 12[|| 5.1%] 18[|| 4.5%] 24[|| 4.5%]
Mem[||||||||||||||||||||||||||||||| 11.2G/31.3G] Tasks: 252, 1803 thr, 341 kthr; 1 running
Swp[|||||||||||||||||||||||||||||||||||||||||||||||| 3.06G/4.00G] Load average: 0.59 1.19 1.13
PID USER PRI NI VIRT RES SHR S CPU%+MEM% TIME+ Command
966 vojta 20 0 448M 9692 5824 S 0.0 0.0 0:01.01 | | | `-
3130 vojta 20 0 85856 3360 3356 S 0.0 0.0 0:00.03 | | `- /usr/lib/eclipse/eclipse -data /home/vojta/mff/eclips
3174 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 2h57:34 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.requi
3175 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 1h12:37 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3176 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 2:44.29 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3177 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 0:00.04 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3178 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 0:00.96 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3179 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 0:00.23 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3180 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 8:46.39 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3181 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 3:53.24 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3182 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 0:01.85 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3183 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 0:01.62 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3184 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 0:00.00 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3185 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 0:00.00 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3186 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 1:55.30 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3187 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 0:20.61 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3188 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 0:01.52 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3189 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 0:01.68 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
3193 vojta 20 0 20.8G 1274M 28068 S 0.0 4.0 17:29.22 | | | `- /usr/lib/jvm/java-11-openjdk/bin/java -Dosgi.re
F1 Help F2 Setup F3 Search F4 Filter F5 List F6 SortBy F7 Nice - F8 Nice + F9 Kill F10 Quit
Similar to MidnightCommander, function keys perform the most important actions and the help is visible in the bottom bar.
Signály
Open two terminals now.
Run the following command in the first one.
sleep 999
In the second terminal, find the PID of this process. Hint.
From the second terminal, we will now kill the program.
kill THE_PID
Looking back at the first terminal, you should see the following message.
Terminated (SIGTERM).
Start the sleep command again, but send KILL this time.
kill -9 THE_PID
The message probably changed to Killed and the process was terminated as
well.
Reakce na signály v Pythonu
Your program would typically react to TERM (the default “soft”
termination), INT (Ctrl-C from the keyboard) and perhaps to USR1 or
USR2 (the only user-defined signals). System daemons (non-interactive
programs handling system services) often react to HUP by reloading their
configuration.
The following Python program reacts to Ctrl-C by terminating (imports
omitted):
# Actual signal callback
def on_signal(signal_number, frame_info):
print("")
print("Caught signal {} ({})".format(signal_number, frame_info))
sys.exit()
def main():
# Setting signal callback
signal.signal(signal.SIGINT, on_signal)
while True:
time.sleep(0.5)
print("Hit Ctrl-C...")
if __name__ == '__main__':
main()
Exercise: write a program that tries to print all prime numbers. When terminating, it stores the highest number found so far and on the next invocation, it continues from there. Solution.
Reacting to a signal in a shell script is also possible using the trap
command. Note that a typical action for a signal handler in a shell script
is clean-up of temporary files.
#!/bin/bash
set -ueo pipefail
on_interrupt() {
echo "Interrupted, terminating ..." >&2
exit 17
}
on_exit() {
echo "Cleaning up..." >&2
rm -f "$MY_TEMP"
}
MY_TEMP="$( mktemp )"
trap on_interrupt INT TERM
trap on_exit EXIT
echo "Running as $$"
counter=1
while [ "$counter" -lt 10 ]; do
date "+%Y-%m-%d %H:%M:%S | Waiting for Ctrl-C (loop $counter) ..."
echo "$counter" >"$MY_TEMP"
sleep 1
counter=$(( counter + 1 ))
done
The command trap receives as the first argument the command to execute on
the signal. Other arguments list the signals to react to. Note that a
special signal EXIT means normal script termination. Hence, we do not
need to call on_exit after the loop terminates.
Using - instead of the handler causes the respective handler to be set to
default. Without this reset in on_exit, the handler would be called twice
after the user would hit Ctrl-C (first for INT caused by Ctrl-C itself
and then by the explicit call to exit).
From now on, your shell scripts shall always include a signal handler for clean-up of temporary files.
Note the use of $$ which prints the current PID.
Alternatively, you can use pgrep <program_name> to find a PIDs for running programs.
Similarly, you can use killall to kill processes by name (but be careful as with great power comes great responsibility).
Consult the manual pages for more details.
Run the above script, note its PID and run the following in a new terminal.
kill THE_PID_PRINTED_BY_THE_ABOVE_SCRIPT
The script was terminated and the clean-up routine was called. Compare with
situation when you comment-out the trap command.
Run the script again but pass -9 to kill to specify that you want to
send signal nine (i.e., KILL).
What happened? Answer.
While signals are a rudimentary mechanism, which passes binary events with
no additional data, they are the primary way of process control in Linux.
(If you need a richer communication channel, you can use
D-Bus instead.)
