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Linux

  • What is Linux?
    Linux is a clone of the operating system Unix, written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX and Single UNIX Specification compliance.
    It has all the features you would expect in a modern fully-fledged Unix, including true multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, proper memory management, and multistack networking including IPv4 and IPv6.
    Although originally developed first for 32-bit x86-based PCs (386 or higher), today Linux also runs on a multitude of other processor architectures, in both 32- and 64-bit variants.
    Read more:
    https://www.kernel.org/linux.html

  • What is Portable Operating System Interface (POSIX)?
    Read more:
    https://en.wikipedia.org/wiki/POSIX

  • What is Single UNIX Specification?
    Read more:
    http://opengroup.org/unix
    https://blog.opengroup.org/2012/05/17/unix-is-still-as-relevant-as-ever/

  • What is the difference between Unix and Linux? ⌘

  • What is *nix? ⌘

  • What is X Window System? ⌘

  • What is man command for? ⌘

  • Explain mandb command. What is it for?
    mandb is used to initialise or manually update index database caches that are usually maintained by man. The caches contain information relevant to the current state of the manual page system and the information stored within them is used by the man-db utilities to enhance their speed and functionality.
    Read more:
    http://man7.org/linux/man-pages/man8/mandb.8.html

  • Lets say we have all or some man pages missing. Can we get and load it, how? ⌘

  • What is the difference between > and >> redirects?
    Both redirect stdout to file. ls > list - if file exists it'll be replaced. ls >> list - if file not exists it'll be created, if it exists, it'll be appended to the end of file.
    Read more:
    http://www.tldp.org/LDP/abs/html/io-redirection.html

  • What is the difference between > (redirect) and | (pipe)? ⌘

  • What is the difference between a hard link and a soft link (aka. symbolic link, symlink, etc.)? ⌘

  • What is inode? ⌘

  • What is the difference between two special files: '.' and '..'?? ⌘

  • What is the difference between *-client, *-server and *-common types of packages? For example, there is a line in a team's project saying Install mariadb via apt-get. However, apt-get finds mariadb-client, mariadb-server and mariadb-common packages only. Which package to install in order to meet the requirement? There is no package called mariadb on apt-get. ⌘

  • What is systemd or upstart for?
    Most current Linux distributions (RHEL, CentOS, Fedora, Ubuntu 16.04 and higher) use systemd to manage which services start when the system boots. Ubuntu 14.10 and below use upstart.
    For example,
    sudo systemctl enable docker starts docker service on system boot,
    sudo systemctl disable docker removes docker service from system boot

  • How can I search the bash history and rerun a command?
    Type Ctrl R at the command line and start typing the previous command. Once a result appears keep hitting Ctrl R to see other matches. When the command you want appears, simply press Enter.
    Read more:
    https://superuser.com/questions/7414/how-can-i-search-the-bash-history-and-rerun-a-command
    https://www.gnu.org/software/bash/manual/bashref.html#Commands-For-History

  • What service --status-all does?
    The service --status-all command tries to figure out for every init script in /etc/init.d if it supports a status command (by grepping the script for status).
    If it doesn't find that string it will print [ ? ] for that service.
    Otherwise it will run /etc/init.d/$application status.
    If the return code is 0 it prints [ + ].
    If it's not 0 it prints [ - ].
    Why may ssh print [ - ] even though it's still running?
    ssh is controlled by upstart in Ubuntu (13.10).
    Running /etc/init.d/ssh status will produce no output and a return code of 1.

  • What is TeleTYpewriter (tty)?
    In computing, tty is a command in Unix and Unix-like operating systems to print the file name of the terminal connected to standard input. Given below is a sample output when the command is run.

