[Linux][System] Learn Linux Command Operations

20220329

Check kernal version of Linux

• uname -a

zn@ubuntu:~$ uname -a
Linux ubuntu 5.4.0-105-generic #119~18.04.1-Ubuntu SMP Tue Mar 8 11:21:24 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
zn@ubuntu:~$ uname --help
Usage: uname [OPTION]...
Print certain system information.  With no OPTION, same as -s.
-a, --all                print all information, in the following order,
                             except omit -p and -i if unknown:
  -s, --kernel-name        print the kernel name
  -n, --nodename           print the network node hostname
  -r, --kernel-release     print the kernel release
  -v, --kernel-version     print the kernel version
  -m, --machine            print the machine hardware name
  -p, --processor          print the processor type (non-portable)
  -i, --hardware-platform  print the hardware platform (non-portable)
  -o, --operating-system   print the operating system
      --help     display this help and exit
      --version  output version information and exit

Check IP address

• ip addr
• ifconfig (apt install net-tool)

zn@ubuntu:~$ ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    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: ens33: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 00:0c:29:32:c3:a7 brd ff:ff:ff:ff:ff:ff
    inet 192.168.219.132/24 brd 192.168.219.255 scope global dynamic noprefixroute ens33
       valid_lft 1506sec preferred_lft 1506sec
    inet6 fe80::a506:7e6f:bf8d:e1d8/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever

Check free disk space / Usage

• df -ah

zn@ubuntu:~$ df -ah
Filesystem      Size  Used Avail Use% Mounted on
sysfs              0     0     0    - /sys
proc               0     0     0    - /proc
udev            1.9G     0  1.9G   0% /dev
devpts             0     0     0    - /dev/pts
tmpfs           391M  2.0M  389M   1% /run
/dev/sda1        20G  5.3G   14G  29% /
securityfs         0     0     0    - /sys/kernel/security
tmpfs           2.0G     0  2.0G   0% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           2.0G     0  2.0G   0% /sys/fs/cgroup

Check folder/file size

• du -sh <targetfolderorfile>

zn@ubuntu:~/Desktop/work/20220329$ du -sh tmp
4.0K tmp

Manage service on system / start / stop / restart

• systemctl status <servicename>
• service <servicename> status

zn@ubuntu:~$ service --help
Usage: service < option > | --status-all | [ service_name [ command | --full-restart ] ]
zn@ubuntu:~$ systemctl --help
systemctl [OPTIONS...] {COMMAND} ...
Query or send control commands to the systemd manager.
-h --help           Show this help
     --version        Show package version
     --system         Connect to system manager
     --user           Connect to user service manager
  -H --host=[USER@]HOST
                      Operate on remote host
  -M --machine=CONTAINER
                      Operate on local container
  -t --type=TYPE      List units of a particular type
     --state=STATE    List units with particular LOAD or SUB or ACTIVE state
  -p --property=NAME  Show only properties by this name
  -a --all            Show all properties/all units currently in memory,
                      including dead/empty ones. To list all units installed on
                      the system, use the 'list-unit-files' command instead.
     --failed         Same as --state=failed
  -l --full           Don't ellipsize unit names on output
  -r --recursive      Show unit list of host and local containers
     --reverse        Show reverse dependencies with 'list-dependencies'
     --job-mode=MODE  Specify how to deal with already queued jobs, when
                      queueing a new job
     --show-types     When showing sockets, explicitly show their type
     --value          When showing properties, only print the value
  -i --ignore-inhibitors

zn@ubuntu:~/Desktop/work/20220329$ service --status-all
 [ + ]  acpid
 [ + ]  alsa-utils
 [ - ]  anacron
 [ + ]  apparmor
 [ + ]  apport
 [ + ]  atd
 [ + ]  avahi-daemon
 [ - ]  bluetooth
 [ - ]  console-setup.sh
 [ + ]  cron
 [ + ]  cups
 [ + ]  cups-browsed

