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--- os_cp:intro [2023/02/27 23:22]
jkonczak [Accessing the shell]
+++ os_cp:intro [2025/03/06 13:54] (current)
jkonczak [Listing directory contents]
@@ Line 22: / Line 22: @@
 POSIX - a standard (published by IEEE and The Open Group) for operating system interface and environment, predominantly built to unify Unix-like systems
-<html><small></html>
+<small>
-GNU/Linux
+Linux, GNU/Linux, Linux distribution…
   * [[https://www.gnu.org/gnu/gnu.html|GNU]] is a collection of free software that altogether forms an operating system supplied with a broad choice of user applications [[https://www.gnu.org/manual/blurbs.html]]
   * the kernel of GNU, [[https://www.gnu.org/software/hurd/|Hurd]], has never been fit enough for the task
   * therefore GNU software is usually run on top of Linux kernel
-  * most Linux distributions, apart form GNU software, has also a multitude of other software (not necessarely free or open source)
+  * most Linux distributions, apart form GNU software, has also a multitude of other software (not necessarily free or open source)
+  * some Linux distributions base on free and/or open source software that is neither GNU software nor uses GNU licenses
-<html></small></html>
+</small>
 ==== Shell and vital utilities ====
@@ Line 59: / Line 60: @@
   * Apple operating systems (e.g. MacOS) ship with modified BSD tools [[https://opensource.apple.com/releases/|[1]]] [[https://github.com/apple-oss-distributions/file_cmds|[2]]] [[https://github.com/apple-oss-distributions/text_cmds|[3]]]
   * Embedded / resource restrained systems often use [[https://www.busybox.net/downloads/BusyBox.html|BusyBox]]
-  * Android uses [[https://www.landley.net/toybox/|toybox]] [[https://cs.android.com/android/platform/superproject/+/master:external/toybox/toys/posix/|[4]]]
+  * Android uses [[https://www.landley.net/toybox/|toybox]] [[https://cs.android.com/android/platform/superproject/main/+/main:external/toybox/toys/posix/|[4]]]
   * For more, see [[https://wiki.archlinux.org/title/core_utilities]]
 <html></small></html>
-POSIX standarizes both the [[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html|shell]]
+POSIX standarizes both the [[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html|shell]]
-and its vital [[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/contents.html|utilities]].
+and its vital [[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/contents.html|utilities]].
 ==== Accessing the shell ====
@@ Line 78: / Line 79: @@
   * terminals bundled with some Linux desktop environment: [[https://docs.xfce.org/apps/terminal/start|xfce4-terminal]], [[https://en.wikipedia.org/wiki/GNOME_Terminal|GNOME Terminal]], [[https://konsole.kde.org/|konsole]]
   * default MacOS [[https://en.wikipedia.org/wiki/Terminal_(macOS)|Terminal]]
-  * drop-down terminals: [[https://apps.kde.org/yakuake/|yakuake]] / [[http://guake-project.org/|guake]] / [[https://github.com/lanoxx/tilda|tilda]]
+  * drop-down terminals: [[https://apps.kde.org/yakuake/|yakuake]] / [[https://guake.github.io/|guake]] / [[https://github.com/lanoxx/tilda|tilda]]
 <html></small></html>
@@ Line 102: / Line 103: @@
 Currently all major operating systems (including Windows) provide ''ssh'' command by default.
-<html><small></html>
+<small>
 Secure SHell encrypts all traffic. Its predecessor – telnet – sends all data (including passwords) in plaintext.
-<html></small></html>
+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
+([[https://docs.ssh-mitm.at/user_guide/authentication.html#id2|ready-to-go tool]])
+and/or hijacks the session.
+\\
+[[https://en.wikipedia.org/wiki/Public-key_cryptography|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.
+</small>
 ~~Exercise.#~~ Run a terminal emulator. Execute command ''date'' and ''echo $SHELL'' within.
-~~Exercise.#~~ Execute command ''sleep 1h'' and interrupt it by pressing //Ctrl + c//.