$ tty
/dev/pts/10

Early user terminals connected to computers were electromechanical teleprinters or teletypewriters (TeleTYpewriter, TTY), and since then TTY has continued to be used as the name for the text-only console although now this text-only console is a virtual console not a physical console.
There are 6 virtual consoles in Ubuntu accessed by the keyboard shortcuts Ctrl+Alt+F1 to Ctrl+Alt+F6. You can move away from a virtual console (move the console to the background) by using the keyboard shortcut Ctrl+Alt+F7.
In Ubuntu 17.10 the login screen now uses virtual terminal 1. In Ubuntu 17.10 press Ctrl+Alt+F3 up to Ctrl+Alt+F7 for accessing a virtual console and press Ctrl+Alt+F2 to go back to the desktop environment.
Read more:
https://askubuntu.com/questions/481906/what-does-tty-stand-for
https://en.wikipedia.org/wiki/Tty_(unix)

  • Explain echo "date.timezone = Europe/Vilnius" >> docker/php/php.ini
    The command echos (prints) the string between "" (and redirects the output) to a file docker/php/php.ini.

  • Explain mkdir -p .docker/{php,apache}
    This will make the directory .docker and two subdirectories within: php and apache.
    The -p flag (or alternative: --parent) stands for "no error if existing, make parent directories as needed".
    https://ss64.com/bash/mkdir.html

  • Explain find . -type f -print | grep php | wc -l
    This will find all files in the current directory (.). The list will be given to a grep command to look for the pattern (php). At the very end, lines matching php pattern will be given to wc utility which will print the newline counts (-l).
    Read more:
    https://ss64.com/bash/find.html
    https://ss64.com/bash/grep.html

  • Explain chmod -R o+rw bootstrap/ storage/
    Read more:
    https://ss64.com/bash/chmod.html

  • Explain export PHP_INI_SCAN_DIR=/usr/local/etc/php/conf.d-dev
    Sets an environment variable (PHP_INI_SCAN_DIR to value /usr/local/etc/php/conf.d-dev). Marks each name (e.g. PHP_INI_SCAN_DIR) to be passed to child processes in the environment.
    Read more:
    https://ss64.com/bash/export.html

  • How to make a search in less?
    / - type to start writting a pattern
    n - for next result
    N - for previous result
    Read more:
    https://ss64.com/bash/less.html

  • What does chmod +x <filename> do and how do I use it?
    chmod +x on a file only means, that you'll make it executable.
    The man page of chmod covers that: u stands for user, g stands for group, o stands for others, a stands for all.
    That means that chmod u+x somefile will grant only the owner of that file execution permissions whereas chmod +x somefile is the same as chmod a+x somefile.
    Read more:
    https://askubuntu.com/questions/443789/what-does-chmod-x-filename-do-and-how-do-i-use-it
    https://askubuntu.com/questions/29589/chmod-ux-versus-chmod-x
    https://ss64.com/bash/chmod.html

  • Explain: sed -i -e "s/%consumer-key%/$BITBUCKET_CONSUMER_KEY/" app/auth.json
    The full format for invoking sed is: sed OPTIONS... [SCRIPT] [INPUTFILE...]
    SED is a stream editor. A stream editor is used to perform basic text transformations on an input stream (a file or input from a pipeline).
    While in some ways similar to an editor which permits scripted edits, SED works by making only one pass over the input(s), and is consequently more efficient. But it is SED's ability to filter text in a pipeline which particularly distinguishes it from other types of editors.
    A single command can be specified as the first argument to sed. Multiple commands can be specified by using the -e or -f options. All commands are applied to the input in the order they are specified regardless of their origin.
    ...
    sed - writes output to standard output.
    -i - to edit files in-place instead of printing to standard output.
    -e script or --expression=script - add the commands in script to the set of commands to be run while processing the input.
    "s/%consumer-key%/$BITBUCKET_CONSUMER_KEY/" - sed-comaptible command replacing all occurrences of %consumer-key% to $BITBUCKET_CONSUMER_KEY (this is the environment variable so the value of the variable will be used).
    app/auth.json - the input file to work on.
    Read more:
    https://www.gnu.org/software/sed/manual/sed.html
    https://stackoverflow.com/questions/18527365/what-does-sed-i-option-do