Check open ports on machine

• ss -tupln (sudo show more content)
• netstat -tupln (sudo show more content) (apt install net-tools)

zn@ubuntu:~$ ss -tupln
Netid    State      Recv-Q      Send-Q            Local Address:Port            Peer Address:Port     
udp      UNCONN     0           0                       0.0.0.0:59614                0.0.0.0:*        
udp      UNCONN     0           0                       0.0.0.0:631                  0.0.0.0:*        
udp      UNCONN     0           0                       0.0.0.0:5353                 0.0.0.0:*        
udp      UNCONN     0           0                 127.0.0.53%lo:53                   0.0.0.0:*        
udp      UNCONN     0           0                       0.0.0.0:68                   0.0.0.0:*        
udp      UNCONN     0           0                          [::]:39352                   [::]:*        
udp      UNCONN     0           0                          [::]:5353                    [::]:*        
tcp      LISTEN     0           128               127.0.0.53%lo:53                   0.0.0.0:*        
tcp      LISTEN     0           5                     127.0.0.1:631                  0.0.0.0:*        
tcp      LISTEN     0           5                         [::1]:631                     [::]:*

zn@ubuntu:~$ ss --help
Usage: ss [ OPTIONS ]
       ss [ OPTIONS ] [ FILTER ]
   -h, --help          this message
   -V, --version       output version information
   -n, --numeric       don't resolve service names
   -r, --resolve       resolve host names
   -a, --all           display all sockets
   -l, --listening     display listening sockets
   -o, --options       show timer information
   -e, --extended      show detailed socket information
   -m, --memory        show socket memory usage
   -p, --processes     show process using socket
   -i, --info          show internal TCP information
   -s, --summary       show socket usage summary
   -b, --bpf           show bpf filter socket information
   -E, --events        continually display sockets as they are destroyed
   -Z, --context       display process SELinux security contexts
   -z, --contexts      display process and socket SELinux security contexts
   -N, --net           switch to the specified network namespace name
   -4, --ipv4          display only IP version 4 sockets
   -6, --ipv6          display only IP version 6 sockets
   -0, --packet        display PACKET sockets
   -t, --tcp           display only TCP sockets
   -S, --sctp          display only SCTP sockets
   -u, --udp           display only UDP sockets
   -d, --dccp          display only DCCP sockets
   -w, --raw           display only RAW sockets
   -x, --unix          display only Unix domain sockets
       --vsock         display only vsock sockets
   -f, --family=FAMILY display sockets of type FAMILY
       FAMILY := {inet|inet6|link|unix|netlink|vsock|help}
-K, --kill          forcibly close sockets, display what was closed
   -H, --no-header     Suppress header line

Check CPU usage for a process

• ps aux | grep <processname>
• top (simular to task manager in Windows)
• htop (sudo apt install htop)

zn@ubuntu:~$ ps --help s
Usage:
 ps [options]
Basic options:
 -A, -e               all processes
 -a                   all with tty, except session leaders
  a                   all with tty, including other users
 -d                   all except session leaders
 -N, --deselect       negate selection
  r                   only running processes
  T                   all processes on this terminal
  x                   processes without controlling ttys
For more details see ps(1).

zn@ubuntu:~$ top
top - 04:30:42 up  1:36,  1 user,  load average: 0.01, 0.01, 0.00
Tasks: 308 total,   1 running, 237 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.3 us,  0.7 sy,  0.0 ni, 99.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem :  3994684 total,  1000840 free,  1266408 used,  1727436 buff/cache
KiB Swap:   969960 total,   969960 free,        0 used.  2454372 avail Mem
PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                          
  4413 zn        20   0  443292  64440  38380 S   1.0  1.6   0:05.67 Xorg                             
  4569 zn        20   0 3401748 186048  81712 S   1.0  4.7   0:12.64 gnome-shell                      
  4887 zn        20   0  796952  38004  28068 S   1.0  1.0   0:03.39 gnome-terminal-                  
  5347 zn        20   0   44216   4028   3284 R   0.3  0.1   0:00.11 top                              
     1 root      20   0  159972   9264   6712 S   0.0  0.2   0:02.20 systemd                          
     2 root      20   0       0      0      0 S   0.0  0.0   0:00.00 kthreadd                         
     3 root       0 -20       0      0      0 I   0.0  0.0   0:00.00 rcu_gp

Mount a USB

• mount <target> /mnt

zn@ubuntu:~$ mount -h
Usage:
 mount [-lhV]
 mount -a [options]
 mount [options] [--source] <source> | [--target] <directory>
 mount [options] <source> <directory>
 mount <operation> <mountpoint> [<target>]
Mount a filesystem.

