linux_basics.md

Terminal

• clear (CTRL+L) to clear the screen
• exit close the current session

Personalize the prompt

The prompt is the string that greets the user every time the system is waiting for input. In Ubuntu, it is usually defined as

usrname@hostname:current_directory$ 

for standard users, or

root@hostname:current_directory# 

for the root user and sudoers.

To change the appearance of the prompt it is sufficient to redefine the environment variable PS1 as in:

PS1 'promptstring'

where promptstring can contain, besides any text, one or more of the following escape sequences:

• \h hostname, up to the first dot
• \H full hostname
• \u username of the current user
• \W current working directory (partial path, $HOME is abbreviated as ~)
• \w current working directory (full path, $HOME is abbreviated as ~)
• \d current date in "Weekday Month Day" format
• \D{format} current date in the specified format (and parsed by strftime(3))
• \t current time in 24-hour HH:MM:SS format
• \T current time in 12-hour HH:MM:SS format
• \@ current time in 12-hour AM/PM format
• \A current time in 24-hour HH:MM format
• \n newline
• \r carriage return
• \\ a backslash
• \nnn the character corresponding to the octal number nnn
• \e an ASCII escape character (033)
• \[ begin a sequence of non-printing characters, which could be used to embed a terminal control sequence into the prompt
• \] end a sequence of non-printing characters
• \# command number of the current command
• \! the history number of the current command
• \j the number of jobs currently managed by the shell
• \a an ASCII bell character (07)

It's also possible to include the result of any command (be careful here!) using backticks to include it wherever in the promptstring.

Notice that when a user wants to display lots of information, but wants anyway to keep the prompt short and unobstrusive, one strategy is to divide that information between two or more lines using the newline character \n . While many people dislike a multi-line prompt, it is often the only way to provide more information in the prompt.

The change of the PS1 variable will take effect immediately, but will be abandoned when that terminal session is ended. To change it permanently you need to change your personal configuration file ~/.bashrc.

• First save a backup copy, just in case

  cp ~/.bashrc ~/.bashrc-backup 
  

• then open it for editing:

  nano ~/.bashrc 
  

• and find the section that starts with:

  if [ "$color_prompt" = yes ]; then
  

• once located, comment the code inside the else part, and add below it the chosen promptstring
• save and exit.

It's also possible to add colours, though it's a bit excruciating if done by the book. But you can actually copy the following snippet:

# ANSI color codes
RS="\[\033[0m\]"    # reset
HC="\[\033[1m\]"    # hicolor
UL="\[\033[4m\]"    # underline
INV="\[\033[7m\]"   # inverse background and foreground
FBLK="\[\033[30m\]" # foreground black
FRED="\[\033[31m\]" # foreground red
FGRN="\[\033[32m\]" # foreground green
FYEL="\[\033[33m\]" # foreground yellow
FBLE="\[\033[34m\]" # foreground blue
FMAG="\[\033[35m\]" # foreground magenta
FCYN="\[\033[36m\]" # foreground cyan
FWHT="\[\033[37m\]" # foreground white
BBLK="\[\033[40m\]" # background black
BRED="\[\033[41m\]" # background red
BGRN="\[\033[42m\]" # background green
BYEL="\[\033[43m\]" # background yellow
BBLE="\[\033[44m\]" # background blue
BMAG="\[\033[45m\]" # background magenta
BCYN="\[\033[46m\]" # background cyan
BWHT="\[\033[47m\]" # background white

and paste it before the if block, and then you can insert colours in the string definition for PS1 using something like $BWHT instead of its ANSI counterpart \[\033[47m\], to indicate a background white.

Help

• man

Date and Time

• date check the system date and time, with timezone
• timedatectl
• date +%Y%m%d -s "yyyymmdd" set the system date
• date +%T -s "hh:mm:ss" set the system time
• sudo dpkg-reconfigure tzdata reconfigure the timezone of the system clock
• sudo hwclock --show check the hardware clock
• sudo hwclock --set --date="yyyy-mm-dd hh:mm:ss" --localtime set the hardware clock to the local time

Folders structure

• / (root of the system)
• bin Essential commands
• boot Boot loader files, Linux kernel
• cdrom
• dev Device files (notice that in Linux everything is a file)
• etc System configuration files, almost everything on how to do things in Linux is stored here
• home host the repositories for all users different from root. Notice that ~ represents the home directory of the current user, so that ~ points in general towards different directories according to the user logged in.
• lib Shared libraries, kernel modules
• lib64
• lost+found Directory for recovered files (if found after a file system check)
• media Mount point for removable media
• mnt Mount point for local, remote file systems
• opt Add-on software packages
• proc Kernel information, process control
• root Super-user (root) home
• run
• sbin System commands (mostly root only)
• snap
• srv Holds information relating to services that run on your system
• sys Real-time information on devices used by the kernel
• tmp Temporary files
• usr Secondary software file hierarchy
• var Variable data, spooled files, they can grew indefinitely so they should kept under control
  • log store all logs for all apps and services
  • ftp
  • www
    • html Apache web server's initial home page directory for the system

Managing Files and Folders

• Names and commands are case sensitive

• . the current directory

• .. the parent directory of the current directory

• / the root directory (notice that the home directory of the root user is /root)

• ~ the home directory of the current user, or /home/usrname/

• - the previous directory, if it exists

• pwd returns the full path of the current directory

• cd [path] allows to navigate through the file system

  • cd, cd ~ and cd $HOME all return to the current user home directory

• rm /path/to/file

  • rm -f
  • rm -r

• mkdir path create a new directory

• rmdir path

  • rmdir -r path

• shred

• ls [path] to list files and folders located in the current directory or in the specified path

