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Essential Commands

Log into local & remote graphical and text mode consoles

Basic concept to know:

  • Text Terminal: text input/output environment.
    • Originally, they meant a piece of equipment through which you could interact with a computer: in the early days of Unix, that meant a teleprinter-style device resembling a typewriter, sometimes called a teletypewriter, or “tty” in shorthand
    • Tty were used to establish a connection to a mainframe computer and share operating system provided by it
    • A typical text terminal produces input and displays output and errors
  • Console: terminal in modern computers that don't use mainframe but have an own operating system. It is generally a terminal in the physical sense that is, by some definition, the primary terminal directly connected to a machine.
    • The console appears to the operating system "like" a remote terminal
    • In Linux and FreeBSD, the console, in realty, appears as several terminals (ttys) called Virtual Consoles
  • Virtual Consoles: to provide several text terminals on a single computer
    • Multiple virtual consoles can be accessed simultaneously
  • Shell: command line interface or CLI
    • It is the primary interface that users see when they log in, whose primary purpose is to start other programs
    • It is presented inside console
    • There are many different Linux shells
    • Command-line shells include flow control constructs to combine commands. In addition to typing commands at an interactive prompt, users can write shell scripts

To summarize: A virtual console is a shell prompted in a non-graphical environment, accessed from the physical machine, not remotely.

  • Pseudo-terminal: Terminal provided by programs called terminal emulators e.g. ssh, tmux

  • X Windows System: is a windowing system for bitmap displays

    • X provides the basic framework for a graphical user interface (GUI) environment: drawing and moving windows on the display device and interacting with a mouse and keyboard
    • X does not mandate the user interface – this is handled by individual programs, like KDE or GNOME
    • It is considered "graphical terminal"
    • When is executed it will substitute one of the text terminal provided by virtual console. In CentOS the terminal will be 1, in other system could be 7.
    • Some applications running inside X Windows System provide pseudo-terminal e.g. Konsole, Gnome Terminal
    • If graphical environment is not started, you can run command startx to execute it

Log in:

  • To log into local environment you must provide, when prompted, userID and password for both graphical and text mode
  • To login into a remote text environment you can use command ssh
  • To login into a remote graphical environment you can use command ssh -X

Once logged command w can be used to show who is logged and what they are doing:

[root@localhost ~]# w
23:41:16 up 2 min,  2 users,  load average: 0.02, 0.02, 0.01
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
root     tty1                      23:40   60.00s  0.01s  0.01s -bash
root     pts/0    192.168.0.34     23:41    1.00s  0.02s  0.00s w

First column shows which user is logged into system and the second one to which terminal.

  • For Virtual Console in terminal is showed tty1, tty2 etc.

  • For ssh remote sessions (pseudo-terminal) in terminal is showed pts/0, pts/1 etc.

  • :0 is for X11server namely used for graphical login

References:

Search for files

  • find is recursive without parameters

  • Base syntax: find PATH PARAMETERS

  • find /etc -name "\*host*"

    Search in /etc all file/directories with host in their name. * is a wildcard

  • find . -perm 777 -exec rm -f '{}' \;

    Search from current position all files/directories with permissions 777 and after remove them

    -exec uses the result of find to do something

    {} will be substitute with result of find

    The exec's command must be contained between -exec and \;.

    ; is treated as end of command character in bash shell. For this I must escape it with \. If escaped it will be interpreted by find and not by bash shell.

  • Some parameter accepts value n with + or - in front. The meaning is:

    • +n - for greater than n
    • -n - for less than n
    • n - for exactly n

  • find /etc -size -100k

    Search in /etc all files/directories with size less of 100 kilobytes

  • find . -maxdepth 3 -type f -size +2M

    Search starting from current position, descending maximum three directories levels, files with size major of 2 megabyte

  • find . \( -name name1 -o -name name2 \)

    • -o or, it is used to combine two conditions. \ is escape to avoid that ( or ) will be interpreted by bash shell

  • find . -samefile file

    • Find all files that have same i-node of file

  • find . \! -user owner

    • It will show all files that aren't owned by user owner. ! means negation, but must be escaped by \ to not be interpreted by bash shell

  • find . -iname name

    • Search name ignoring case

  • find . -perm 222

    • Find all files with permissions equal to 222. E.g. only file with permissions 222 will be showed

  • find . -perm -222

    • Find all files with at least permissions 222. E.g. 777 match as valid.

