Terminal and console were a piece of hardware that let you interface with a computer.
https://thevaluable.dev/guide-terminal-shell-console/
'Terminal' and 'console' are nowadays used to name any (way of accessing a) text interface (of the operating system).
https://en.wikipedia.org/wiki/Unix-like
POSIX - a standard (published by IEEE and The Open Group) for operating system interface and environment, predominantly built to unify Unix-like systems
Linux, GNU/Linux, Linux distribution…
Shell is a program that provides a (text) interface to an operating system.
There is a bunch of such programs https://en.wikipedia.org/wiki/Comparison_of_command_shells [1]
Shells for Unix-like systems include at least sh, csh, tcsh, ksh, bash, zsh, dash, busybox, fish
Windows provides cmd.exe and PowerShell shells.
A shell reads commands from input and executes them. An example of a command is a program name followed by a list of arguments (separated by space).
Apart from a shell, to control the operating system, one needs also a set of programs that realize basic tasks. Such programs are sometimes called utilities.
As well as there are multiple implementations of a shell, so there are multiple implementations of the basic utilities.
When one uses a graphical user interface, to access a shell one can run a terminal emulator.
There are numerous terminal emulators (consult a list on Wikipedia). A choice of terminal emulators:
Linux typically starts a program that enables logging into shell in several virtual consoles alongside a graphical interface.
To switch to a virtual console one may use Ctrl+Alt+Fn shortcut, where Fn is a function key (F1÷F12).
Typically numbers 1÷6 are text terminals, 7 stands for first graphical console, and some systems are configured to output system log messages to virtual console 12
A standard way to work with remote UNIX-like systems is to log into a shell of the system using a SSH (Secure SHell) client.
The basic command to log into a shell on a remote system is:
ssh user@host
where user is the user name and host is the name or address of the target device.
For instance, if used john would like to log into a computer named polluks.cs.put.poznan.pl, he would have to input
ssh john@polluks.cs.put.poznan.pl.
Currently all major operating systems (including Windows) provide ssh command by default.
Secure SHell encrypts all traffic. Its predecessor – telnet – sends all data (including passwords) in plaintext.
Upon password authentication, the client must send the password to the server.
It is vital for security to confirm the authenticity of the server – the client
must verify that it does not contact a rogue server that can learn the password
(ready-to-go tool)
and/or hijacks the session.
Asymmetric cryptography
is used to this end: the server has a (secret) private key, and upon each
connection it sends the public key to the client, and the client verifies whether
the public key pairs with the private.
Hence, upon the first connection to a new server the SSH client program requests
the user to verify the authenticity of the public key and stores the key (in
OpenSSH in ~/.ssh/known_hosts) to verify it automatically upon subsequent
connections.
Exercise 1 Run a terminal emulator. Execute command date and echo $SHELL within.
Exercise 2 Execute command sleep 1h and interrupt it by pressing Ctrl + c.
Exercise 3 Switch to second virtual console and log in. Execute pwgen. Return to graphical console.
Exercise 4 Use SSH to access polluks.cs.put.poznan.pl.
Most commands will display help when run with --help argument. Some commands also use -h for this.
Exercise 5 Display help for cat and bat commands by running them with help option. (NB: bat is installed by default only in some Linux distros.)
Usually together with a program comes with a set of system manual pages. The pages can be displayed using the man pagename command. To leave the manual, type q.
Exercise 6 Display manual page for timeout command.
Manual pages are organized into sections. Use man man to learn about the standard sections.
Page names are unique within a section, but there may be pages with the same name in different sections.
You may use the whatis pagename command to learn which sections contain given page name and display the full page title.
Exercise 7 List all pages that are named time.
To select a page from a given section, one has to use man section pagename syntax.
Exercise 8 The man itself is described on two manual pages. Display both. What information is contained in each one?
man uses another tool, called less, to actually display the manual pages. less will be discussed in details later.
To search for text in less, one can type / followed by the search term and Enter. Next / previous occurrence of the search term can be found by typing n / N. To jump to the beginning of the document press g. h displays a summary of less commands.
Exercise 9 Display manual page for bash and search for printf there. Then, search for explanation of dirs command.
For searching within manual there is also the apropos what command that searches within page names and titles.
Apart from man, there is an alternative (and less popular) documentation system called info. To see an Info page, type info name. Some programs ship with a more detailed Info document than manual page.
Manuals, documentation and examples for commands traditionally use a set of square and curly brackets to indicate (respectively) optional parts and choice of one of alternatives.
Sometimes arbitrary brackets are used to list alternatives: this is the case if they contain tokens separated by a | character. Angle brackets are sometimes used to name elements.
wait [pid] means that both wait and wait 1234 is a correct command (provided 1234 is a pid)grep --color[=WHEN] means that both grep --color and grep --color=always are correcttar {c|x} means that one must write either tar c or tar x
Ellipsis is used to indicate that a part of command may repeat, e.g. cat [FILE]... says that cat accepts multiple filenames.
Many programs accept options (called also switches) in the argument list.