  • ls -l
  • ls -a also returns hidden files
  • ls -R use a recursive scan into all sub-directories. If you know it'll be a lot of information, or just to stay on the safe side, you may want to redirect the output to a text file (see below)

• cp copy files

• mv move or rename files

• scp secure copy between different servers

• cat filename print the content of any file

  • cat -n numbers the lines in the output
  • cat -s prints a maximum of one blank line at a time, that's any contiguous multiple blank lines in the input are translated in a single blank line i the output. Notice that combining -s and -n, in the output the numbers relate only to the lines that are actually printed
  • cat fname1 fname2 ... print the content of all the specified files chained together as a single file

• touch

• less filename filters for paging through output

• tail filename prints the last lines of a file

• sort filename prints the contents of a file in the specified order

• nano filename basic editor for text files

• emacs filename powerful editor for text files

• vim filename powerful editor for text files

• grep

• apropos

• find

• which prgname prints the location of a command

• locate find files from an index

• ln creates links between files

• wget url download files using HTTP, HTTPS, and FTP protocols

  • wget -O filename url save the downloaded file as filename
  • wget -m url mirror the content of

• curl

• tar

  • tar -x
  • tar -z
  • tar -v
  • tar -f

• gzip

• bash

Piping, Redirections

• >

• >>

• |

Users and Groups, Permissions, Ownership

• the root user is the first user created during installation and it's the

• useradd usrname

• su

• sudo

  • sudo -i

• sudo passwd to change password for the root user

• sudo passwd usrname to change password for the user usrname

Apart from root, there are three sets of users in every Linux system:

• the user who created the file/directory
• the group the owner belong
• other, meaning anyone different from the owner and the group(s)

Each user has

To set the permissions on a file and/or directory we use the following commands:

• chown
• chmod change file and directories access permissions
• chgrp

Package management

• what's apt /

• what's dpkg

• apt-cache search

• apt-cache show pkgname

• sudo apt-get update

• sudo apt-get upgrade

• sudo apt-get dist-upgrade

• sudo apt-get install pkgname

• sudo apt-get remove pkgname

• sudo apt-get autoremove

• sudo apt-get clean

• apt -qq list list all packages installed in the system

• apt -qq list <pkgname> or dpkg -l <pkgname> checks if the package <pkgname> is already installed

• dpkg -l list everything installed in the system

• make Compiles and installs program from source

Services

• top

• ps -acx

• kill pid

• service --status-all list all installed services, and if they are running or not

• sudo /etc/init.d/srvname start or sudo service srvname start

• sudo /etc/init.d/srvname stop or sudo service srvname stop

• sudo /etc/init.d/srvname restart or sudo service srvname restart

• ``

• less /etc/services list all ports used by standard services

Firewall

All the following commands must be run with root/sudo admin rights:

• ufw enable
• ufw disable
• ufw reset
• ufw status
  • ufw status verbose
• ufw allow XXXX
• ufw allow app list
• ufw allow app_profile where app_profile must be present in the above list. Some default profiles present are: ssh, http, https, ftp
• ufw allow XXXX/tcp
• ufw allow XXXX/udp
• ufw allow XXXX:YYYY
• ufw allow from www.xxx.yyy.zzz
  • ufw allow from www.xxx.yyy.zzz to port
  • ufw allow from www.xxx.yyy.zzz to any port
• ufw show added
• ufw delete allow XXXX

Monitoring

• du print disk usage
• free print information about RAM
• watch free
• watch sensors (if not present run: sudo apt-get install )
• ``

Check Internet Speed with speedtest

• Install pre-requisite pip:

  sudo apt-get install python-pip
  

• Install the app:

  sudo pip install speedtest-cli 
  speedtest-cli
  

• To upgrade the speedtest-cli application in future:

  pip install speedtest-cli –-upgrade
  

• It's possible to create an internet speed log by simply scheduling a cron job (see below) to the system, adding the following line in the crontab file:

  min hour dom mon dow  /usr/local/bin/speedtest-cli >> /tmp/speedlog.txt
  

  where min, hour, dom, mon, dow should be replaced with the appropriate desired values, and assuming that the script speedtest_cli.py is installed in /usr/local/bin.

Scheduling

The package cron allows Linux users to run commands or scripts at a given date and/or time, or periodically at given dates and/or times. Cron is one of the most useful tool in a Linux OS, notably for sysadmin jobs such as backups or cleaning /tmp/ directories.

• crontab -u usrname -e

The general format of a line in the crontab file is the following:

{minute} {hour} {day-of-month} {month} {day-of-week} {user} {/path/to/shell-script}

Every line consists in a call to a command or scripts, where the first five arguments should be in order:

• minute(s) when the process will be started [0-59]
• hour(s) when the process will be started [0-23]
• day(s) of the month when the process will be started [1-31]
• month(s) when the process will be started [1-12], or [JAN-DEC]
• day(s) of the week when the process will be started [0-6], or [SUN-SAT] (Note here that Sunday can actually be specified with a 7)

Additionally, there are some general options valid for all the temporal arguments:

• * any value
• */n every n period
• , value list separator
• - range of values

See here for many real cases examples.

It's important to keep in mind that cronjobs can actually fail. To monitor a cronjob ...

bash programming

Measuring Performance / Benchmarking

Variables

Remote Access

SSH

Telnet

VNC

Desktop Environment over Ubuntu Server

see this post

• sudo apt-get install ubuntu-desktop install the default Unity desktop environment:

• sudo aptitude install --without-recommends ubuntu-desktop install the Unity desktop without addons (email, OpenOffice, ...)

• sudo apt-get install xubuntu-desktop install XFCE a very lightweight desktop environment, just the basic GUI

• sudo apt-get install lubuntu-desktop : install LXDE an even lighter GUI

• ``

• ``