Preface
Hello everyone! My name is Sasha, and I have been involved in backend testing (Linux services and APIs) for over six years. The idea for this article came to me after yet another request from a fellow tester asking for reading recommendations on Linux commands before an interview. Typically, a QA engineer candidate is expected to know the basic commands (provided, of course, that the work involves Linux), but how can someone with little or no experience in Linux know which commands to study during interview preparation?
Therefore, even though this topic has been covered many times before, I've decided to write another article titled 'Linux for Beginners' and list the basic commands that one should know before any interview in a department (or company) that uses Linux. I thought about which commands and utilities I use most frequently, gathered feedback from colleagues, and compiled everything into one article. The article is roughly divided into three parts: first, a brief overview of the basics of input and output in the Linux terminal, then an overview of the most basic commands, and in the third part, a description of how to solve typical tasks in Linux.
Each command has many options, but not all of them will be listed here. You can always enter `man` or `--help` to learn more about a specific command.
Example:
[user@testhost ~]$ mkdir --help Usage: mkdir [OPTION]... DIRECTORY... Create the DIRECTORY(ies), if they do not already exist. Mandatory arguments to long options are mandatory for short options too. -m, --mode=MODE set file mode (as in chmod), not a=rwx - umask -p, --parents no error if existing, make parent directories as needed -v, --verbose print a message for each created directory -Z set SELinux security context of each created directory to the default type --context[=CTX] like -Z, or if CTX is specified then set the SELinux or SMACK security context to CTX --help display this help and exit --version output version information and exit GNU coreutils online help: For complete documentation, run: info coreutils 'mkdir invocation'
If any command takes too long to execute, you can terminate it by pressing Ctrl+C (this sends a signal to the process SIGINT).
A bit about command output
When a process is launched in Linux, three standard data streams are created for that process: stdin, stdout and stderr. They are numbered 0, 1, and 2, respectively. However, we are currently interested in stdout and, to a lesser extent, stderr. From their names, it is easy to guess that stdout is used for outputting data, while stderr — for outputting error messages. By default, when a command is run in Linux stdout and stderr it displays all information on the console, however, if the command output is large, it may be convenient to redirect it to a file. This can be done, for example, like this:
[user@testhost ~]$ man signal > man_signal
If we display the contents of the file man_signal, we will see that it is identical to what would have been displayed from a simple execution of the command `man signal`.
The redirection operation `>` by default uses stdout. You can specify redirection stdout explicitly: `1>`. Similarly, you can specify the redirection stderr: `2>`. These operations can be combined to separate the normal output of the command from the error messages:
[user@testhost ~]$ man signal 1> man_signal 2> man_signal_error_log
You can redirect and stdout, and stderr into one file as follows:
[user@testhost ~]$ man signal > man_signal 2>&1
The redirection operation `2>&1` means redirection stderr to the same place that stdout.
Another handy tool for working with input-output (or rather, it is a convenient means of inter-process communication) is pipe (or pipeline). Pipelines are often used to connect multiple commands: stdout the output of one command is redirected to stdin the next one, and so on in a chain:
[user@testhost ~]$ ps aux | grep docker | tail -n 2
root 1045894 0.0 0.0 7512 3704 ? Sl 16:04 0:00 docker-containerd-shim -namespace moby -workdir /var/lib/docker/containerd/daemon/io.containerd.runtime.v1.linux/moby/2fbfddaf91c1bb7b9a0a6f788f3505dd7266f1139ad381d5b51ec1f47e1e7b28 -address /var/run/docker/containerd/docker-containerd.sock -containerd-binary /usr/bin/docker-containerd -runtime-root /var/run/docker/runtime-runc
531 1048313 0.0 0.0 110520 2084 pts/2 S+ 16:12 0:00 grep --color=auto docker
Basic Linux commands
pwd
Display the current (working) directory.
[user@testhost ~]$ pwd
/home/user
date
Display the current date and system time.
[user@testhost ~]$ date
Mon Dec 16 13:37:07 UTC 2019
[user@testhost ~]$ date +%s
1576503430
w
This command shows who is logged in to the system. It also displays uptime and LA (load average) on the screen.
[user@testhost ~]$ w
05:47:17 up 377 days, 17:57, 1 user, load average: 0.00, 0.01, 0.05
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
user pts/0 32.175.94.241 05:47 2.00s 0.01s 0.00s w
ls
Display the contents of a directory. If no path is provided, the contents of the current directory will be shown.
