Linux Basics

May 3, 2016

Linux Basics

Things I learned from just reading linux essentials

Pwd print working directory shows current path

Ls list what is currently in our directory

Ls [option] [directory]
-l flag is long listing

drwxr-xr-x 12 TaiRui staff 408 4 Apr 19:46 Applications

D <-directory type if nothing there it is not a directory
RWXR permissions
12 -> number of blocks
TaiRui owner of the directory / file
Group that the owner is part of
File size in MiB
The time it was last changed
Folder name / File name
Paths
Absolute which are absolute locations based from your home level ~
- Relative which can be achieved from current / or from above ../, or from your user home level ~
- And change directories using cd [directory]
Important folders in your NIX system
- /etc -> stores configuration files for the system
- /var/log -> stores logs for various system components
- /bin -> location of several commonly used programs
- /usr/bin -> Another location for programs [http://unix.stackexchange.com/questions/5915/difference-between-bin-and-usr-bin]
Everything is a file in UNIX
- Everything in a NIX system is a file, this was set as a core principal design
- File extensions don’t matter to the system, as it relies on the file header within the file to tell the nix system what type of file it is
  - We can check a file’s type using the file command
- Files are case sensitive and even commands are case sensitive as well
- Spaces!
  - How do we deal with spaces in the file or directory? Either by using quotes cd ‘Family Photos’
  - Or via an escape character cd Family\ Photos the escape was the \ <- there is a space after that escape character
- Hidden files in nix systems all begin with a full stop period, so in your home directory configurations can be seen by doing a ls -a
Man Pages
- Man [command] which will show you a list of all configuration with the command shows you how to use it
- Searching can be done with man -k or while in a man page / then press n to browse next and next
File and Directory Manipulation
- Mkdir [options]
  - The creation of directory
  - -p will create the parent directory if you where to create a directory /parent1/folder
  - -v will log the creation of the folders
  - -pv combination p and v, lol so we list out the creation of multiple parent folders and such
- Rmdir removes the directory =
- Touch [option] we don't just create a file we TOUCH ONE
- Cp copying a file or folder
  - Cp source_file new_file
  - Cp -r source_folder new_folder
- MV moving a file or folder
  - Mv source file new_source_location
  - Mv -r source_folder new_source_location_folder
  - To rename we can just do the same for moving folders and files just change the name
- Rm removes a file
  - Rm [option]
  - Rm -r <- removes recursively
Command line Text Editors
- Vi - Vim the thing that everyone is scared of

Open a file vi [options]
Read only view
Text commands
i will change the opened mode to insertion mode which allows for the modification of the file
ZZ save and exit
:q! quit don’t save
:w save
:wq save and quit
Arrow keys - move the cursor around
j, k, h, l - move the cursor down, up, left and right (similar to the arrow keys)
^ (caret) - move cursor to beginning of current line
$ - move cursor to end of the current line
nG - move to the nth line (eg 5G moves to 5th line)
G - move to the last line
w - move to the beginning of the next word
nw - move forward n word (eg 2w moves two words forwards)
b - move to the beginning of the previous word
nb - move back n word
{ - move backward one paragraph
} - move forward one paragraph
x - delete a single character
nx - delete n characters (eg 5x deletes five characters)
dd - delete the current line
dn - d followed by a movement command. Delete to where the movement command would have taken you. (eg d5w means delete 5 words)
u - Undo the last action (you may keep pressing u to keep undoing)
U (Note: capital) - Undo all changes to the current line * Cat (concatenate) -> used to view files and logs right in the cmdline * Less -> used to view large files with ability to scroll * Wildcards * * All seeing star, matches n characters of whatever * ? Matches one unknown character ?A* matches the second letter with A * [] range operator, matches any letter within the specified range ?[sv]* matches second letter with either s or v or [0-9] matches numbers from 0-9 * Permissions * 3 things for a file
- Read [r]
- Write [w]
- Execute [x]
  - 3 different types of permissions
- Owner single person owns the file, permission could be granted to other users to access the file
- Group
- Others
  - Chmod to change the permission to the file
- 3 components :
Who are we changing the permission for user, owner or all?
Are we granting or revoking permissions
Which permission are we setting (R W X)
- And we can do short hand such as 777 which in octal speak will set the individual bit values for the permissions at the [owner,group,others] group, and within that the bit representation of the value is used to determine the permission level
  - Filters
    - Filter is a program that accepts textual data and transforms it in a particular way
    - Head prints out the head of the document up to n number of lines
    - Tail prints out the bottom of the document up to n number of lines
    - Sort sorts the printout of the document by default alphabetically
    - Nl shows line numbers for the print out
    - Wc word count, character count.. Etc look in man pages
    - Cut useful for column data, csv… tabbed data, but you can cut out a certain column using this command
    - Sed stream editor, can edit an input stream’s data, or just do search and replace within a document
    - Uniq gives output of only unique data, really useful for column data again
    - Tac <-reverse of cat, going from the bottom to the top
  - Regex
    - . (dot) - a single character.
    - ? - the preceding character matches 0 or 1 times only.
    - * - the preceding character matches 0 or more times.
    - + - the preceding character matches 1 or more times.
    - {n} - the preceding character matches exactly n times.
    - {n,m} - the preceding character matches at least n times and not more than m times.
    - [agd] - the character is one of those included within the square brackets.
    - [^agd] - the character is not one of those included within the square brackets.
    - [c-f] - the dash within the square brackets operates as a range. In this case it means either the letters c, d, e or f.
    - () - allows us to group several characters to behave as one.
    - (pipe symbol) - the logical OR operation.
    - ^ - matches the beginning of the line.
    - $ - matches the end of the line.
  - Piping!! And Redirection
    - Every program that we run in the commandline has 3 different streams that can be connected to it
    - STDIN standard input data fed into the program
    - STDOUT standard output data outputted from the program
    - STDERR which are the error output stream
    - Redirecting to a file
- If we wish to take the output stream of a application and save it to a file we will use the ‘>’ greater than symbol to a file that you want as the output
- If we redirect to afile that’s name isn’t already created then a new file will be created for you, however if redirecting to an existing file that file will be over written

But we can get the data appended to the file using the » operator * Redirecting from a file

Using the ‘<’ less than operator we can actually redirect a file to be as an input to a program, we redirect into the stdin stream
- Redirecting to standard error
Using the 2> operator where we identify the stream by its number stream 2 being the err stream, we can direct an error the error stream which will be printed out on the commandline
ls -l video.mpg blah.foo > myoutput 2>&1 will redirect the output stream of err into the output file stream to be saved into the file myoutput
- Piping

We can pipe the output of one file into another file using the

operator, it will pipe the output from the application on the left into the one on the right

Process management
- Top <-shows processes in ram gives a live output
- PS aux to show all processes running
- Killing a process can be done with kill or we can truly kill by kill -9 which is a force kill which will not wait and kill it
- Running processes in the background

You can run processes in the background by adding the & operator at the end of a commandline to move the process in the background
To move a job or process back into the foreground use the ‘fg’ command with the job id to bring that process into the foreground
Ctrl z will bring the currently running foreground application into the background
- Bash Scripting
  - http://ryanstutorials.net/linuxtutorial/scripting.php

Linux Basics

Ls list what is currently in our directory

Paths