  • Explain: chmod -R o+rw the/folder/
    Change access permissions, change mode of the/folder/.
    -R (or --recursive) - change files and directories recursively.
    o+rw - make a file readable and writable by others.
    Read more:
    https://ss64.com/bash/chmod.html

  • Explain the following:

curl --silent --show-error --fail --location \
    --header "Accept: application/tar+gzip, application/x-gzip, application/octet-stream" -o - \
    "https://caddyserver.com/download/linux/amd64?plugins=http.expires,http.realip&license=personal" \
    | tar --no-same-owner -C /usr/bin/ -xz caddy \
    && chmod 0755 /usr/bin/caddy \
    && /usr/bin/caddy -version

curl is a tool to transfer data from or to a server, using one of the supported protocols (DICT, FILE, FTP, FTPS, GOPHER, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, POP3, POP3S, RTMP, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET and TFTP). The command is designed to work without user interaction.
curl offers a busload of useful tricks like proxy support, user authentication, FTP upload, HTTP post, SSL connections, cookies, file transfer resume, Metalink, and more. (To transfer multiple files use wget or FTP.).
--silent or -s - Silent or quiet mode. Don't show progress meter or error messages. Makes Curl mute. It will still output the data you ask for, potentially even to the terminal/stdout unless you redirect it.
--show-error or -S - When used with -s, --silent, it makes curl show an error message if it fails.
--fail or -f - (HTTP) Fail silently (no output at all) on server errors. This is mostly done to better enable scripts etc to better deal with failed attempts. In normal cases when an HTTP server fails to deliver a document, it returns an HTML document stating so (which often also describes why and more). This flag will prevent curl from outputting that and return error 22.
This method is not fail-safe and there are occasions where non-successful response codes will slip through, especially when authentication is involved (response codes 401 and 407).
--location or -L - (HTTP) If the server reports that the requested page has moved to a different location (indicated with a Location: header and a 3XX response code), this option will make curl redo the request on the new place. If used together with -i, --include or -I, --head, headers from all requested pages will be shown. When authentication is used, curl only sends its credentials to the initial host. If a redirect takes curl to a different host, it won't be able to intercept the user+password. See also --location-trusted on how to change this. You can limit the amount of redirects to follow by using the --max-redirs option.
When curl follows a redirect and the request is not a plain GET (for example POST or PUT), it will do the following request with a GET if the HTTP response was 301, 302, or 303. If the response code was any other 3xx code, curl will re-send the following request using the same unmodified method.
You can tell curl to not change the non-GET request method to GET after a 30x response by using the dedicated options for that: --post301, --post302 and --post303.
--header <header/@file> (e.g. --header "Accept: application/tar+gzip, application/x-gzip, application/octet-stream") or -H - (HTTP) Extra header to include in the request when sending HTTP to a server. You may specify any number of extra headers. Note that if you should add a custom header that has the same name as one of the internal ones curl would use, your externally set header will be used instead of the internal one. This allows you to make even trickier stuff than curl would normally do. You should not replace internally set headers without knowing perfectly well what you're doing. Remove an internal header by giving a replacement without content on the right side of the colon, as in: -H "Host:". If you send the custom header with no-value then its header must be terminated with a semicolon, such as -H "X-Custom-Header;" to send "X-Custom-Header:".