[user@testhost ~]$ pwd
/home/user
[user@testhost ~]$ ls
qqq
[user@testhost ~]$ ls /home/user
qqq
[user@testhost ~]$ ls /
bin boot cgroup dev etc home lib lib64 local lost+found media mnt opt proc root run sbin selinux srv swap sys tmp usr var
Personally, I often use the options -l (long listing format — output in a column with additional information about files), -t (sorting by file/directory modification time) and -r (reverse sorting — together with -t the most "recent" files at the bottom):
[user@testhost ~]$ ls -ltr /
total 4194416
drwxr-xr-x 2 root root 4096 Jan 6 2012 srv
drwxr-xr-x 2 root root 4096 Jan 6 2012 selinux
drwxr-xr-x 2 root root 4096 Jan 6 2012 mnt
drwxr-xr-x 2 root root 4096 Jan 6 2012 media
drwx------ 2 root root 16384 Oct 1 2017 lost+found
drwxr-xr-x 2 root root 4096 Oct 1 2017 local
drwxr-xr-x 13 root root 4096 Oct 1 2017 usr
drwxr-xr-x 11 root root 4096 Apr 10 2018 cgroup
drwxr-xr-x 4 root root 4096 Apr 10 2018 run
-rw------- 1 root root 4294967296 Sep 10 2018 swap
dr-xr-xr-x 10 root root 4096 Dec 13 2018 lib
drwxr-xr-x 6 root root 4096 Mar 7 2019 opt
drwxr-xr-x 20 root root 4096 Mar 19 2019 var
dr-xr-xr-x 10 root root 12288 Apr 9 2019 lib64
dr-xr-xr-x 2 root root 4096 Apr 9 2019 bin
dr-xr-xr-x 4 root root 4096 Apr 9 2019 boot
dr-xr-xr-x 2 root root 12288 Apr 9 2019 sbin
dr-xr-xr-x 3229 root root 0 Jul 2 10:19 proc
drwxr-xr-x 34 root root 4096 Oct 28 13:27 home
drwxr-xr-x 93 root root 4096 Oct 30 16:00 etc
dr-xr-x--- 11 root root 4096 Nov 1 13:02 root
dr-xr-xr-x 13 root root 0 Nov 13 20:28 sys
drwxr-xr-x 16 root root 2740 Nov 26 08:55 dev
drwxrwxrwt 3 root root 4096 Nov 26 08:57 tmp
There are 2 special directory names: "." and "..". The first refers to the current directory, and the second refers to the parent directory. These can be useful in various commands, especially, ls:
[user@testhost home]$ pwd
/home
[user@testhost home]$ ls ..
bin boot cgroup dev etc home lib lib64 local lost+found media mnt opt proc root run sbin selinux srv swap sys tmp usr var
[user@testhost home]$ ls ../home/user/
qqq
There is also a useful option for viewing hidden files (starting with ".«) — -a:
[user@testhost ~]$ ls -a
. .. 1 .bash_history .bash_logout .bash_profile .bashrc .lesshst man_signal man_signal_error_log .mongorc.js .ssh temp test .viminfo
You can also use the option -h — output in a human-readable format (note the file sizes):
[user@testhost ~]$ ls -ltrh
total 16K
-rwxrwx--x 1 user user 31 Nov 26 11:09 temp
-rw-rw-r-- 1 user user 6.0K Dec 3 16:02 1
drwxrwxr-x 2 user user 4.0K Dec 4 10:39 test
cd
Change the current directory.
[user@testhost ~]$ pwd
/home/user
[user@testhost ~]$ cd /home/
[user@testhost home]$ pwd
/home
If no directory name is provided as an argument, the environment variable $HOME, that is, the home directory, will be used. It can also be convenient to use `~` — a special character meaning $HOME:
[user@testhost etc]$ pwd
/etc
[user@testhost etc]$ cd ~/test/
[user@testhost test]$ pwd
/home/user/test
mkdir
Create a directory.
[user@testhost ~]$ mkdir test
[user@testhost ~]$ ls -ltr
total 38184
-rw-rw-r-- 1 user user 39091284 Nov 22 14:14 qqq
drwxrwxr-x 2 user user 4096 Nov 26 10:29 test
Sometimes, it is necessary to create a specific directory structure: for example, a directory inside another directory that does not exist. To avoid repeatedly typing mkdir, you can use the option -p — it allows you to create all the missing directories in the hierarchy. Also with this option mkdir it won't return an error if the directory exists.
[user@testhost ~]$ ls
qqq test
[user@testhost ~]$ mkdir test2/subtest
mkdir: cannot create directory ‘test2/subtest’: No such file or directory
[user@testhost ~]$ mkdir -p test2/subtest
[user@testhost ~]$ ls
qqq test test2
[user@testhost ~]$ ls test2/
subtest
[user@testhost ~]$ mkdir test2/subtest
mkdir: cannot create directory ‘test2/subtest’: File exists
[user@testhost ~]$ mkdir -p test2/subtest
[user@testhost ~]$ ls test2/
subtest
rm
Delete the file.
[user@testhost ~]$ ls
qqq test test2
[user@testhost ~]$ rm qqq
[user@testhost ~]$ ls
test test2
Option -r allows recursively removing directories along with all their contents, the option -f allows ignoring errors during deletion (for example, about a nonexistent file). These options allow, roughly speaking, to guarantee the removal of the entire hierarchy of files and directories (if the user has the rights), hence, they must be used with caution (a classic joke example — “rm -rf /«, under certain circumstances, will delete not just the whole system, but a significant number of important files necessary for its functionality).
[user@testhost ~]$ ls
test test2
[user@testhost ~]$ ls -ltr test2/
total 4
-rw-rw-r-- 1 user user 0 Nov 26 10:40 temp
drwxrwxr-x 2 user user 4096 Nov 26 10:40 temp_dir
[user@testhost ~]$ rm -rf test2
[user@testhost ~]$ ls
test
cp
Copy a file or directory.
[user@testhost ~]$ ls
temp test
[user@testhost ~]$ cp temp temp_clone
[user@testhost ~]$ ls
temp temp_clone test
This command also has options -r and -f, which can be used to guarantee the copying of the directory hierarchy and folders to another location.
mv
Move or rename a file or directory.
