[Linux] Complete Linux Training Course to Get Your Dream IT Job 2022

이향기·2022년 1월 13일
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Section3. System Access and File Systme

Things to remember in Linux (39)

  • root
  • case-sensitive
  • avoid spacing in files and directory (use hipens or dashes)
  • Linux kernel ≠\ne Linux OS(operating system)
  • Linux : mostly CLI(command line interface), not GUI(Graphical user interface)
  • very flexible

Linux file system (40)

  • File system ≈\approx Closet in your home
    • commands = /usr/bin
    • Attached Devices = /dev
    • Applications = /etc or /var
  • Linux filesystems store information in a hierachy of directories and files

File system structure and its description (41)

File system navigation commands (42)

  • cd (change directory)

    • .. : one step back (upper directory)

  • pwd (print working directory)

  • ls (list)

    • -l : with all info
    • -ltr : in modified time order

What is root? (43)

  • root account

  • root as / : the very first directory, refered as root directory

  • root home directory : /root

Absolute and Relative Paths (44)

  • absolute path
    • start with "/"
$ cd /var/log/samba
  • relative path
    • not start with "/"
    • relative to my current position
$ cd /var
$ cd log
$ cd samba

Directory Listing Attributes (45)

  • ls -a : gives all attributes!
    • Type : begins with
      • d : directory
      • l : link
      • '-' : regular files
    • Number of Links
    • Owner
    • Group
    • Size
    • Month
    • Day
    • Time
    • Name

Creating Files and Directories (46)

  • Creating files
    • touch : create file (of course, empty file)
$ touch {name1} {name2} ...

: create empty file name1, name2, ...

  • cp : copying an existing file and creating a new file
$ cp {name1} {name2}

: copy file {name1} as {name2}

  • vi : an editor, explained later
$ vi {name}

: bring you to file editor, shift + :wq! (write and quit)

  • Creating directories
$ mkdir {name1} {name2}

: make empty directory name1, name2,...
: cf. if you do not have permission, it can be denied

Copying Directories (47)

  • Copying directories
    • in Linux, cp with the "-R" option for recursive
$ cp -R {name1} {name2}

: copy folder {name1} to {name2}

Linux file types (48)

Fining files and directories (49)

  • Two main commands are used to find files/directories
    • find
    • locate
  • find
    • starting directory
      • . : relative path, from current directory
      • / : absolute path
$ find {starting directory} -name "{finding name}"
  • locate
$ locate {}

  • What if "Permission denied"?
    • get root 권한 by su -

Difference between find and locate (50)

  • locate use a prebuilt database,
    • which should be regularly updated
  • find iterates over a filesystem to locate files
  • locate is much faster than find, but can be inaccurate if the database is not updated
  • To update locate database, run updatedb

Changing password (51)

  • should change my initial password as soon as I login
  • $ passwd {userid}

Wildcards(*,?,^[]) (52)

  • Examples
    • $ touch abcd{1..9}-xyz : create 9 files!
    • $ rm a* : remove all the files starting with "a"
    • $ rm *xyz : remove all the files ending with "xyz"
    • $ ls -l ?bcd*
    • $ ls -l *[cd]*

  • inode
  • Soft Link : $ ln -s
  • Hard Link : $ ln
  • Link ≈\approx Shortcut
  • How softlink works?
$ touch hulk
$ cd /tmp
$ ls -s /home/iafzal/hulk


  • How hardlink works?
$ touch hulk
$ echo "hulk is superhero" > hulk
$ cat hulk
$ cd /tmp
$ ln /home/iafzal/hulk


: there is no pointing in hardlink!

hard linksoft link
한 파일의 복사본을 만드는 것
cf. cp랑은 다른 개념!
-cp는 원본과 복사본 파일의 inode가 다르다
-따라서, 복사본/원본의 수정여부가 서로에게 영향을 미치지 않음		바로가기 아이콘을 만드는 것
원본-하드링크 파일은 같은 inode를 가짐원본-소프트링크 파일은 다른 inode를 가짐
어디에서 수정을 하던지, 함께 수정이 이루어짐어디에서 수정을 하던지, 함께 수정이 이루어짐
원본을 지워도 하드링크 파일을 실행가능/내용보존원본을 지우면 연결이 끊겨, 링크 파일 실행 불가능
Type : "-"Type : "l"