The options are often written as short options in form -o where o is a single character and long options --option where option is the option name.
Some programs use only long options preceded with single dash (e.g., find, convert, ffmpeg).
Short options can usually be combined in the following way: ls -color is identical to ls -c -o -l -o -r (and different than ls --color)
Options may require an argument or may accept an optional argument.
Apart form command name, options (and keywords, if any) all other text is a placeholder for concrete values.
For instance, in find -amin N and find -anewer FILE the N and FILE are such placeholders: N must be replaced by number of minutes, and FILE must be replaced by a name of a file.
Exercise 10 Run:
man zip and review the SYNOPSIS,
findmnt --help and explain its syntax
taskset -h and explain its syntax
Almost any shell supports autocompletion. It is invoked by pressing TAB key.
Autocompletion is supposed to type the rest of a word for you, and does so if only one possibility is found.
If autocompletion recognizes multiple possibilities, it does nothing. If one presses TAB key again, most autocompletion procedures display all possibilities.
Autocompletion implementation differs across shells. Many shells support context-aware helpers.
Exercise 11 Type vu and press TAB. Type ba and press TAB twice.
Exercise 12 Type man -- and press TAB twice. Type man fs and press TAB twice.
In Unix-like systems all files and directories are organized in a tree, i.e., any two files share the same top-level directory called root directory. The path to root directory is /.
Filesystems other than the root one (e.g., other partitions, compact disks, flash drives), are mounted inside a selected directory located within / (rather than being a separate file hierarchy).
A path is a chain of names separated by /.
. stands for current directory and .. stands for parent directory.
A path that starts with / is called absolute. A path that is not absolute is called relative.
In Unix-like system each process has its working directory. Relative paths are resolved relative to the working directory.
Each user has a home directory. Shell (and some other programs) replaces the word ~ with home directory of the current user. ~user stands for home directory of user, so ~/x/a.txt is within home directory of current user, and ~x/b.txt is within home directory of user x.
The command ls [dir] lists file in directory dir. When run with no arguments, it lists the current working directory.
By default ls does not list hidden files. The -a switch changes this behaviour.
Switch -l prints list of files with details such as file owner, size, modification date, etc.
Exercise 13 List files in /usr/share/zoneinfo/ and ../../../../../bin.
Exercise 14 List all files in your home directory.
Exercise 15 Test the following switches with ls command: -A -d -s -h -t -1
Apart from ls command, there are standard dir and vdir commands that list files.
Shell is often configured to use colors in ls by default and understand ll as ls -l, and la as ls -l -a.
The tree command outputs recursively contents of the directory in a tree form.
To print the working directory one can execute the pwd command.
Exercise 16 Check in which directory you end up upon logging into the system.
To change directory the command cd [target] is used. If no target is provided, cd changes current directory to the home directory.
If - is uses as the target, the directory is changed to the previously visited.
Exercise 17 Enter .config directory (located in your home directory) and list files within.
Exercise 18 Navigate to /var/log. What is stored there?
Exercise 19 Go back to where you've been before with cd -
To change directory and save the previous one can use pushd dir command.
pushd dir pushes the previous directory on a directory stack. The directory stack can be displayed with dirs command.
To change back one can either execute popd (that removes a directory from the stack and enters it) or pushd (that swaps current directory with the directory on the stack). pushd and popd accept arguments +n/-n that select directory on the stack.
In computer science patterns/expressions that describe the expected data can be
expressed and used to match against some data, usually to find fitting data or
check if the data matches the description.
For instance, lines beginning with a word followed by an equals sign might
be written as a regular expression ^\w+=.
There are countless syntaxes and implementations of such patterns/expressions.
In the shell, wildcards and regular expressions are commonplace.
Syntax and implementation of both wildcards and regexes is language/application
specific.
The shell uses syntax summarized e.g., in
man 7 glob and
man 7 regex
Upon parsing the user input, the shell attempts to replace with filenames any
word containing wildcard characters *, ? and expressions in
square brackets […]. A word that contains such wildcards is called
glob.
* expands to any text
? expand to single character
[ace] expands to either a, or c, or e
[f-h] expands to either f, or g, or h
If there is at least one file that matches the glob, the shell replaces the glob with all filenames that match. If no file matches, then the glob is left unaltered.
Exercise 20 Change directory to /usr/bin/ directory. List all files that end with cat.
Exercise 21 List all files that end with cat in /usr/bin/ while being in your home directory.
Exercise 22 List all files that are in the /usr/bin/ directory that have names consisting of two chatacters.
Exercise 23 List contents of /usr/share/zoneinfo/ and its subdirectories using a single command.
Exercise 24 List all files named UTC that are in any subdirectory of /usr/share/zoneinfo/.
Exercise 25 List all files starting with L that are in any subdirectory of /usr/share/zoneinfo/.
Exercise 26 List all files starting with F or H that are in any subdirectory of /usr/share/zoneinfo/. Use a single glob.
Exercise 27 Navigate to /usr/bin and attempt to execute ls*. What has happened?