[user@testhost ~]$ ls -ltr
total 4
drwxrwxr-x 2 user user 4096 Nov 26 10:29 test
-rw-rw-r-- 1 user user 0 Nov 26 10:45 temp
-rw-rw-r-- 1 user user 0 Nov 26 10:46 temp_clone
[user@testhost ~]$ ls test
[user@testhost ~]$ mv test test_renamed
[user@testhost ~]$ mv temp_clone test_renamed/
[user@testhost ~]$ ls
temp test_renamed
[user@testhost ~]$ ls test_renamed/
temp_clone
cat
Display the contents of a file (or files).
[user@testhost ~]$ cat temp
Content of a file.
Lalalala...
It's also worth paying attention to the commands head (to display n the first lines or bytes of a file) and tail (more on that later).
tail
Output n the last lines or bytes of a file.
[user@testhost ~]$ tail -1 temp
Lalalala...
A very useful option is -f — it allows displaying new data in the file in real-time.
less
Sometimes a text file is too large, and it’s inconvenient to display it with the command cat. In such cases, it can be opened with the command less: the file will be displayed in parts, navigation through these parts will be available, search and other simple functionalities.
[user@testhost ~]$ less temp
It might also be convenient to use an option less with a pipeline (pipe):
[user@testhost ~]$ grep "ERROR" /tmp/some.log | less
ps
Display the list of processes.
[user@testhost ~]$ ps
PID TTY TIME CMD
761020 pts/2 00:00:00 bash
809720 pts/2 00:00:00 ps
I usually use the BSD options "aux" — to display all processes in the system (since there can be many processes, I only output the first 5 of them, using a pipeline (pipe) and the command head):
[user@testhost ~]$ ps aux | head -5
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.0 0.0 19692 2600 ? Ss Jul02 0:10 /sbin/init
root 2 0.0 0.0 0 0 ? S Jul02 0:03 [kthreadd]
root 4 0.0 0.0 0 0 ? I< Jul02 0:00 [kworker/0:0H]
root 6 0.0 0.0 0 0 ? I< Jul02 0:00 [mm_percpu_wq]
Many also use the BSD options "axjf", which allows displaying the process tree (I trimmed part of the output for demonstration):
[user@testhost ~]$ ps axjf
PPID PID PGID SID TTY TPGID STAT UID TIME COMMAND
0 2 0 0 ? -1 S 0 0:03 [kthreadd]
2 4 0 0 ? -1 I< 0 0:00 _ [kworker/0:0H]
2 6 0 0 ? -1 I< 0 0:00 _ [mm_percpu_wq]
2 7 0 0 ? -1 S 0 4:08 _ [ksoftirqd/0]
...
...
...
1 4293 4293 4293 tty6 4293 Ss+ 0 0:00 /sbin/mingetty /dev/tty6
1 532967 532964 532964 ? -1 Sl 495 0:00 /opt/td-agent/embedded/bin/ruby /usr/sbin/td-agent --log /var/log/td-agent/td-agent.log --use-v1-config --group td-agent --daemon /var/run/td-agent/td-agent.pid
532967 532970 532964 532964 ? -1 Sl 495 803:06 _ /opt/td-agent/embedded/bin/ruby /usr/sbin/td-agent --log /var/log/td-agent/td-agent.log --use-v1-config --group td-agent --daemon /var/run/td-agent/td-agent.pid
1 537162 533357 532322 ? -1 Sl 0 5067:43 /usr/bin/dockerd --default-ulimit nofile=262144:262144 --dns=172.17.0.1
537162 537177 537177 537177 ? -1 Ssl 0 4649:28 _ docker-containerd --config /var/run/docker/containerd/containerd.toml
537177 537579 537579 537177 ? -1 Sl 0 4:48 | _ docker-containerd-shim -namespace moby -workdir /var/lib/docker/containerd/daemon/io.containerd.runtime.v1.linux/moby/0ee89b20deb3cf08648cd92e1f3e3c661ccffef7a0971
537579 537642 537642 537642 ? -1 Ss 1000 32:11 | | _ /usr/bin/python /usr/bin/supervisord -c /etc/supervisord/api.conf
537642 539764 539764 537642 ? -1 S 1000 0:00 | | _ sh -c echo "READY"; while read -r line; do echo "$line"; supervisorctl shutdown; done
537642 539767 539767 537642 ? -1 S 1000 5:09 | | _ php-fpm: master process (/etc/php73/php-fpm.conf)
539767 783097 539767 537642 ? -1 S 1000 0:00 | | | _ php-fpm: pool test
539767 783131 539767 537642 ? -1 S 1000 0:00 | | | _ php-fpm: pool test
539767 783185 539767 537642 ? -1 S 1000 0:00 | | | _ php-fpm: pool test
...
...
...
This command has many different options, so if you're using it actively, I recommend checking the documentation. For most cases, it's enough to just know “ps aux«.
kill
Send a signal to a process. By default, it sends the signal SIGTERM, which terminates the process.