출처 : 파일 링크 : ln - 하드 링크(Hard Link), 소프트 링크(Soft Link)

Section4. Linux Fundamentals

Linux Command Syntax (57)

  • command have options and arguments
$ command option(s) argumnet(s)
  • options
    • modify the way that a command works
    • usually consist of hypen or dash followed by a single letter
    • some commands accept multiple options which can be grouped together after a single hypen
  • arguments
    • most commands are used together with (one or more) argumnets
    • some commands have default option (if none is supplied)
    • argmuments may be required/optional
  • ls : list files and directorys
    • -l : list
    • -t : order by time
    • -r : reverse order
    • ls -l {file or directory name} : list only the argument
  • rm : remove
    • -f : forcefully
    • -r : recursively
    • rm -rf {file or directory name}
  • mkdir : make directory
    • mkdir {file or directory name}
  • man {command} : manual for command
    • to go to bottom, keep padding Space key
    • to kill, press Esc + q

Files and Directory Permissions (chmod) (58)

  • chmod : change mode
    • chmod g-w {file name} : remove write permissions from group in {file name}
    • chmod a-r {file name} : remove read permissions from all in {file name}
    • chmod u-w {file name} : remove write permissions from user in {file name}
  • rm {file name} :
  • cat {file name} : read file
  • chmod u+rw {file name}
  • chmod a-x {folder name}/

File Permissions Using Numeric Mode (59)

  • Numbers assigned to permissions types
  • User types : User, Group, Others
  • chmod
    • chmod 777 {file name}
  • Online calculator

File ownership (60)

  • 2 types of owner
    • user : who create the file/directory
    • group : group which the user belongs to
  • Command to change file ownership
    • chown : changes the ownership of a file
    • chgrp : changes the group ownership of a file
    • option -R : recursive option
  • cf. login as root
    • su - : log in as root
    • exit : exit root

Access Control list (61)

  • What is ACL?
    • an additional & more flexible permission mechanism for file systems
    • to assist with UNIX file permissions
    • allow you to give permissions for any user or group to any disc resource
  • Use of ACL
  • Commands for ACL
    • setfacl : set file ACL
      • setfacl -m u:{user name}:rwx /path/to/file : change user's permissions
      • setfacl -m g:{group name}:rwx /path/to/file : change group's permissions
      • setfacl -Rm "entry" /path/to/dir : to allow all files or directories to ingerit ACL entries from the directory it is within
      • setfacl -x u:{user name} /path/to/file : to remove a specific entry (for a specific user)
      • setfacl -b /path/to/file : to remove a specific entry (for all users)
      • note
    • getfacl : get (current) file ACL

Help commands (62)

  • There are 3 types of help commands
    • $ whatis {command}
    • $ {command} --help
    • $ man {command}

cf. MAC deos not support $ {command} --help any more!

TAB Completion and Up Arrow (63)

Adding text to files(Redirects) (64)

  • 3 ways
    • $ echo "{text}" > {fileName} : override text!
    • $ echo "{text}" >> {fileName} : keep lines and add line
  • read files
    • $ cat {fileName}

Input and Output Redirects (65)

  • 3 redirects in Linux

  • Output

  • Input

  • Error

Standard Output to a File (tee) (66)

  • command tee
    • $ echo "{your text}" | tee {filename}
    • This overrides the extisting text!
  • How to add/append text to a file?
    • $ echo "{your text}" | tee -a {filename}
  • How to count # of words in a file?
    • $ wc -c {filename}

Pipes (|) (67)


  • e.g.
    • $ ls -ltr | more
    • $ ls -l |

File Maintenance Commands (68)

  • $ cp : copy

  • $ rm : remove

  • $ mv : move or rename

  • $ mkdir : make directory

  • $ rmdir or $ rm -r : remove directory

  • $ chgrp : change group

  • $ chown : chagne ownership (at the user level)

  • How to use?