curl will make sure that each header you add/replace is sent with the proper end-of-line marker, you should thus not add that as a part of the header content: do not add newlines or carriage returns, they will only mess things up for you.
Starting in 7.55.0, this option can take an argument in @filename style, which then adds a header for each line in the input file. Using @- will make curl read the header file from stdin.
See also the -A, --user-agent and -e, --referer options.
Starting in 7.37.0, you need --proxy-header to send custom headers intended for a proxy.
Example: curl -H "X-First-Name: Joe" http://example.com/
WARNING: headers set with this option will be set in all requests - even after redirects are followed, like when told with -L, --location. This can lead to the header being sent to other hosts than the original host, so sensitive headers should be used with caution combined with following redirects.
This option can be used multiple times to add/replace/remove multiple headers.
-o or --output <file> - Write output to <file> instead of stdout. If you are using {} or [] to fetch multiple documents, you can use # followed by a number in the <file> specifier. That variable will be replaced with the current string for the URL being fetched. Like in: curl http://{one,two}.example.com -o "file_#1.txt" or use several variables like: curl http://{site,host}.host[1-5].com -o "#1_#2". You may use this option as many times as the number of URLs you have. For example, if you specify two URLs on the same command line, you can use it like this: curl -o aa example.com -o bb example.net and the order of the -o options and the URLs doesn't matter, just that the first -o is for the first URL and so on, so the above command line can also be written as curl example.com example.net -o aa -o bb See also the --create-dirs option to create the local directories dynamically.
Specifying the output as - (a single dash) - will force the output to be done to stdout. See also -O, --remote-name and --remote-name-all and -J, --remote-header-name.
"https://caddyserver.com/download/linux/amd64?plugins=http.expires,http.realip&license=personal" - the link to download Caddy for linux, using query string parameters to define the Caddy plugins we want to use and the license type. We can adjust the plugins based on our needs by visiting https://caddyserver.com/download and copying the download link after selecting our configuration. We include the http.expires, http.git, and http.realip plugins here.
tar - Store, list or extract files in an archive (originally on tape - Tape ARchiver).
--no-same-owner - extract files as yourself (default for ordinary users).
-C /usr/bin/ or --directory=DIR - change to directory DIR.
-xz caddy - -x, --extract, --get - extract files from an archive.
-xz caddy - -z or --gzip - filter the archive through gzip.
chmod 0755 /usr/bin/caddy - make file /usr/bin/caddy permissions like that: user (execute, write, read), group (execute, read), other (execute, read).
/usr/bin/caddy -version - Print the version of Caddy (HTTP/2 web server with automatic HTTPS). It also prints build information if not from a tagged release. Caddy will terminate after printing; it does not serve sites if this option is used.
Read more:
https://curl.haxx.se/docs/manpage.html
https://ss64.com/bash/curl.html
https://linux.die.net/man/1/tar
https://ss64.com/bash/tar.html
https://ss64.com/bash/chmod.html
https://caddyserver.com/docs/cli
https://caddyserver.com/download