[user@testhost ~]$ ps ux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
531 1027147 0.0 0.0 119956 4260 ? S 14:51 0:00 sshd: user@pts/1
531 1027149 0.0 0.0 115408 3396 pts/1 Ss 14:51 0:00 -bash
531 1027170 0.0 0.0 119956 4136 ? R 14:51 0:00 sshd: user@pts/2
531 1027180 0.0 0.0 115408 3564 pts/2 Ss 14:51 0:00 -bash
531 1033727 0.0 0.0 107960 708 pts/1 S+ 15:17 0:00 sleep 300
531 1033752 0.0 0.0 117264 2604 pts/2 R+ 15:17 0:00 ps ux
[user@testhost ~]$ kill 1033727
[user@testhost ~]$ ps ux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
531 1027147 0.0 0.0 119956 4260 ? S 14:51 0:00 sshd: user@pts/1
531 1027149 0.0 0.0 115408 3396 pts/1 Ss+ 14:51 0:00 -bash
531 1027170 0.0 0.0 119956 4136 ? R 14:51 0:00 sshd: user@pts/2
531 1027180 0.0 0.0 115408 3564 pts/2 Ss 14:51 0:00 -bash
531 1033808 0.0 0.0 117268 2492 pts/2 R+ 15:17 0:00 ps ux
Since a process may have signal handlers, kill it does not always result in the expected outcome — an immediate termination of the process. To “kill” a process for sure, you need to send it the signal SIGKILL. However, this may lead to data loss (for example, if the process needs to save some information to disk before terminating), so this command should be used carefully. The signal number SIGKILL is 9, so the short version of the command looks like this:
[user@testhost ~]$ ps ux | grep sleep
531 1034930 0.0 0.0 107960 636 pts/1 S+ 15:21 0:00 sleep 300
531 1034953 0.0 0.0 110516 2104 pts/2 S+ 15:21 0:00 grep --color=auto sleep
[user@testhost ~]$ kill -9 1034930
[user@testhost ~]$ ps ux | grep sleep
531 1035004 0.0 0.0 110516 2092 pts/2 S+ 15:22 0:00 grep --color=auto sleep
In addition to the ones mentioned, SIGTERM and SIGKILL there are many different signals, and you can easily find a list of them online. And don't forget that signals SIGKILL and SIGSTOP cannot be caught or ignored.
ping
Send an ICMP packet to the host ECHO_REQUEST.
[user@testhost ~]$ ping google.com
PING google.com (172.217.15.78) 56(84) bytes of data.
64 bytes from iad23s63-in-f14.1e100.net (172.217.15.78): icmp_seq=1 ttl=47 time=1.85 ms
64 bytes from iad23s63-in-f14.1e100.net (172.217.15.78): icmp_seq=2 ttl=47 time=1.48 ms
64 bytes from iad23s63-in-f14.1e100.net (172.217.15.78): icmp_seq=3 ttl=47 time=1.45 ms
64 bytes from iad23s63-in-f14.1e100.net (172.217.15.78): icmp_seq=4 ttl=47 time=1.46 ms
64 bytes from iad23s63-in-f14.1e100.net (172.217.15.78): icmp_seq=5 ttl=47 time=1.45 ms
^C
--- google.com ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4006ms
rtt min/avg/max/mdev = 1.453/1.541/1.850/0.156 ms
the net/http ping works until it is manually terminated. Therefore, the option may be useful -c — the number of packets after which ping it will terminate automatically. Another option that I sometimes use is -i, the interval between packet transmissions.
[user@testhost ~]$ ping -c 3 -i 5 google.com
PING google.com (172.217.5.238) 56(84) bytes of data.
64 bytes from iad30s07-in-f238.1e100.net (172.217.5.238): icmp_seq=1 ttl=47 time=1.55 ms
64 bytes from iad30s07-in-f14.1e100.net (172.217.5.238): icmp_seq=2 ttl=47 time=1.17 ms
64 bytes from iad30s07-in-f14.1e100.net (172.217.5.238): icmp_seq=3 ttl=47 time=1.16 ms
--- google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 10006ms
rtt min/avg/max/mdev = 1.162/1.295/1.551/0.181 ms
ssh
OpenSSH SSH client, allows you to connect to a remote host.
MacBook-Pro-User:~ user$ ssh user@11.11.22.22
Last login: Tue Nov 26 11:27:39 2019 from another_host
[user@testhost ~]$ hostname
testhost
There are many nuances in using SSH, and this client has many features, so if desired (or necessary) you can read more about this .
scp
Copying files between hosts (for this, the ssh).
[user@testhost ~]$ pwd
/home/user
[user@testhost ~]$ ls
temp test_renamed
[user@testhost ~]$ exit
logout
Connection to 11.11.22.22 closed.
MacBook-Pro-Aleksandr:~ user$ scp user@11.11.22.22:/home/user/temp Downloads/
temp 100% 31 0.2KB/s 00:00
MacBook-Pro-Aleksandr:~ user$ cat Downloads/temp
Content of a file.
Lalalala...
rsync
Also, for synchronizing directories between hosts, you can use rsync (-a — archive mode, allows you to copy all contents of a directory 'as is', -v — displays additional information in the console):
MacBook-Pro-User:~ user$ ls Downloads/user
ls: Downloads/user: No such file or directory
MacBook-Pro-User:~ user$ rsync -av user@testhost:/home/user Downloads
receiving file list ... done
user/
user/.bash_history
user/.bash_logout
user/.bash_profile
user/.bashrc
user/.lesshst
user/.mongorc.js
user/.viminfo
user/1
user/man_signal
user/man_signal_error_log
user/temp
user/.ssh/
user/.ssh/authorized_keys
user/test/
user/test/created_today
user/test/temp_clone
sent 346 bytes received 29210 bytes 11822.40 bytes/sec
total size is 28079 speedup is 0.95
MacBook-Pro-User:~ user$ ls -a Downloads/user
. .bash_history .bash_profile .lesshst .ssh 1 man_signal_error_log test
.. .bash_logout .bashrc .mongorc.js .viminfo man_signal temp
echo
Display a line of text.