  • find . -perm /222

    • Find all files with write for owner or write for group or write for others (at least one)

  • find . -perm -g=w

    • Find all files with at least permission write for group

  • find . -atime +1

    • Show all files accessed at least two days ago (more than 24 hours)

Evaluate and compare the basic file system features and options

References:

Compare and manipulate file content

  • diff file1 file2

    Compare file1 and file 2

  • diff -y file1 file2

    Compare file1 and file 2 with output in two columns

  • vi file

    It is used to manipulate a file

    Inside vi:

    • i - switch between command mode to insert mode
    • Esc - switch between insert to command mode

    In command mode:

    • o - open a new line and enter in insert mode

    • O - open a new line above current position and enter in insert mode

    • :wq - write and quit

    • :q! - quit without save

    • :w! - force write

    • u - undo

    • ctrl + r - redo

    • gg - go to file begin

    • G - go to last line

    • Search

      • :/texttosearch
      • n - next occurence
      • N - previous occurence
      • :300 - go to line 300

    • dd - delete current line

    • x - delete current character

    • d$ - delete from current point to end of line

    • Replace:

      • :%s/one/ONE/g - replace all occurrences of one with ONE

        :%s/one/ONE - replace first occurrences of one with INE

    • Cut and paste:

      • v - select text
      • y - copy text selected text
      • p - paste copied text
      • d - delete selected text

    In insert mode:

    • It's possible to insert text

  • uniq fileRemove equal consecutive rows

    • uniq -w 2 file

      Remove equal consecutive rows comparing only first two characters

    • uniq -c file

      Remove equal consecutive rows and show number of occurrences

  • sort file order file content

    • sort -k 2 file

      Order file content using as reference second word

  • cut -d delimiter -f column

    • cut -d ' ' -f 1 file

      Print first word of each line. Delimiter will be space

    • cut -d ' ' -f 1,3 file

      Print first and third word of each line. Delimiter will be space

  • cat filePrint file content

  • tail file Print last 10 file lines

    • tail -n 5 file Print last 5 file lines
    • tail -f file Print last 10 file lines and append. Useful to monitor log files

  • head file Print first 10 file lines

    • head -n 2 file Print first 2 file lines

  • tr SET1 SET2 translate set of characters one to set of characters 2

    • cat file | tr test sub

      It will replace all occurrences of test with sub

    • cat file | tr -s ' '

      It will replace all consecutive occurrences of space with one space

  • file namefile print the type of namefile

Use input-output redirection (e.g. >, >>, |, 2>)

All Unix-based operating systems provide at least three different input and output channels - called stdin, stdout and stderr respectively - that allow communication between a program and the environment in which it is run.

In Bash each of these channels is numbered from 0 to 2, and takes the name of file descriptor, because it refers to a particular file: as it happens with any other file stored in the system, you can manipulate it, copy it, read it or write it on its.

When a Bash environment is started, all three default descriptor files point to the terminal where the session was initialized: the input (stdin - 0) corresponds to what is typed in the terminal, and both outputs - stdout ( 1) for traditional messages and stderr (2) for error messages - they are sent to the terminal. In fact, an open terminal in a Unix-based operating system is usually itself a file, commonly stored in /dev/tty0; when a new session is opened in parallel with an existing one, the new terminal will be /dev/tty1 and so on. Therefore, initially the three file descriptor all point to the file representing the terminal in which they are executed.

There are operator to redirect input, ouput and error.

  • < - redirect stdin

    • wc < file

      Execute wc using the content of file as input

  • > and >> - redirect stdout

    • echo test > file1

      Write test in a file1. The content of file1 will be replaced

    • echo test >> file1

      Append test in file1

  • 2> - redirect stderr

    • find /proc -name "cpu*" 2> /dev/null

      Find in /proc file/directory that begin with cpu and redirect all errors, like 'Permission Denied' to special file /dev/null (virtual file that discard all data)

  • | - the stdout is transformed in stdin

    • cat file | wc

      Use the output of 'cat file' as input of wc

  • 2>&1 - redirect stderr to same place of stdout

  • All redirections can be combined

    • find /etc -name '\*a\*' 2> /dev/null | less

References:

Analyze text using basic regular expressions

  • File Globbing in Linux

    File globbing is a feature provided by the UNIX/Linux shell to represent multiple filenames by using special characters called wildcards with a single file name. A wildcard is essentially a symbol which may be used to substitute for one or more characters. Therefore, we can use wildcards for generating the appropriate combination of file names as per our requirement.