Reasonable reaction to basic signals is a must for server-style applications
(e.g., a web server should react to TERM by completing outstanding
requests without accepting new connections, and terminating afterwards). It
shell scripts, it is considered good manners to always clean up temporary
files.
Správa uživatelských účtů
So far, we used /etc/passwd for getting information about accounts
directly. In practice, things can be more complicated as the information
may come from different sources. For example, in IMPAKT labs, the
information is somehow fetched from the CAS.
Generally, there can be multiple sources of user accounts. Thus, it is
better to use getent to query them all instead of relying on /etc/passwd
only:
getent passwd YOUR_GITLAB_LOGIN
You can specify group instead of passwd to query groups, too.
Note that the output is actually in the same format as /etc/passwd. This
is on purpose.
Without the login parameter, the command will (usually) list all accounts.
How many student accounts are on the machine u-pl17.ms.mff.cuni.cz?
Hint.
Solution.
Vytváření nových účtů
Creating a new account in Linux is rather straightforward. The utility
useradd creates a new user by adding the appropriate entry into
/etc/passwd and by creating a home directory.
Technically, nothing more needs to be done: the entry in /etc/passwd means
that user id (numerical) is assigned to a human-readable name, creation of
the home directory ensures that the user has a writable directory to start
with.
Practically, one can create a new user by editing /etc/passwd manually,
but it is not recommended.
Hodnocené úlohy (deadline: 24. dubna)
09/passwd.txt (20 bodů)
Na stroji linux.ms.mff.cuni.cz získejte informace o vašem účtu.
Vložte odpovídající řádek o vašem účtu z passwd databáze do souboru
09/passwd.txt.
Automatické testy nekontrolují skutečnou správnost, ale jen formát odpovědi.
Hint.09/signals.txt (25 bodů)
Spusťte příkaz nswi177-signals na linux.ms.mff.cuni.cz.
Budete mu muset ve správném pořadí poslat různé signály, abyste splnili tuto úlohu.
Program vás během provede: vždy vytiskne, jaký signál máte poslat.
Zkopírujte poslední řádek výstupu (budou to dvě čísla) z tohoto programu do
09/signals.txt.
Tato úloha je pouze částečně testovaná automatickými testy.
09/countdown.sh (35 bodů)
Napište skript, který obdrží jeden parametr: počet sekund (nezáporné číslo) k odpočtu a každou vteřinu vypíše, kolik času zbývá.
Můžete předpokládat, že program bude vždy korektně spuštěn.
Pokud uživatel stiskne Ctrl-C za běhu (nebo pošle TERM signál), skript
se ukončí se slovem Aborted a s exit kódem 17.
Příklady:
$ ./countdown.sh 10
10
9
8
7
6
5
4
3
2
1
Každá řádka se objeví o vteřinu později než předchozí. Využijte sleep 1 na
čekání mezi tisky a před ukončením.
První číslo se tiskne ihned, zatímco po poslední řádce s 1 má program
ještě vteřinku počkat, než skončí.
Prosím, uvědomte si, že skript bude o něco pomalejší, protože i spuštění
sleepu něco trvá, ale to nám nevadí.
Ukázkové spuštění, kde uživatel stiskne Ctrl-C (^C označuje místo stisku
Ctrl-C a nejde o výstup z program).
Zpráva FAILED je vytištěna mimo program a jen zdůrazňuje nenulový
návratový kód programu.
$ ./countdown.sh 5 || echo "FAILED"
5
4
^C
Aborted
FAILED
09/dnf.txt (20 bodů)
Jaká verze balíčku (nikoliv verze programu!) msim-git je nainstalovaná na
linux.ms.mff.cuni.cz?
Tato úloha je pouze částečně testovaná automatickými testy.
Použijte příkaz dnf help, abyste zjistili seznam podpříkazů a našli ten
správný pro vyřešení úlohy (budete potřebovat jeden z hlavních příkazů:
main commands).
Je v pořádku napsat jen verzi nebo verzi včetně vydání (version, release).
Učební výstupy
Znalosti konceptů
Znalost konceptů znamená, že rozumíte významu a kontextu daného tématu a jste schopni témata zasadit do většího rámce. Takže, jste schopni …
-
vysvětlit jaké druhy účtů existují na Linuxu a čím se liší
-
vysvětlit, proč jsou potřeba mechanismy jako je sudo a proč jsou lepší než přímá práce pod superuživatelem (root)
-
vysvětlit co je to balíček (package) a jak se na Linuxu používá
-
vysvětlit, co je to signál (u procesů)
Praktické dovednosti
Praktické dovednosti se obvykle týkají použití daných programů pro vyřešení různých úloh. Takže, dokážete …
-
použít
getentna získání informací o existujících účtech -
použít
useraddpro vytvoření nového uživatelského účtu -
použít správce balíčků (package manager) pro instalaci a odinstalaci balíčků a upgrade celého systému
-
použít
psapgrep -
použít
htop -
poslat signál běžícímu procesu