[user@testhost ~]$ echo "Hello"
Hello
Here it is worth noting the options -n — do not end the line with a new line, and -e — enable interpretation of escape sequences with "".
[user@testhost ~]$ echo "tHellon"
tHellon
[user@testhost ~]$ echo -n "tHellon"
tHellon[user@testhost ~]$
[user@testhost ~]$ echo -ne "tHellon"
Hello
You can also use this command to display the values of variables. For example, in Linux, the exit code of the last completed command is stored in a special variable. $?, and this way you can find out what error occurred in the last executed application:
[user@testhost ~]$ ls # there will be no error
1 man_signal man_signal_error_log temp test
[user@testhost ~]$ echo $? # we get 0 — no error
0
[user@testhost ~]$ ls qwerty # there will be an error
ls: cannot access qwerty: No such file or directory
[user@testhost ~]$ echo $? # we get 2 — Misuse of shell builtins (according to Bash documentation)
2
[user@testhost ~]$ echo $? # the last echo executed without errors, we get 0
0
telnet
Client for the TELNET protocol. Used for communication with another host.
[user@testhost ~]$ telnet example.com 80
Trying 93.184.216.34...
Connected to example.com.
Escape character is '^]'.
GET / HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
Cache-Control: max-age=604800
Content-Type: text/html; charset=UTF-8
Date: Tue, 26 Nov 2019 11:59:18 GMT
Etag: "3147526947+gzip+ident"
Expires: Tue, 03 Dec 2019 11:59:18 GMT
Last-Modified: Thu, 17 Oct 2019 07:18:26 GMT
Server: ECS (dcb/7F3B)
Vary: Accept-Encoding
X-Cache: HIT
Content-Length: 1256
... here was the body of the response that I cut out by hand ...
If you need to use the TLS protocol (reminding you that SSL is outdated), then telnet it won't be suitable for these purposes. However, the openssl:
Example of using openssl with output response to a GET request
[user@testhost ~]$ openssl s_client -connect example.com:443
CONNECTED(00000003)
depth=2 C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert Global Root CA
verify return:1
depth=1 C = US, O = DigiCert Inc, CN = DigiCert SHA2 Secure Server CA
verify return:1
depth=0 C = US, ST = California, L = Los Angeles, O = Internet Corporation for Assigned Names and Numbers, OU = Technology, CN = www.example.org
verify return:1
---
Certificate chain
0 s:/C=US/ST=California/L=Los Angeles/O=Internet Corporation for Assigned Names and Numbers/OU=Technology/CN=www.example.org
i:/C=US/O=DigiCert Inc/CN=DigiCert SHA2 Secure Server CA
1 s:/C=US/O=DigiCert Inc/CN=DigiCert SHA2 Secure Server CA
i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert Global Root CA
2 s:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert Global Root CA
i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert Global Root CA
---
Server certificate
-----BEGIN CERTIFICATE-----
MIIHQDCCBiigAwIBAgIQD9B43Ujxor1NDyupa2A4/jANBgkqhkiG9w0BAQsFADBN
MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMScwJQYDVQQDEx5E
aWdpQ2VydCBTSEEyIFNlY3VyZSBTZXJ2ZXIgQ0EwHhcNMTgxMTI4MDAwMDAwWhcN
MjAxMjAyMTIwMDAwWjCBpTELMAkGA1UEBhMCVVMxEzARBgNVBAgTCkNhbGlmb3Ju
aWExFDASBgNVBAcTC0xvcyBBbmdlbGVzMTwwOgYDVQQKEzNJbnRlcm5ldCBDb3Jw
b3JhdGlvbiBmb3IgQXNzaWduZWQgTmFtZXMgYW5kIE51bWJlcnMxEzARBgNVBAsT
ClRlY2hub2xvZ3kxGDAWBgNVBAMTD3d3dy5leGFtcGxlLm9yZzCCASIwDQYJKoZI
hvcNAQEBBQADggEPADCCAQoCggEBANDwEnSgliByCGUZElpdStA6jGaPoCkrp9vV
rAzPpXGSFUIVsAeSdjF11yeOTVBqddF7U14nqu3rpGA68o5FGGtFM1yFEaogEv5g
rJ1MRY/d0w4+dw8JwoVlNMci+3QTuUKf9yH28JxEdG3J37Mfj2C3cREGkGNBnY80
eyRJRqzy8I0LSPTTkhr3okXuzOXXg38ugr1x3SgZWDNuEaE6oGpyYJIBWZ9jF3pJ
QnucP9vTBejMh374qvyd0QVQq3WxHrogy4nUbWw3gihMxT98wRD1oKVma1NTydvt
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-----END CERTIFICATE-----
subject=/C=US/ST=California/L=Los Angeles/O=Internet Corporation for Assigned Names and Numbers/OU=Technology/CN=www.example.org
issuer=/C=US/O=DigiCert Inc/CN=DigiCert SHA2 Secure Server CA
---
No client certificate CA names sent
Peer signing digest: SHA256
Server Temp Key: ECDH, P-256, 256 bits
---
SSL handshake has read 4643 bytes and written 415 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES128-GCM-SHA256
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
Protocol : TLSv1.2
Cipher : ECDHE-RSA-AES128-GCM-SHA256
Session-ID: 91950DC50FADB57BF026D2661E6CFAA1F522E5CA60D2310E106EE0E0FD6E70BD
Session-ID-ctx:
Master-Key: 704E9145253EEB4E9DC47E3DC6725D296D4A470EA296D54F71D65E74EAC09EB096EA1305CBEDD9E7020B8F72FD2B68A5
Key-Arg : None
Krb5 Principal: None
PSK identity: None
PSK identity hint: None
TLS session ticket lifetime hint: 7200 (seconds)
TLS session ticket:
0000 - 68 84 4e 77 be e3 f5 00-49 c5 44 40 53 4d b9 61 h.Nw....I.D@SM.a
0010 - c9 fe df e4 05 51 d0 53-ae cf 89 4c b6 ef 6c 9e .....Q.S...L..l.