Auto mount on boot

• nano /etc/fstab (config for mounting drives while boot)

Infos for command / on-line reference manuals

• man <commandtocheck>

zn@ubuntu:~$ man man
MAN(1)                  Manual pager utils                  MAN(1)
NAME
       man - an interface to the on-line reference manuals
SYNOPSIS
       man  [-C file] [-d] [-D] [--warnings[=warnings]] [-R encod‐
       ing] [-L locale] [-m system[,...]] [-M path] [-S list]  [-e
       extension] [-i|-I] [--regex|--wildcard] [--names-only] [-a]
       [-u] [--no-subpages] [-P pager] [-r prompt] [-7] [-E encod‐
       ing]  [--no-hyphenation]  [--no-justification]  [-p string]
       [-t] [-T[device]] [-H[browser]] [-X[dpi]]  [-Z]  [[section]
       page[.section] ...] ...
       man -k [apropos options] regexp ...

Check free memory on machine

• free -h
• vmstat
• top

zn@ubuntu:~/Desktop/work/20220329$ free -h
              total        used        free      shared  buff/cache   available
Mem:           3.8G        1.3G        743M         16M        1.8G        2.2G
Swap:          947M          0B        947M

zn@ubuntu:~$ vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  0      0 911392 167532 1567788    0    0   107    46   65  119  1  0 99  0  0

Change permission for file

• chmod 777 <target>
• chmod +rwx <target>

Linux Directory Commands

• pwd
• ls <targetDir?>
• cd <targetDir>
• mkdir <targetDir>
• rmdir <targetDir>

Redirection operator / Write output to file

• echo “hi-01” > tmp.txt
• echo “hi-02” >> tmp.txt

zn@ubuntu:~/Desktop/work/20220329$ echo "hi-01" > tmp.txt
zn@ubuntu:~/Desktop/work/20220329$ echo "hi-02" >> tmp.txt
zn@ubuntu:~/Desktop/work/20220329$ cat tmp.txt
hi-01
hi-02

Extract / create archived file

• tar -xvzf sample.tar.gz
• tar -cvzf filename.tar.gz
• (c: create archive, x: extract, v: verbose, f: file)

Copy a file

• cp <source> <destination>

Rename / Move a file in Linux

• mv <oldname> <newname>

Remove file/ directory

• rm -rf <target>
• rmdir <directory name>

Terminate a running process

• kill <PID>

Find the difference in two files

• diff <file1> <file2>

zn@ubuntu:~/Desktop/work/20220329$ echo "hi-02" >> tmp2.txt
zn@ubuntu:~/Desktop/work/20220329$ diff tmp.txt tmp2.txt
1c1
< hi-01
---
> hi

Echo all files in here with names including “tmp”

• $ for i in *tmp*; do echo $i; done

zn@ubuntu:~/Desktop/work/20220329$ for i in *tmp*; do echo $i; done
tmp
tmp2.txt
tmp.txt

Create a text file without opening it

• touch <filename>

Schedule a task in Linux

• cron
• at (sudo apt install at)

CRON(8)              System Manager's Manual              CRON(8)
NAME
       cron - daemon to execute scheduled commands (Vixie Cron)
SYNOPSIS
       cron [-f] [-l] [-L loglevel]
DESCRIPTION
       cron is started automatically from /etc/init.d on entering
       multi-user runlevels.
OPTIONS
       -f      Stay in foreground mode, don't daemonize.
-l      Enable LSB compliant names for /etc/cron.d  files.
               This setting, however, does not affect the parsing
               of files under /etc/cron.hourly,  /etc/cron.daily,
               /etc/cron.weekly or /etc/cron.monthly.

