04-First_glimpse_of_shell.md

title	author	date	lang
A first glimpse of the shell	CSC Training	2020-04	en

Contents of this session

This session shall give you some insight on:

• What is a shell?
• What is a command?
• Basic introduction how to navigate and change the filesystem from the shell
• Following shell commands will be discussed ls, cd, pwd, cat, less, mkdir, rmdir

What is a shell?

• A shell in computing provides a user interface for access to an operating system’s kernel services. (Wikipedia)
• Remote login:
  • Often no GUI (Graphical User Interface) available
• Shell: runs in terminal with a set of commands
• Different flavours:
  • bash (default), tcsh (old default), zsh, corn-shell, ...

What is a shell?

Lets open the a **terminal** and the **desktop GUI**

What is a command?

• A command is a small program provided by the shell

• The over-all structure of a command is:

  $ command -option [optional input]

• Example (listing of a directory, we will see details later)

  • MIND: $ depicts the shell's command prompt (is not part of the command)

  $ ls –lsh /etc/init.d

  • Are commands case sensitive? Try: $ Ls –lsh /etc/init.d
  • How to find a command? $ apropos list
  • How to find all options for a command? $ $ man ls

Listing of directories and files

Print contents of a directory or information on a file

• Detailed list of directory (in this case /etc):

  $ ls -lhtr /etc/

• the options are:
  • -l displays additional information (detailed list in GUI)
  • -h displays size in human readable format (in particular file-sizes)
  • -t orders by date (use -r to reverse order, i.e., oldest first)
  • -d omit listings of sub-directories (not used above)

Listing of directories and files: Wildcards

• Only print directory/filenames matching a expression:

  $ ls -d /etc/*.d

  • * is a wildcard for any combination of alpha-numeric characters

• Only print directory/filenames with a 4 char suffix:

  $ ls –l /etc/*.????

  • ? is a wildcard for any single alpha-numeric character

Moving around in directories

• change directory:

  $ cd /etc/

• print work directory:

  $ pwd 
  -> /etc

  • NB: output in shell is indicated by ->
• go to subdirectory:

  $ cd init.d
  $ pwd
  -> /etc/init.d

  • the above is the same as using relative path: $cd ./init.d

Moving around in directories

• Relative paths:

• a single dot . indicates the local directory

• double dots .. indicate the directory above

  • in last slide it was mentioned that one can use (relative path)

  $ cd ./init.d

  • our current directory is /etc/init.d

• to go back to /etc one can also use the relative path:

  $ cd ..

  • mind the space between cd and .. or .

Moving around in directories

• Absolute paths:
  • always starting with a leading / (indication root-directory)

$ cd /etc/init.d

- brings you there from everywhere in the file-system

• Combination of relative paths:

  $ cd ../../usr

  • brings you from /etc/init.d to /usr (check with pwd)

• The way home:

  $ cd ~/

  • brings you into home-directory (alternatively, just cd)

Creating directories

• Make a new directory

  $ mkdir mydir1

  • make first sure you are in your home-directory: cd ~/

• Create a sub-directory relative to existing path:

  $ mkdir mydir1/subdir1

• Recursively (whole sub-tree in one):

  $ mkdir -p mydir2/subdir2

  • try first without -p option; What happens?

Renaming/moving directories

• The command mv does both, renaming and moving directories:
• To rename mydir2 into mydir3

  $ ls
  -> mydir1 mydir2
  $ mv mydir2 mydir3
  $ ls
  -> mydir1 mydir3

  • Mind: renaming mydir2 works only if mydir3 does not exist!
  • Exactly the same strategy presented above applies also to renaming of regular files

Renaming/moving directories

• To move a directory into another directory, naturally, the destination has to already exist

   $ mkdir subdir3
   $ ls
   ->  mydir1 mydir3 subdir3
   $ mv subdir3 mydir3
   $ ls
   ->  mydir1 mydir3
   $ ls mydir3
   ->  subdir2  subdir3

Removing directories

• The command rmdir only removes empty directories

  $ cd mydir1
  $ rmdir subdir1
  $ cd ..
  $ rmdir mydir1
  $ ls
  -> mydir3

• Try to remove whole branch of empty directory tree

  $ rmdir mydir3
  -> rmdir: failed to remove 'mydir3/': Directory not empty

  • Exercise: Why is there an error message like the one above?