  • Explain scp -r -P 20021 -i /home/paulius/.ssh/ uname@example.com:/etc/php5 /home/paulius/dev/example-config/
    This will use Secure CoPy command to copy the content of the directory (/etc/php5) inside remote machine (example.com) to the directory (/home/paulius/dev/example-config/) inside the local machine. All files within the directory (-r) will be copied by connecting to the remote with the username (uname) and public ssh key (-i) where the valid public key is about to be found in /home/paulius/.ssh/ local directory.
    Read more:
    https://www.youtube.com/watch?v=fmMg6cyww14
    https://ss64.com/bash/scp.html
    https://linux.die.net/man/1/scp

  • Explain curl -LOk https://github.com/cakephp/cakephp/archive/2.6.0.zip && unzip 2.6.0.zip -d ./ && rm 2.6.0.zip
    This downloads the archive ( https://github.com/cakephp/cakephp/archive/2.6.0.zip && unzip 2.6.0.zip ) by using curl. Then the archive is extracted to the same directory ( ./ ) and the original archive file is removed ( rm 2.6.0.zip ).
    curl is used with three parameters:
    -L or --location - Follow redirects if the server reports that the requested page has moved (indicated with a Location: header and a 3XX response code).
    o or --output - Write output to file instead of stdout. If you are using {} or [] to fetch multiple documents, you can use # followed by a number in the file specifier. That variable will be replaced with the current string for the URL being fetched. Like in: curl https://{one,two}.site.com -o "file_#1.txt" or use several variables like: curl https://{site,host}.host[1-5].com -o "#1_#2". You may use this option as many times as the number of URLs you have. See also --create-dirs option to create the local directories dynamically. Specify - to force the output to stdout.
    k or --insecure - This option explicitly allows curl to perform "insecure" SSL connections and transfers. All SSL connections are attempted in secure mode using the CA certificate bundle installed by default. This makes all connections considered "insecure" fail unless -k, --insecure is used.(SSL).
    Read more:
    https://ss64.com/bash/curl.html
    https://linux.die.net/man/1/unzip
    https://linux.die.net/man/1/rm

  • Explain set -eu in a bash script
    Set both -e and -u so that the script will exit on an Error or if an Unset variable is referenced.
    This is good practice when writing and debugging shell scripts.
    Change the value of a shell option and set the positional parameters, or display the names and values of shell variables.
    If no options or arguments are supplied, set displays the names and values of all shell variables and functions, sorted according to the current locale, in a format that can be reused as input. When options are supplied, they set or unset shell attributes.
    Options: Using + rather than - will cause the option to be turned off.
    Read more:
    https://ss64.com/bash/set.html

  • Explain openssl rand -base64 40
    openssl-rand, rand - generate pseudo-random bytes.
    Synopsis: openssl rand [-help] [-out file] [-rand file...] [-writerand file] [-base64] [-hex] num
    The rand command outputs num pseudo-random bytes after seeding the random number generator once. As in other openssl command line tools, PRNG seeding uses the file $HOME/.rnd or .rnd in addition to the files given in the -rand option. A new $HOME/.rnd or .rnd file will be written back if enough seeding was obtained from these sources.
    -base64 - Perform base64 encoding on the output.
    Read more:
    https://www.openssl.org/docs/manmaster/man1/openssl-rand.html
    https://en.wikipedia.org/wiki/Base64

  • What is so common about true and : in Bourne shells (bash)?
    Historically, Bourne shells didn't have true as built-in command. true was instead simply aliased to :.
    : is slightly better than true for portability to ancient Bourne-derived shells. As a simple example, consider having neither the ! pipeline operator nor the || list operator (as was the case for some ancient Bourne shells). This leaves the else clause of the if statement as the only means for branching based on exit status:
    if command; then :; else ...; fi
    Since if requires a non-empty then clause and comments don't count as non-empty, : serves as a no-op.
    Nowadays (that is: in a modern context) you can usually use either : or true. Both are specified by POSIX, and some find true easier to read. However there is one interesting difference: : is a so-called POSIX special built-in, whereas true is a regular built-in.

    • Special built-ins are required to be built into the shell; Regular built-ins are only "typically" built in, but it isn't strictly guaranteed. There usually shouldn't be a regular program named : with the function of true in PATH of most systems.
    • Probably the most crucial difference is that with special built-ins, any variable set by the built-in - even in the environment during simple command evaluation - persists after the command completes, as demonstrated here using ksh93: 
      $ unset x; ( x=hi :; echo "$x" )
hi
$ ( x=hi true; echo "$x" )

$
    • Note that Zsh ignores this requirement, as does GNU Bash except when operating in POSIX compatibility mode, but all other major "POSIX sh derived" shells observe this including dash, ksh93, and mksh.
    • Another difference is that regular built-ins must be compatible with exec - demonstrated here using Bash: 
      $ ( exec : )
-bash: exec: :: not found
$ ( exec true )
$
    • POSIX also explicitly notes that : may be faster than true, though this is of course an implementation-specific detail.