    • * - Every character

      ls -l a*

      List all file/directories that begin with a

    • ? - Every single character

      ls -l a?

      List all file/directories formed by two character that begin with a

    • [ab] - list of characters

      ls -l a[ab]

      List file/directories called aa or ab

    • [a-c]

      ls -l a[a-c]

      List file/directories called aa, ab and ac

    • Wildcards can be combined

      ls -l a[a-c]*

      List all file/directories that begins aa, ab and ac

  • grep pattern path/*

    Search pattern inside the strings of the files in path/*. Show file name and row matching pattern

    It is no recursive and key sensitive. To have recursion -r must be added

    Pattern can be a regular expression. The regular expression must be surrounded by ' ' otherwise content could match bash globing.

    • grep -l patter path/*

      Search pattern inside file in path/*. Show only file name

    • grep -lr patter path/*

      Search pattern inside file in path/* and path subdirectories. Show only file name

    • grep -ilr patter path/*

      Search pattern ignoring case inside file in path/* and path subdirectories. Show only file name

Regular Expressions

Character Definition Example Result
^ Start of a string ^abc abc, abcd, abc1
$ End of a string abc$ abc, rasabc, 2aabc
. Any character except newline a.c abc, acc, a1c
Alteration a
{...} Explicit quantity of preceding character ab{2}c abbc
[...] Explicit set of characters to match a[bB]c abc,aBc
[a-z0-9] One lower case characters or number a[a-z0-9]c aac,a1c
(...) Group of characters (abc){2} abcabc
* Null or more of the preceding characters a*bc bc, abc, aabc, aaaabc
+ One or more of the preceding character a+bc abc, aabc
? Null or one of the preceding character a?bc bc, abc
^$ Empty string

  • Not all regular expressions are supported by grep. As alternative can be used egrep

  • sed - Without -i the results of file alteration won't be permanent

    • sed 's/source/target/' file

      In any row of file, it will change first occurrence of source to target. Print all rows

    • sed 's/source/target/g' file

      In any row of file, it will change all occurrences of source to target. Print all rows

    • sed 's/source/target/gI'

      In any row of file, it will change all occurrences of source to target. Ignore case = case insensitive. Print all rows

    • sed '10s/source/target/' file

      For row 10, it will change first occurrence of source to target. Print all rows

    • sed -n 's/source/target/p'

      In any row of file, it will change first occurrence of source to target. Print only changed rows

    • sed -n '/source/p' file

      It will print only rows that contain source

      It is equal to grep source file

    • sed -n 2,4p file

      It prints rows from 2 to 4

    • sed '/source/d' file

      Delete rows with source

    • sed -n 12d file

      Delete row 12

    • sed '11inewline' file

      It will insert newline as line 11

    • sed -i 's/source/target/g' file

      In any row of file, it will change all occurrences of source to target. Save result to file

    • sed -i.orign 's/source/target/g' file

      In any row of file, it will change all occurrences of source to target. Save result to file but keep an copy of original file with name file.orign

References:

Archive, backup, compress, unpack, and uncompress files

  • tar Save many files into a single file

    File permissions are maintained by default only for file users. For other user I must explicit say to maintain permission during decompression using -p parameter

    • tar jcfv file.tar.bz2 *

      Save all files of current directory in new bzip2 compressed file called file.tar.bz2