    • $ cp {file1} {file2} : copy file1 to file2
    • $ cp {file1} {directory} : copy file1 in directory (with the same file name)
    • $ mv {file1} {file2} : change file name from file1 to file2
    • $ mv {file1} {directory} : change location of file
    • $ rm {filename} : remove file
    • $ rm -Rf : forcefully remove the sub-directories and its contents as well
    • $ chown {ownership} {file1} : change ownership of file1 to ownership (ownership - user, root)

File Display Commands (69)

  • How can you view INSIDE of a file?
    • $ cat : views the entire content (regardless of 1 page, 2 page, ...)
    • $ more : go over 1 page at a time
    • $ less : go over 1 line at a time, in the reverse order
      • press j : 1 line below
      • press k : 1 line above (go back up)
    • $ head : views top of the file
      • $ head -{#lines} {filename}
    • $ tail : views bottom of the file
      • $ tail -{#lines} {filename}

Filters - Text Processing Commands (70)

  • cut
  • awk
  • grep and egrep
  • sort
  • uniq : unique
  • wc : word counts

cut - Text Processors Commands (71)

awk - Text Processors Commands (72)

grep/egrep - Text Processors Commands (73)

sort/uniq - Text Processors Commands (74)

wc - Text Processors Commands (75)

Compare files (diff and cmp) (76)

  • diff : $ diff {file1} {file2}
  • cmp : $ diff {file1} {file2}

Compress and uncompress (tar, gzip, gunzip) (77)

  • tar : actually do not compress (nearly)
    • tar cvf {file_name}.tar {directory} : taring files in {directory} to {file_name}.tar file
    • tar xvf {file_name}.tar : untaring file {file_name}.tar
  • gzip : compress files
    • gzip {file_name} : compress {file_name}
  • gzip -d OR gunzip : compress / uncompress files
    • gzip -d {file_name} : uncompress {file_name}

Truncate file size (truncate) (78)

  • truncate -s {size} {filen_name}

Combining and splitting files (79)

  • split -l {# of lines} {file_name} {splitted_file_name}

Section5. System Administration

"sed" Command (86)

[Replace strings]

  • $ sed 's/{from_word}/{to_word}/g' {file_name}
    • s : subtitute
    • g : globally (multiple cases, at the same time)
    • only print changed in terminal, not actually change the file!
    • $ sed -i 's/{from_word}/{to_word}/g' {file_name}
    • add option -i : insert

[Remove strings]

  • $ sed 's/{from_word}//g' {file_name}
  • $ sed -i 's/{from_word}//g' {file_name}

[Remove all the lines containing strings]

  • $ sed '/{word}/d' {file_name}
  • $ sed -i '/{word}/d' {file_name}

[Remove empty lines]

  • $ sed '/^$/d' {file_name}
    • ^ : starts with
    • $ : ends with

[Remove 1st lines]

  • $ sed '1d' {file_name}

[Replace tab to space]

  • $ sed 's/\t/ /g' {file_name}
  • $ sed -i 's/\t/ /g' {file_name}

[Print some lines only]

  • $ sed -n {starting_num},{ending_num}p {file_name}

[Print file except some lines]

  • $ sed {starting_num},{ending_num}d {file_name}

[Add empty line between lines]

  • $ sed G {file_name}

[Replace strings except 8th lines]

  • $ sed '8!s/{from_word}/{to_word}/g' {file_name}

User Account Management (87)

  • First, become root! : $ su -
    • What if it does not work?
      • $ sudo -s : login sudo
      • $ passwd root : change password for root
    • 그래도 안되네...

