Jan Kończak

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

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]

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.

POSIX standarizes both the shell and its vital utilities.

When one uses a graphical user interface, to access a shell one can run a terminal emulator.

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 has to 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.put.poznan.pl, he would have to input
ssh john@polluks.put.poznan.pl.

Currently all major operating systems (including Windows) provide ssh command by default.

Exercise 1 Run a terminal emulator. Execute command date and echo $SHELL within.

Exercise 2 Switch to second virtual console and log in. Execute pwgen. Return to graphical console.

Exercise 3 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 4 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.

Exercise 5 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 6 List all pages that are named time.

To select a page from a given section, one has to use man section pagename syntax.

Exercise 7 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 8 Display manual page for bash and search for printf there. Then, search for explanation of dirs command.

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 correct
• tar {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 9 Run:
man zip and review the SYNOPSIS,
more --help and explain its syntax
taskset -h and explain its syntax

Almost any shell supports autocompletion. Autocompletion 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 10 Type bad and press TAB. Type ba and press TAB twice.

Exercise 11 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.

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 and --all switch changes this behaviour.
Switch -l / --list prints list of files with details such as file owner, size, modification date, etc.

Exercise 12 List files in /usr/share/zoneinfo/ and ../../../../../bin.

Exercise 13 List all files in your home directory.

To print the working directory one can execute the pwd command.

Exercise 15 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 16 Enter .config directory (located in your home directory) and list files within.

Exercise 17 Navigate to /var/log. What is stored there?

Upon parsing the user input, the shell attempts to replace text containing wildcard characters *, ? and expressions in square brackets […]. Any text that contains such wildcards is called glob.

* expands to any text, ? expand to single character, [ace] expands to either a, or c, or e, and [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 19 Change directory to /usr/bin/ directory. List all files that end with cat.

Exercise 20 List all files that end with cat in /usr/bin/ while being in your home directory.

Exercise 21 List all files that are in the /usr/bin/ directory that have names consisting of two chatacters.

Exercise 22 List contents of /usr/share/zoneinfo/ and its subdirectories using a single command.

Exercise 23 List all files named UTC that are in any subdirectory of /usr/share/zoneinfo/.

Exercise 24 List all files starting with L that are in any subdirectory of /usr/share/zoneinfo/.

mkdir dir creates a new directory.
mkdir -p dir1/dir2/dir3 creates directory dir1, and directory dir2 within dir1, and dir3 within dir1/dir2. With the -p switch mkdir does not print an error if a directory already exists.

To remove an empty directory one can use rmdir dir.

Exercise 27 Create the following directory tree:

.
├── foo
│   ├── baz
│   │   └── bar
│   └── bar
└── foobaz
    └── bar

Exercise 28 Remove foo/baz directory

Multiple directory entries can point to the same file. This is called a hard link.
A directory entry can also point to another directory entry. This is called a soft link or symbolic link (usually abbreviated as symlink).

All Linux filesystems support hard links for ordinary files, and soft links to arbitrary path (e.g., a file or a directory).

To create a hard link, one can use ln source destination.
To create a symlink, one can use ln -s source destination.

Exercise 29 Create file file1 with some contents. Create a hard link called file2 of the file file1. Modify file2. Display file1.

Exercise 31 Create in your home directory a symlink called TMP pointing to /tmp. Change directory to TMP. What does pwd output?

Exercise 32 Create in your home directory a symlink called loop that points to your home directory. Enter it. And enter it again.

Exercise 33 Create a symlink to a non-existent path. List the directory containing it.

To copy files, one can use the cp command. To move (or rename) file, one can use the mv command.

The basic syntax is cp/mv source… destination.
Multiple source files can be provided if the destination is a directory.
If the destination is a file, it will be overwritten without warning (unless -i or -n switch is used).

When cp copies a file, it creates a new file with current date, default permissions, etc.
To copy recursively, and preserve dates, permissions and more, one can use the -a switch (that stands for --archive).

Exercise 34 Run mkdir someDir && for F in file1 file2 file3; do echo $RANDOM$RANDOM > someDir/$F; done to create someDir directory with three files inside.

Exercise 35 Copy the someDir directory recursively under a new name.

Exercise 36 Move the newly copied directory into the someDir directory.

Exercise 37 Rename someDir to a name of your choice.

It is possible to copy files via SSH. Whenever one has SSH access to a remote machine, one can copy files with scp command.
scp accepts file as file path on local machine and user@host:file as a file path on a remote machine. Remote relative paths are relative to home directory.
scp accepts the -r switch for copying recursively.

Most SSH servers enable also the SFTP protocol that allows copying files more conveniently.
sftp user@host launches sftp command line. You can use ls and cd to navigate the remote filesystem, and get and put to copy files. Type help to see all supported commands.

Exercise 39 Create a file in /tmp directory in your computer. Copy the file to home directory of user student on another computer.

Exercise 40 Copy the file to /tmp directory on another computer.

Exercise 41 Copy the file to /tmp directory on another computer using sftp.

The program that removes files is called rm.

rm by default won't remove directories (regardless if empty) and write-protected files.

To remove a directory with rm (recursively with their contents) one has to add a -r switch.

To remove write-protected files (and stop printing warnings whenever a file to be removed already does not exist) the switch -f (--force) can be used.

A misused rm -rf … command is a notorious source of data loss. Beware especially of asterisk and what it expands to.

rm accepts -I and -i switches that ask for confirmation. -I asks once upon attempt to remove multiple files, and -i asks upon each file.

Exercise 42 Create multiple files with touch file_{a..z}.
Remove all files with a single rm command with 1) no switches 2) -f switch 3) -I switch 4) -i switch.

Exercise 43 Repeat the remove command from the previous exercise when the files are gone.
Then repeat the command again with -f switch.

Exercise 44 Create a directory and remove it with rm.