0020 - fe 12 9a f0 e8 e5 4e 87-42 89 ac af ca e5 4a 85 ......N.B.....J.
0030 - 38 08 26 e3 22 89 08 b5-62 c0 8b 7e b8 05 d3 54 8.&."...b..~...T
0040 - 8c 24 91 a7 b4 4f 79 ad-36 59 7c 69 2d e5 7f 62 .$...Oy.6Y|i-..b
0050 - f6 73 a3 8b 92 63 c1 e3-df 78 ba 8c 5a cc 82 50 .s...c...x..Z..P
0060 - 33 4e 13 4b 10 e4 97 31-cc b4 13 65 45 60 3e 13 3N.K...1...eE`>.
0070 - ac 9e b1 bb 4b 18 d9 16-ea ce f0 9b 5b 0c 8b bf ....K.......[...
0080 - fd 78 74 a0 1a ef c2 15-2a 0a 14 8d d1 3f 52 7a .xt.....*....?Rz
0090 - 12 6b c7 81 15 c4 c4 af-7e df c2 20 a8 dd 4b 93 .k......~.. ..K.
Start Time: 1574769867
Timeout : 300 (sec)
Verify return code: 0 (ok)
---
GET / HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
Cache-Control: max-age=604800
Content-Type: text/html; charset=UTF-8
Date: Tue, 26 Nov 2019 12:04:38 GMT
Etag: "3147526947+ident"
Expires: Tue, 03 Dec 2019 12:04:38 GMT
Last-Modified: Thu, 17 Oct 2019 07:18:26 GMT
Server: ECS (dcb/7EC8)
Vary: Accept-Encoding
X-Cache: HIT
Content-Length: 1256
<!doctype html>
<html>
<head>
<title>Example Domain</title>
<meta charset="utf-8" />
<meta http-equiv="Content-type" content="text/html; charset=utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<style type="text/css">
body {
background-color: #f0f0f2;
margin: 0;
padding: 0;
font-family: -apple-system, system-ui, BlinkMacSystemFont, "Segoe UI", "Open Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
}
div {
width: 600px;
margin: 5em auto;
padding: 2em;
background-color: #fdfdff;
border-radius: 0.5em;
box-shadow: 2px 3px 7px 2px rgba(0,0,0,0.02);
}
a:link, a:visited {
color: #38488f;
text-decoration: none;
}
@media (max-width: 700px) {
div {
margin: 0 auto;
width: auto;
}
}
</style>
</head>
<body>
<div>
<h1>Example Domain</h1>
<p>This domain is used for illustrative examples in documents. You may use this domain in literature without prior coordination or asking for permission.</p>
<p><a href="https://www.iana.org/domains/example">More information...</a></p>
</div>
</body>
</html>
Solving typical tasks in Linux
Change file ownership
You can change the ownership of a file or directory using the command chown:
[user@testhost ~]$ chown user:user temp
[user@testhost ~]$ ls -l temp
-rw-rw-r-- 1 user user 31 Nov 26 11:09 temp
The parameter for this command needs to specify the new owner and group (optional), separated by a colon. Also, when changing the ownership of a directory, the option -R — then ownership will change for all contents of the directory.
Change file permissions
This task is solved using the command chmod. As an example, I will provide setting permissions: 'owner has read, write and execute; group has read and write; everyone else has nothing':
[user@testhost ~]$ ls -l temp
-rw-rw-r-- 1 user user 31 Nov 26 11:09 temp
[user@testhost ~]$ chmod 760 temp
[user@testhost ~]$ ls -l temp
-rwxrw---- 1 user user 31 Nov 26 11:09 temp
The first 7 (which is 0b111 in binary) in the parameter means 'all rights for the owner', the second 6 (0b110 in binary) means 'read and write', and 0 means no rights for others. The bitmask consists of three bits: the least significant ('rightmost') bit is for execution, the next one ('middle') is for writing, and the most significant ('leftmost') is for reading.
You can also set permissions using special symbols (mnemonic syntax). For example, in the following example, execution rights are first removed for the current user, and then restored:
[user@testhost ~]$ ls -l temp
-rwxrw---- 1 user user 31 Nov 26 11:09 temp
[user@testhost ~]$ chmod -x temp
[user@testhost ~]$ ls -l temp
-rw-rw---- 1 user user 31 Nov 26 11:09 temp
[user@testhost ~]$ chmod +x temp
[user@testhost ~]$ ls -l temp
-rwxrwx--x 1 user user 31 Nov 26 11:09 temp
This command has many usage options, so I recommend reading more about it (especially about the mnemonic syntax, for example, ).
Display the contents of a binary file
This can be done using the utility hexdump. Below are examples of its usage.
[user@testhost ~]$ cat temp
Content of a file.
Lalalala...