Read more:
https://stackoverflow.com/questions/3224878/what-is-the-purpose-of-the-colon-gnu-bash-builtin

  • What is so common about false and let 0 in Bourne shells (bash)?
    Historically, Bourne shells didn't have false as built-in command. false was instead simply aliased to something like let 0.
    Read more:
    https://stackoverflow.com/questions/3224878/what-is-the-purpose-of-the-colon-gnu-bash-builtin

  • ?? What is the difference between quoted and not quoted code blocks in bash?
    Just for example, : can also be for block comment (similar to /* */ in C language). Let say, if you want to skip a block of code in your script, you can do this:

: << SKIP
your code block here
SKIP

However, this way command substitutions inside the here document are still processed.
Anyways, variable resolution/substitution in here docs can be avoided by single-quoting the delimiter:

: << 'SKIP'
your code block here
SKIP

Read more:
https://stackoverflow.com/a/18929997/2026314

  • What is the difference between Interrupts and Signals on OS like Linux?
    Interrupts can be viewed as a mean of communication between the CPU and the OS kernel.
    Signals can be viewed as a mean of communication between the OS kernel and OS processes.
    Interrupts may be initiated by the CPU (exceptions - e.g.: divide by zero, page fault), devices (hardware interrupts - e.g: input available), or by a CPU instruction (traps - e.g: syscalls, breakpoints). They are eventually managed by the CPU, which "interrupts" the current task, and invokes an OS-kernel provided ISR/interrupt handler.
    Signals may be initiated by the OS kernel (e.g: SIGFPE, SIGSEGV, SIGIO), or by a process(kill()). They are eventually managed by the OS kernel, which delivers them to the target thread/process, invoking either a generic action (ignore, terminate, terminate and dump core) or a process-provided signal handler.
    Read more:
    https://stackoverflow.com/questions/13341870/signals-and-interrupts-a-comparison

  • Explain man -k signal
    man - an interface to the on-line reference manuals.
    -k - Equivalent to apropos. apropos - Each manual page has a short description available within it. apropos searches the manual page names and descriptions for instances of keyword.
    signal - keyword to search for. It is usually a regular expression, as if (-r) was used, or may contain wildcards (-w), or match the exact keyword (-e). Using these options, it may be necessary to quote the keyword or escape (\) the special characters to stop the shell from interpreting them. The standard matching rules allow matches to be made against the page name and word boundaries in the description.
    Read more:
    https://ss64.com/bash/man.html
    http://man7.org/linux/man-pages/man1/apropos.1.html