zn@ubuntu:~/Desktop/work/20220329$ echo "hi now" > tmp3.txt | at -m 05:17
warning: commands will be executed using /bin/sh
job 2 at Tue Mar 29 05:17:00 2022
zn@ubuntu:~/Desktop/work/20220329$ cat tmp3.txt
hi now

Get current time

• date

zn@ubuntu:~/Desktop/work/20220329$ date
Tue Mar 29 05:13:59 PDT 2022

Identify which shell you are using

• echo $SHELL

zn@ubuntu:~/Desktop/work/20220329$ echo $SHELL
/bin/bash

Login to another system in your network

• ssh <username>@<ip address>

zn@ubuntu:~/Desktop/work/20220329$ ssh
usage: ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
           [-D [bind_address:]port] [-E log_file] [-e escape_char]
           [-F configfile] [-I pkcs11] [-i identity_file]
           [-J [user@]host[:port]] [-L address] [-l login_name] [-m mac_spec]
           [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address]
           [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]]
           [user@]hostname [command]

Show sort lines in a file

• sort <filename>

zn@ubuntu:~/Desktop/work/20220329$ echo "hi-00" >> tmp.txt
zn@ubuntu:~/Desktop/work/20220329$ sort tmp.txt
hi-00
hi-01
hi-02
zn@ubuntu:~/Desktop/work/20220329$ cat tmp.txt
hi-01
hi-02
hi-00

Check username / Check usergroup

• whoami
• id

zn@ubuntu:~/Desktop/work/20220329$ whoami
zn

zn@ubuntu:~/Desktop/work/20220329$ id
uid=1000(zn) gid=1000(zn) groups=1000(zn),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),126(sambashare)

Run 1.sh, if success then run 2.sh

• 1.sh && 2.sh

Cut command & Pipeline example

• cat tmp4.txt | cut -d: -f2

zn@ubuntu:~/Desktop/work/20220329$ nano tmp4.txt
zn@ubuntu:~/Desktop/work/20220329$ cat tmp4.txt
name:Zn
email:xxx@xxx
phone:x#x#$
zn@ubuntu:~/Desktop/work/20220329$ cat tmp4.txt | cut -d: -f2
Zn
xxx@xxx
x#x#$

Print Uniq lines

• cat <targetname> | uniq

Count file charaters, lines

• wc <targetfile>

zn@ubuntu:~/Desktop/work/20220329$ wc tmp4.txt
 3  3 34 tmp4.txt

Grep command example

• cat <filename> | grep <target>
• GREP = Global Regular Expression Print

zn@ubuntu:~/Desktop/work/20220329$ cat tmp4.txt | grep name
name:Zn

Start bash as root

• sudo -i

zn@ubuntu:~$ ls -l
total 32
drwxr-xr-x 3 zn zn 4096 Mar 29 04:20 Desktop
drwxr-xr-x 2 zn zn 4096 Mar 29 03:08 Documents
drwxr-xr-x 2 zn zn 4096 Mar 29 03:08 Downloads
drwxr-xr-x 2 zn zn 4096 Mar 29 03:08 Music
drwxr-xr-x 2 zn zn 4096 Mar 29 03:08 Pictures
drwxr-xr-x 2 zn zn 4096 Mar 29 03:08 Public
drwxr-xr-x 2 zn zn 4096 Mar 29 03:08 Templates
drwxr-xr-x 2 zn zn 4096 Mar 29 03:08 Videos
zn@ubuntu:~$ sudo -i
root@ubuntu:~# ls -l
total 4
drwx------ 6 root root 4096 Mar 29 05:36 snap
root@ubuntu:~#

Reference

• https://www.youtube.com/watch?v=l0QGLMwR-lY&ab_channel=tutoriaLinux
• https://www.edureka.co/blog/interview-questions/linux-interview-questions-for-beginners/