Recursively removing directory trees

• use -p option to recursively remove empty (!!) directory trees

  $ rmdir -p mydir3/subdir?
  -> rmdir: failed to remove 'mydir3/': Directory not empty
  $ ls mydir3
  -> ls: cannot access 'mydir3': No such file or directory

  • Exercise: Explain why, despite the error message, mydir3 is gone.

Contents of a file

• Printing contents of (text) file to screen:

   $ cat /etc/group

  • -n to precede lines with line numbers
  • What if the file does not fit on the screen?

• Open a scroll-/pageable view of a file:

  $ less /etc/group

  • Press q to quit
  • / to search forward, ? for backwards
  • n to find the next match, N for previous

Creating files, redirecting output

• In UNIX/Linux: everything is text
  • even directories are only special text-files (containing a list of filenames)
  • Also the output of commands is text
• Output can be redirected into files using the symbol > or >>
• In case of creating a new file creating new file:

  $ echo "hello world" > myfile1.txt

  • MIND: An existing file with that name will be overwritten
  • to prevent that from happen, set (use +o to revert)

  $ set -o noclobber

• Exercise: Show contents of myfile1.txt on screen

Creating files, redirecting output

• In case of creating a new file appending a file:

  $ echo "hello again" >> myfile1.txt

  • appends to an existing file
  • creates a new one, if file does not exist
• create an empty file:

  $ touch myfile1.txt

  • If myfile1.txt exists, only its access date will be altered
  • Remark: suffixes (like here .txt) are not really important in Linux, but useful for the user. They can have an impact on what actions the desktop GUI takes (e.g., opening PDF's with a PDF reader)

Copying files

• Make a copy of a file

  $ cp myfile1.txt myfile2.txt

  • MIND: If no special precausions are taken, an already existing destination myfile2.txt would be overwritten
  • one can play save by providing the option -i
  • this option will ask before overwriting exsiting files

Copying directories

• Recursively copying a directory

  $ mkdir -p mydir4/subdir4
  $ ls -d mydir?
  -> mydir4
  $ cp -r mydir4 mydir5
  $  ls -d mydir?
  -> mydir4 mydir5

• Exercise: Create another directory mydir6 and try the following commands:

  $ cp myfile2.txt mydir5
  $ cp -r mydir5 mydir6

  • What will be the contents of the directory mydir6?

Renaming/moving files

• Like for directories, the command mv has different results:
  1. destination does not exist (renaming)

   $ ls myfile?.txt
     -> myfile1.txt myfile2.txt
   $ mv myfile2.txt myfile3.txt
   $ ls
     -> myfile1.txt myfile3.txt

  2. destination exists and is a file (overwrite)

   $ mv -i myfile1.txt myfile3.txt
     -> mv: overwrite 'myfile3.txt'? n

  3. destination exists and is a directory (moving)

   $ mv -i  myfile3.txt mydir5
   $ ls myfile?.txt
     -> myfile1.txt

Renaming/moving files

• Exercises:
  • Create another empty file myfile4.txt
  • Make a listing (ls) to verify that it exists
  • Come up with a single command that moves myfile4.txt into the directory mydir5 and renames it at the same time into myfile5.txt
  • As result, show the listing of mydir5

Removing files

• Removing a file (again with playing save with -i option)

  $ rm -i  myfile1.txt
  -> rm: remove regular file ' myfile1.txt'?  y
  $ ls myfile1.txt
  -> ls: cannot access 'myfile1.txt': No such file or directory

  • MIND: There is no such thing as a trashbin in Linux. Once a file is gone, it usually is gone for good!

Recursively removing (non empty) directories

• Removing recursively a whole directory tree

  $ rm -r mydir5

  • the option -f would force to recursively remove the whole tree behind the directory
  • MIND: rm -f -r is a very dangerous command and you should execute it with care (and you always should have a backup of your important files)