[Add user]

  • $ useradd {user_name}
  • $ id {user_name}
  • $ cd /home and $ ls -ltr

[Add groups]

  • $ groupadd {group_name}

[Change group of user]

  • $ usermod -G {group_name} {user_name}

  • $ chgrp -R {group_name} {user_name}

  • $ cat /etc/passwd

  • $ cat /etc/group

  • $ cat /etc/shadow

[Add user with all options]

  • $ useradd -g {group_name} -s {shell_path} -c "{Description}" -m -d {home_directory} {user_name}

Enable Password Aging (88)

  • $ chage : change age, used to set parameters around password (cf. usermod)

Switch Users and Sudo Access (89)

  • $ whoami
  • $ su - {user_name}
  • $ exit
  • $ su - : change user to root

[sudo commands]

  • $ dmidecode
  • $ fdisk -l
  • $ usermod -aG wheel {user_name}

Linux directory service - Account Authentification (92)

  • What if you have to create a user account of 10000 different servers?
    • Then, you have to have a domain or a directory account on a server
  • LDAP
    • Not only for Linux! (Window, Mac, ...)
    • a protocol used for authentification against directory

Processes, Jobs and Scheduling (95)







systemctl command (96)

  • How to start an application in Linux?
    • There is no GUI!
  • systemctl : system control

  • $ systemctl status firewalld.service
  • $ systemctl stop firewalld.service
  • $ systemctl status firewalld.service
  • $ systemctl disable firewalld.service
  • $ systemctl list-units
    • only active units
  • $ systemctl list-units -all

ps command (97)

  • ps : process status

top command (98)

  • top : show a real-time view of the running system
  • You don't need to be root user, to use top command
    • But if you want to be kill a prcoess, you have to be root!

  • $ top -U {user name}
  • $ sudo top →\to press shift + s →\to write kill as a signal →\to write the PID that you want to kill

kill command (99)

  • $ killall
  • $ pkill

crontab command (100)

  • cf. crontab gives vi editing mode!
  • crontab - e : edit crontab
  • crontab -l : list crontab
  • crontab -r : remove crontab

Process management (bg, fg, nice) (103)

  • Check my IP address
    • $ ipconfig getifaddr en0 : local IP
    • $ curl ifconfig.me : public IP
  • Put in background
    • ctrl + z
    • $ bg
  • Put in foreground
    • $ fg
  • Run process even after eixt terminal
    • $ nohup {process command}
    • $ nohup {process command} > /dev/null 2>&1 &
  • Process priority
    • $ nice : e.g. $ nice -n 5 {process command}

System monitoring commands (df, dmesg, iostat, netstat, free, top) (104)

  • All these commands will list all the system resources. These will tell you
    • How the system is doing
    • Hou is it behaving
    • Whether it's running high or low
  • These are system monitoring commands!
  • $ top
    • CPU, memory, which process is on, which user is running
    • To quit, press q
  • $ df : disk partition information
    • $ df -h : more human readable
  • $ sudo dmesg
  • $ iostat : input output statistics
    • $ iostat 1 : iostat with refreshing every 1 second
    • To quit, press ctrl + c
  • $ netstat
    • $ netstat -rnv
    • $ netstat | more
  • $ top -l 1 -s 0 | grep PhysMem | sed 's/, /\n /g'
    • There is no free command in Mac!
  • $ sysctl -n machdep.cpu.brand_string
    • There is no result from $ cat /proc/cpuinfo in Mac!
    • cf. $ sysctl hw.ncpu : the number of CPUs
  • $ sysctl hw.memsize

System log monitor (/var/log) (105)

  • Linux ≠\ne Mac in Log system ㅠ
  • boot
    • There is no boot.log in /var/log
    • $ log show --predicate "processID == 0" --debug
  • cron
    • There is no cron directory in /var/log
    • $ whereis syslog gives /usr/bin/syslog
    • $ whereis cron gives /usr/sbin/cron
  • secure

Terminal control keys (110)

Terminal commands (clear, exit, script) (111)

Environment variables (114)

  • 순서가
    • 없는
      • 목록
    • 순서가
  • 없어용

Section6. Shell Scripting

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