+~~Exercise.#~~ Execute command ''sleep 1h'' and interrupt it by pressing //Ctrl + c//.
 ~~Exercise.#~~ Switch to second virtual console and log in. Execute ''pwgen''. Return to graphical console.
@@ Line 178: / Line 196: @@
 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.#~~ Run: \\ ''man zip'' and review the SYNOPSIS, \\ ''more --help'' and explain its syntax \\ ''taskset -h'' and explain its syntax
+~~Exercise.#~~ Run: \\ ''man zip'' and review the SYNOPSIS, \\ ''findmnt --help'' and explain its syntax \\ ''taskset -h'' and explain its syntax
 ==== Autocompletion ====
@@ Line 188: / Line 206: @@
 Autocompletion implementation differs across shells. Many shells support context-aware helpers.
-~~Exercise.#~~ Type ''gu'' and press //TAB//. Type ''ba'' and press //TAB// twice.
+~~Exercise.#~~ Type ''vu'' and press //TAB//. Type ''ba'' and press //TAB// twice.
 ~~Exercise.#~~ Type ''man --'' and press //TAB// twice. Type ''man fs'' and press //TAB// twice.
@@ Line 213: / Line 231: @@
 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.
+By default ''ls'' does not list hidden files. The **''-a''** switch changes this behaviour.
 \\
-Switch **''-l''** / ''--list'' prints list of files with details such as file owner, size, modification date, etc.
+Switch **''-l''** prints list of files with details such as file owner, size, modification date, etc.
 ~~Exercise.#~~ List files in ''/usr/share/zoneinfo/'' and ''../../../../../bin''.
@@ Line 256: / Line 274: @@
 <html></small></html>
+==== Wildcard, regex, … ====
+<small>
+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, [[https://en.wikipedia.org/wiki/Wildcard_character|wildcards]]
+and [[https://en.wikipedia.org/wiki/Regular_expression|regular expressions]]
+are commonplace.
+Syntax and implementation of both wildcards and regexes is language/application
+specific.
+\\
+The shell uses syntax summarized e.g., in
+''[[https://man7.org/linux/man-pages/man7/glob.7.html|man 7 glob]]'' and
+''[[https://man7.org/linux/man-pages/man7/regex.7.html|man 7 regex]]''
+</small>
 ==== Glob names and wildcards ====
-Upon parsing the user input, the shell attempts to replace any word containing wildcard characters **''*''**, **''?''** and expressions in square brackets **''[…]''**. A word that contains such wildcards is called [[https://en.wikipedia.org/wiki/Glob_(programming)|glob]].
+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
+[[https://en.wikipedia.org/wiki/Glob_(programming)|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''.
+''*'' 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.
@@ Line 286: / Line 336: @@
 <html></small></html>
-==== Copying, moving and erasing files ====
-<html><small></html>
-To create an empty file, you can use ''touch //file//''. To create file with some contents, you can use ''echo //contents// > //file//''. The ''touch'' and ''echo'' commands as well as meaning of ''>'' will be explained later on.
-To display a file, use ''cat //filename//''.
-<html></small></html>
-=== Directories ===
-**''mkdir //dir//''** creates a new directory.
-\\
-<html><small></html>
-''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.
-<html></small></html>
-To remove an empty directory one can use ''rmdir //dir//''.
-~~Exercise.#~~ Create the following directory tree:
-  .
-  ├── foo
-  │   ├── baz
-  │   │   └── bar
-  │   └── bar
-  └── foobaz
-      └── bar
-~~Exercise.#~~ Remove ''foo/baz'' directory
-=== Hard and soft links ===
-Multiple directory entries can point to the same file. This is called a [[https://en.wikipedia.org/wiki/Hard_link|hard link]]. \\
-A directory entry can also point to another directory entry.  This is called a [[https://en.wikipedia.org/wiki/Symbolic_link|soft link or symbolic link]] (usually abbreviated as symlink).