    • tar jxfv file.tar.bz2

      Extract content of file.tar.bz2

    • tar tf file.tar

      Show content of file.tar. Note: the file.tar isn't compressed

    • tar --delete -f test.tar file

      Delete file from test.tar. Note: the test.tar isn't compressed

    • tar --update -f test.tar file

      Update file in test.tar. Note: the test.tar isn't compressed

    • tar X<(command that generate list) -c -f file.tar *

      tar X<(ls | file -f - | grep -i MPEG | cut -d: -f 1) -c -f file.tar *

      Exclude file MPEG from content of file.tar

  • Backup a device

    Device must be unmounted

    dd if=/dev/sda of=/system_images/sda.img

  • Restore device

    dd if=/system_images/sda.img of=/dev/sda

  • rsync it is used to keep synchronized the content of two directories

    • yum -y install rsync Install rsync command

    • rsync -av source dest

      Synchronize source with dest. -a archive, provide a series of default option

    • rsync -avz /tmp [email protected]:/dest

      Synchronize tmp with dest that it's contained in a remote machine with IP 123.123.123.123.

      -z means that content will be compressed during transfer

    • rsync -avzhe ssh source root@remote_host:/remote_directory/

      Synchronize source with remote_directory using ssh

Create, delete, copy, and move files and directories

You must be able to check results of activities.

  • ls list directory content

    • ls -l long output. It will print more columns

      File Type+Permissions - Number of links - Owner - Group - Dimension - Creation date - Creation hour - Name

      First letter of first column indicate file type:

      • - : file
      • d: directory
      • l: link

    • ls -la long output plus hidden files

    • ls -lR long output recursive (show subdirectories content)

    • ls -lt long output sorted by modification time

    • ls -ld /etc show the directory properties and not its content

  • du file show disk usage

    • du directory show space used by directory and each subdirectory. It is recursive
    • du -s directory summarize space used by directory and subdirectory
    • du * show space of each file in current directory

  • pwd print current directory

  • touch file

    It creates an empty file

  • cp source destination copy source file to destination

    • cp file1 file2 ./dest

      Copy file2 and file2 to directory dest

    • cp * ./dest

      Copy all file of current directory to directory dest

    • cp -r dir1 dir2

      Copy dir1 in dir2. -r recursive

  • mkdir dir create directory dir

    • mkdir -p dir/dir2

      Create a directory dir with a subdirecotory dir2

  • rmdir dir remove dir. Note: dir must be empty

  • tree show directories tree

    • yum -y install tree to install tree

  • mv file file2 rename file in file2

    • mv file dir move file in directory dir
    • mv dir .. move directory dir at the upper directory level

  • rm file delete file

    • rm -f file remove read-only file
    • rm -r dir remove directory dir and all subdirectories and files

Create and manage hard and soft links

inode

The i-node (index node) is a data structure in a Unix-style file system that describes a file-system object such as a file or a directory. Each i-node stores the attributes and disk block location(s) of the object's data.

File-system object attributes may include metadata (times of last change, access, modification), as well as owner and permission data.

Directories are lists of names assigned to i-nodes. A directory contains an entry for itself, its parent, and each of its children.

Each i-nodes is identified by a unique i-node numbers

To summarize: directory contains filenames, that is associated to i-node, that contains reference to data block.

Hard link

  • The filenames is an hard link.

  • I can have two filenames that point to same i-node.

  • Hardlink limits:

    • Must point to same device
    • Hardlinks pointing a directory cannot be created

Symbolic link

  • It's a pointer to a filename

  • This means that there will by this chain: link -> filename -> i-node

    • If filename will be removed, link will become invalid

  • Note: permissions on a link are "open", because real permission are associate to i-node

  • ls -li in first column show the i-node number

  • ln target newname It will create and hard link to the same i-node of target with name (filename) newname

  • ln -s target newlink It will create a symbolic link to target called newlink

    • ln -s /var . It will create a symbolic link to var in current directory. The name of link will be var

Note: A file is considered deleted when they don't exist anymore hard link to same i-node. This means that rm remove link, hard or symbolic.

References:

List, set, and change standard file permissions

To see user, group and permission use ls -l. Permissions are in the first column, name in third and group in fourth.

Each file/directory will have an owner and will be associated to a group.

The permissions for each file/directory are given for each of this category:

  • Owner
  • Group
  • Others

Others are all other users that are not the owner and are not member of group.

NOTE: The order matters.