[user@testhost ~]$ hexdump -c temp
0000000 C o n t e n t o f a f i l
0000010 e . n L a l a l a l a . . . n
000001f
[user@testhost ~]$ hexdump -x temp
0000000 6f43 746e 6e65 2074 666f 6120 6620 6c69
0000010 2e65 4c0a 6c61 6c61 6c61 2e61 2e2e 000a
000001f
[user@testhost ~]$ hexdump -C temp
00000000 43 6f 6e 74 65 6e 74 20 6f 66 20 61 20 66 69 6c |Content of a fil|
00000010 65 2e 0a 4c 61 6c 61 6c 61 6c 61 2e 2e 2e 0a |e..Lalalala....|
0000001f
Using this utility, you can also output data in other formats, but the most commonly needed variants are exactly these.
Search for files
You can find a file by part of its name in the directory tree using the command find:
[user@testhost ~]$ find test_dir/ -name "*le*"
test_dir/file_1
test_dir/file_2
test_dir/subdir/file_3
Other search options and filters are also available. For example, you can find files in the folder test, created more than 5 days ago:
[user@testhost ~]$ ls -ltr test
total 0
-rw-rw-r-- 1 user user 0 Nov 26 10:46 temp_clone
-rw-rw-r-- 1 user user 0 Dec 4 10:39 created_today
[user@testhost ~]$ find test/ -type f -ctime +5
test/temp_clone
Search for text in files
You can tackle this task with the command grep. It has many usage options; here, as an example, the simplest one is shown.
[user@testhost ~]$ grep -nr "content" test_dir/
test_dir/file_1:1:test content for file_1
test_dir/file_2:1:test content for file_2
test_dir/subdir/file_3:1:test content for file_3
One of the popular ways to use the command grep — its use in the pipeline (pipe):
[user@testhost ~]$ sudo tail -f /var/log/test.log | grep "ERROR"
Option -v allows you to create the effect grepreverse — it will only display lines that do not contain the pattern passed in grep.
View installed packages
There is no universal command as it all depends on the Linux distribution and the package manager used. One of the following commands will likely help you:
yum list installed
apt list --installed
zypper se --installed-only
pacman -Qqe
dpkg -l
rpm -qa
View how much space the directory tree occupies
One of the options for using the command du:
[user@testhost ~]$ du -h -d 1 test_dir/
8.0K test_dir/subdir
20K test_dir/
You can change the parameter value -d, to get more detailed information about the directory tree. You can also use the command in combination with sort:
[user@testhost ~]$ du -h -d 1 test_dir/ | sort -h
8.0K test_dir/subdir
16K test_dir/subdir_2
36K test_dir/
[user@testhost ~]$ du -h -d 1 test_dir/ | sort -h -r
36K test_dir/
16K test_dir/subdir_2
8.0K test_dir/subdir
Option -h the command sort allows sorting sizes recorded in human-readable format (e.g., 1K, 2G), the option -r allows you to sort the data in reverse order.
‘Find and replace’ in a file, in files within the directory
This operation is performed using the utility sed (without the flag g at the end, only the first occurrence of ‘old-text’ in the line will be replaced):
sed -i 's/old-text/new-text/g' input.txt
You can use it for multiple files at once:
[user@testhost ~]$ cat test_dir/file_*
test content for file_1
test content for file_2
[user@testhost ~]$ sed -i 's/test/edited/g' test_dir/file_*
[user@testhost ~]$ cat test_dir/file_*
edited content for file_1
edited content for file_2
Output a column from the output
This task can be handled by awk. In this example, the second column of the output of the command `ps ux`:
[user@testhost ~]$ ps ux | awk '{print $2}'
PID
11023
25870
25871
25908
25909
Keep in mind that awk has a much richer functionality, so if you need to work with text in the command line, it's worth reading more about this command.
Find the IP address by hostname
One of the following commands will help with this:
[user@testhost ~]$ host ya.ru
ya.ru has address 87.250.250.242
ya.ru has IPv6 address 2a02:6b8::2:242
ya.ru mail is handled by 10 mx.yandex.ru.
[user@testhost ~]$ dig +short ya.ru
87.250.250.242
[user@testhost ~]$ nslookup ya.ru
Server: 8.8.8.8
Address: 8.8.8.8#53
Non-authoritative answer:
Name: ya.ru
Address: 87.250.250.242
Network information
You can use ifconfig:
[user@testhost ~]$ ifconfig
eth0: flags=4163 mtu 1500
inet 47.89.93.67 netmask 255.255.224.0 broadcast 47.89.95.255
inet6 fd90::302:57ff:fe79:1 prefixlen 64 scopeid 0x20
ether 04:01:57:79:00:01 txqueuelen 1000 (Ethernet)
RX packets 11912135 bytes 9307046034 (8.6 GiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 14696632 bytes 2809191835 (2.6 GiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73 mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10
loop txqueuelen 0 (Local Loopback)
RX packets 10 bytes 866 (866.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 10 bytes 866 (866.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
You can also ip:
[user@testhost ~]$ ip a
1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 04:01:57:79:00:01 brd ff:ff:ff:ff:ff:ff
inet 47.89.93.67/19 brd 47.89.95.255 scope global eth0
valid_lft forever preferred_lft forever
inet6 fd90::302:57ff:fe79:1/64 scope link
valid_lft forever preferred_lft forever
3: ip_vti0: mtu 1500 qdisc noop state DOWN group default
link/ipip 0.0.0.0 brd 0.0.0.0
If you are interested only in IPv4, you can add the option -4:
[user@testhost ~]$ ip -4 a
1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
2: eth0: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
inet 47.89.93.67/19 brd 47.89.95.255 scope global eth0
valid_lft forever preferred_lft forever
View open ports
To do this, you use the utility netstat. For example, to see all listening TCP and UDP ports along with the PID of the process listening on the port, and with numerical representation of the port, you need to use it with the following options:
[user@testhost ~]$ netstat -lptnu
System information
You can get this information with the command uname.