-All Linux filesystems [[https://en.wikipedia.org/wiki/Comparison_of_file_systems#File_capabilities|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.#~~ Create file ''file1'' with some contents. Create a hard link called ''file2'' of the file ''file1''. Modify ''file2''. Display ''file1''.
-<html><small></html>
-~~Exercise.#~~ The ''ls'' command can display for each file its link count. Discover how to do it.
-The disk space used by a file is reclamied once link count drops to 0 (all directory entries that link to the file are erased) and the file is no longer open in any process.
-<html></small></html>
-~~Exercise.#~~ Create in your home directory a symlink called ''TMP'' pointing to ''/tmp''. Change directory to ''TMP''. What does ''pwd'' output?
-~~Exercise.#~~ Create in your home directory a symlink called ''loop'' that points to your home directory. Enter it. And enter it again.
-~~Exercise.#~~ Create a symlink to a non-existent path. List the directory containing it.
-<html><small></html>
-The ''readlink [-f] //target//'' command resolves all symlinks and prints a [[https://en.wikipedia.org/wiki/Canonicalization|canonical]] path.
-The Windows NTFS filesystem supports links as well. Creating the links is possible e.g., with [[https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/mklink|mklink]] command.
-<html></small></html>
-=== Copying and moving files ===
-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 <html><small></html>(unless ''-i'' or ''-n'' switch is used)<html></small></html>.
-<html><small></html>
-By default ''cp'' will deny copying a directory. Use ''-r'' to copy a directory recursively.
-<html></small></html>
-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'').
-<html><small></html>
-With the ''-l'' switch ''cp'' creates a hard link instead of copying a file. Notice that this can be combined with ''--recursive''.
-<html></small></html>
-~~Exercise.#~~ Copy, using a single command, the files ''/etc/os-release'' and ''/etc/SUSE-brand'' to the current working directory.
-~~Exercise.#~~ Run ''mkdir someDir && for F in file{1..3}; do echo $RANDOM$RANDOM > someDir/$F; done'' to create ''someDir'' directory with three files inside.
-~~Exercise.#~~ Copy the ''someDir'' directory recursively under a new name.
-~~Exercise.#~~ Move the newly copied directory into the ''someDir'' directory.
-~~Exercise.#~~ Rename ''someDir'' to a name of your choice.
-<html><small></html>
-~~Exercise.#~~ Copy the renamed directory with ''-al'' switches. Modify a selected file with any of the directories. Which files changed contents? <html><small></html> (You can ''cat //filename//'' or display modification dates with second accuracy using ''ls -l --time-style=+%H:%M:%S …'' or ''tree -D --timefmt=%H:%M:%S …'')<html></small></html>
-<html></small></html>
-=== Copying files to/from a remote machine ===
-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.
-<html><small></html>
-Microsoft Windows now ships with ''scp'' command, but usually [[https://en.wikipedia.org/wiki/Comparison_of_FTP_client_software#Protocol_support|various file commanders]] are more convenient.
-<html></small></html>
-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.#~~ Create a file in ''/tmp'' directory in your computer. Copy the file to home directory of user ''student'' on another computer.
-~~Exercise.#~~ Copy the file to ''/tmp'' directory on another computer.
-~~Exercise.#~~ Copy the file to ''/tmp'' directory on another computer using ''sftp''.
-<html><small></html>
-=== Rsync ===
-The ''[[https://en.wikipedia.org/wiki/Rsync|rsync]]'' program is widely used to copy files and directories. It efficiently compares source files with destination files and copies only the differences. It can copy data to/from remote machines, and can compress the data sent via network to increase throughput. ''rsync'' is also commonly used to make backups.
-<html></small></html>
-=== Removing files ===
-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.#~~ 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.#~~ Repeat the remove command from the previous exercise when the files are gone. \\ Then repeat the command again with ''-f'' switch.
-~~Exercise.#~~ Create a directory and remove it with ''rm''.

Jan Kończak

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