For each category can be set below permissions

  • Read
    • Octal value: 4
  • Write
    • Octal value: 2
  • Exec (Execution)
    • Octal value: 1

The right that each permission provide are different and depends if target is a file or a directory:

File Directory
Read (4) Read or Exec List (ls)
Write (2) Modify Create Delete
Exec (1) Run cd

Note: When exec is set for group of other, file will be executed with identity of the user that are executing command (user ID) and group of user (group ID)

Absolute mode:

  • Use numbers for each permission, that must be added if more that a permission

  • chmod 760 file Change file permission

    • Owner: grant read, write and exec
    • Group: grant read, write
    • Others: no permission

Relative mode:

  • chmod +x file Add exec to owner, group and other
  • chmod g+w file Add write to group
  • chmod o-rw file Remove read and write to others

Advanced permissions

There are other special permissions that can be granted to file/dirctories

File Directory
suid (4) Run as owner of file N/A
sgid (2) Run as group owner Inherit directory group when a file is created
sticky bit (1) N/A A file can be deleted only by owner or by directory's owner

  • Suid: When a file with setuid is executed, the resulting process will assume the effective user ID given to the owner class. This enables users to be treated temporarily as root (or another user). E.g passwd has suid setted

  • Sgid: When a file with setgid is executed, the resulting process will assume the group ID given to the group class

  • Sticky bit is applied to /tmp

  • Suid cannot be applied to Bash scripts

Absolute mode:

  • chmod 4760 file Change file permission
    • Add suid
    • Owner: grant read, write and exec
    • Group: grant read, write
    • Others: no permission

Relative mode:

  • chmod u+s file set suid
  • chmod g+s file set guid
  • chmod +t dir set sticky bit

References:

Read, and use system documentation

  • commad --help

    • Show help of a command

  • man command

    • Show command manual

    • man -k keyword

      Search a manual for provided keywork

      • sudo mandbCreate database used by man -k command

  • /usr/share/doc

    • It contains configuration files examples

  • info command

    • It shows info document

  • bash completion

    • During the digitalization of a command can be used the pressed two time Tab key to show possible value or parameter
    • yum -y install bash-completion must be installed

Manage access to the root account

  • root is the system administrator

  • When logged as root, shell prompts # character. Otherwise $

  • su Used to become root. It will continue to use the current session with user and group id substituted

    • It will ask root password

  • su - Used to become root. It is same as logging into a fresh session on a terminal

    • It will ask root password

  • su - user Login as user.

    • It will be required user password
    • If command is executed by root, password won't be required

  • sudo command to allow an ordinary user to execute commands as a different user (usually the superuser)

  • In default configuration, group wheel is authorized to act as root. If a user is member of wheel can execute all command as root with this syntax:

    • sudo command
    • NOTE: user password must be provided

  • To add user to wheel execute:

    • usermod -aG wheel username

  • visudo Modify the sudo configuration

    • Basic configuration:
    • demo ALL=(ALL:ALL) ALL ​ The first field indicates the username that the rule will apply to.
    • demo ALL=(ALL:ALL) ALL ​ The first "ALL" indicates that this rule applies to all hosts.
    • demo ALL=(ALL:ALL) ALL ​ This "ALL" indicates that user demo can run commands as all users.
    • demo ALL=(ALL:ALL) ALL ​ This "ALL" indicates that user demo can run commands as all groups.
    • demo ALL=(ALL:ALL) ALL ​ The last "ALL" indicates these rules apply to all commands.

    Whit this row inserted in sudo configuration, demo user can execute this command:

    sudo -u user command

    This means that it will execute command with the identity of user.

    If -u is not specified, this means that command will be executed as root.

    demo user can open a root session running:

    sudo su -

    The powerfulness of this command is that a root session can be opened only providing user password (in this case the password of user demo).

    This means that root direct login (with user and password) could be disabled and root session will be opened using only sudo. Some Linux distribution use this method as default configuration (e.g Ubuntu).

    The advance is that root password is not shared if I need to add a new system administrator.

  • In sudo configuration % indicate group

    • %users localhost=/sbin/shutdown -h now

      The users in group users can execute command /sbin/shutdown -h now on localhost as root

  • To simplify configuration in sudo configuration can be used alias

    • Cmnd_Alias SOFTWARE = /bin/rpm,/usr/bin/up2date, /usr/bin/yum

    SOFTWARE can be used in sudo configuration rows