[user@testhost ~]$ uname -a
Linux alexander 3.10.0-123.8.1.el7.x86_64 #1 SMP Mon Sep 22 19:06:58 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
To understand the output format, you can refer to helpthis command:
[user@testhost ~]$ uname --help
Usage: uname [OPTION]…
Prints certain information about the system. If no OPTION is specified,
it defaults to -s.
-a, --all print all information, in the following order,
except -p and -i, if unknown:
-s, --kernel-name print the kernel name
-n, --nodename print the network node hostname
-r, --release print the operating system release
-v, --kernel-version print the kernel version
-m, --machine print the machine hardware name
-p, --processor print the processor type or 'unknown'
-i, --hardware-platform print the hardware platform or 'unknown'
-o, --operating-system print the operating system name
--help display this help and exit
--version output version information and exit
Memory Information
To understand how much RAM is used or free, you can use the command free.
[user@testhost ~]$ free -h
total used free shared buff/cache available
Mem: 3.9G 555M 143M 56M 3.2G 3.0G
Swap: 0B 0B 0B
File System Information (free space on disks)
The command df allows you to check how much space is free and used on mounted file systems.
[user@testhost ~]$ df -hT
Filesystem Type Size Used Avail Use% Mounted on
/dev/vda1 ext4 79G 21G 55G 27% /
devtmpfs devtmpfs 2.0G 0 2.0G 0% /dev
tmpfs tmpfs 2.0G 0 2.0G 0% /dev/shm
tmpfs tmpfs 2.0G 57M 1.9G 3% /run
tmpfs tmpfs 2.0G 0 2.0G 0% /sys/fs/cgroup
tmpfs tmpfs 396M 0 396M 0% /run/user/1001
Option -T indicates to output the filesystem type.
Information about tasks and various system statistics
This is done using the command top. It can display various information, such as the top processes by memory usage or the top processes by CPU time. It also shows memory, CPU, uptime, and load average (LA) information.
[user@testhost ~]$ top | head -10
top - 17:19:13 up 154 days, 6:59, 3 users, load average: 0.21, 0.21, 0.27
Tasks: 2169 total, 2 running, 2080 sleeping, 0 stopped, 0 zombie
Cpu(s): 1.7%us, 0.7%sy, 0.0%ni, 97.5%id, 0.0%wa, 0.0%hi, 0.1%si, 0.0%st
Mem: 125889960k total, 82423048k used, 43466912k free, 16026020k buffers
Swap: 0k total, 0k used, 0k free, 31094516k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
25282 user 20 0 16988 3936 1964 R 7.3 0.0 0:00.04 top
4264 telegraf 20 0 2740m 240m 22m S 1.8 0.2 23409:39 telegraf
6718 root 20 0 35404 4768 3024 S 1.8 0.0 0:01.49 redis-server
This utility has a rich set of features, so if you need to use it frequently, it's better to familiarize yourself with its documentation.
Network traffic dump
To intercept network traffic in Linux, the utility tcpdumpis used. To dump traffic on port 12345, you can use the following command:
[user@testhost ~]$ sudo tcpdump -i any -A port 12345
Option -A indicates that we want to see the output in ASCII (which is good for text protocols), -i any specifies that we are not interested in a specific network interface, port — which port's traffic to dump. Instead of port you can use host, or a combination of host and port (host A and port X). And another useful option might be -n — do not convert addresses to hostnames in the output.
What if the traffic is binary? Then the option -X will help us — to output data in hex and ASCII:
[user@testhost ~]$ sudo tcpdump -i any -X port 12345
Keep in mind that in both usage scenarios, IP packets will be output, so at the beginning of each of them, there will be binary headers for IP and TCP. Here's an example output for a request "123" sent to a server listening on port 12345:
[user@testhost ~]$ sudo tcpdump -i any -X port 12345
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
14:27:13.224762 IP localhost.49794 > localhost.italk: Flags [P.], seq 2262177478:2262177483, ack 3317210845, win 342, options [nop,nop,TS val 3196604972 ecr 3196590131], length 5
0x0000: 4510 0039 dfb6 4000 4006 5cf6 7f00 0001 E..9..@.@......
0x0010: 7f00 0001 c282 3039 86d6 16c6 c5b8 9edd ......09........
0x0020: 8018 0156 fe2d 0000 0101 080a be88 522c ...V.-........R,
0x0030: be88 1833 3132 330d 0a00 0000 0000 0000 ...3123.........
0x0040: 0000 0000 0000 0000 00 .........
Instead of output
Of course, there are many other interesting things in Linux that you can read about on Habr, StackOverflow, and other sites (as an example, I'll mention , which is also available in ). System administrators and DevOps use many more commands and utilities to configure servers, but even testers might find the listed commands insufficient. You may need to check the correctness of some tricky client-server timeout or the server's operation when there is no free disk space. Not to mention, for example, Docker, which is currently actively used by many companies. It would be interesting to explore several examples of using various Linux console utilities in the testing process in the continuation of this reference article. Please share your top commands in the comments 🙂
Source: habr.com