  • Explain trap : TERM INT; sleep infinity & wait found at the end of bash script
    In short:
    We are running sleep infinitely and then exiting on an interrupt signal.
    In details:
    There might be situations when you don't want users of your scripts to exit untimely using keyboard abort sequences, for example because input has to be provided or cleanup has to be done. The trap statement catches these sequences and can be programmed to execute a list of commands upon catching those signals.
    The syntax for the trap statement is straightforward: trap [COMMANDS] [SIGNALS]
    In the case of our example, all goes like this:
    trap - catches the [SIGNALS] and runs the [COMMANDS] after. When Bash receives a signal for which a trap has been set while waiting for a command to complete, the trap will not be executed until the command completes.
    : - a command to execute on signals catch. : is equivalent to true. Which does nothing except return an exit status of 0, meaning "success". It can be used as a place holder in shell scripts where a successful command is needed. Although the shell built-in command : (colon) does the same thing faster.
    TERM - a signal name without SIG prefix equivalent to SIGTERM and 15 (signal int value). This is Termination signal. The default behavior is to terminate the process, but it also can be caught or ignored. The intention is to kill the process, gracefully or not, but to first allow it a chance to cleanup. The SIGTERM signal is a generic signal used to cause program termination. Unlike SIGKILL, this signal can be blocked, handled, and ignored. It is the normal way to politely ask a program to terminate. The shell command kill generates SIGTERM by default.
    INT - a signal name without SIG prefix equivalent to SIGINT and 15 (signal int value). The SIGINT (“program interrupt”) signal is sent when the user types the INTR character (normally C-c, most shells bind Ctrl + C to "send a SIGINT signal to the program that currently runs in the foreground".).
    ; - When the shell sees a semicolon (;) on a command line, it's treated as a command separator -- basically like pressing the ENTER key to execute a command.
    sleep infinity - delay for a specified amount of time which is specified as "infinity". TL;DR: sleep infinity actually sleeps the maximum time allowed, which is finite.
    & - A single ampersand & can delimit a list of commands to be run asynchronously. For example, ./script.py & ./script2.py & ./script3.py & , - all 3 python scripts are run at the same time, in separate sub-shells. Their stdout will still be attached to the parent shell, so if running this from a Linux terminal, you will still see the outputs. This can also be used as a quick hack to take advantage of multiple cores with shell scripts, but be warned, it is a hack!
    wait - await process completion.
    When an asynchronous list is started by the shell, the process ID of the last command in each element of the asynchronous list shall become known in the current shell execution environment.
    If the wait utility is invoked with no operands, it shall wait until all process IDs known to the invoking shell have terminated and exit with a zero exit status.
    If one or more pid operands are specified that represent known process IDs, the wait utility shall wait until all of them have terminated. If one or more pid operands are specified that represent unknown process IDs, wait shall treat them as if they were known process IDs that exited with exit status 127. The exit status returned by the wait utility shall be the exit status of the process requested by the last pid operand.
    The known process IDs are applicable only for invocations of wait in the current shell execution environment.
    Read more:
    https://explainshell.com/explain?cmd=trap+%3A+TERM+INT%3B+sleep+infinity+%26+wait
    http://www.tldp.org/LDP/Bash-Beginners-Guide/html/sect_12_02.html
    https://bash.cyberciti.biz/guide/Trap_statement
    https://ss64.com/bash/trap.html
    https://stackoverflow.com/questions/13341870/signals-and-interrupts-a-comparison
    https://ss64.com/bash/true.html
    https://linux.die.net/Bash-Beginners-Guide/sect_12_01.html
    https://www.quora.com/What-is-the-difference-between-the-SIGINT-and-SIGTERM-signals-in-Linux-What%E2%80%99s-the-difference-between-the-SIGKILL-and-SIGSTOP-signals
    https://www.gnu.org/software/libc/manual/html_node/Termination-Signals.html
    https://www.gnu.org/software/libc/manual/html_node/Special-Characters.html#Special-Characters
    https://www.gnu.org/software/libc/manual/html_node/Signal-Characters.html
    https://man.cx/signal(7)
    https://superuser.com/questions/243460/what-to-do-when-ctrl-c-cant-kill-a-process
    https://docstore.mik.ua/orelly/unix3/upt/ch28_16.htm
    https://stackoverflow.com/questions/2935183/bash-infinite-sleep-infinite-blocking
    https://github.com/coreutils/coreutils/blob/master/src/sleep.c
    http://bashitout.com/2013/05/18/Ampersands-on-the-command-line.html
    https://man.cx/wait

  • Does the exec command of Bash replace the current process without forking a new process?
    Yes, it does.

  • Explain:

until confd -onetime -backend consul -node consul:8500; do
    echo "Waiting for the initial confd configuration"
    sleep 5
done

until
The until loop is very similar to the while loop, except that the loop executes until the TEST-COMMAND executes successfully. As long as this command fails, the loop continues. The syntax is the same as for the while loop:
until TEST-COMMAND; do CONSEQUENT-COMMANDS; done
The return status is the exit status of the last command executed in the CONSEQUENT-COMMANDS list, or zero if none was executed. TEST-COMMAND can, again, be any command that can exit with a success or failure status, and CONSEQUENT-COMMANDS can be any UNIX command, script or shell construct.
confd
confd is a lightweight configuration management tool that allows you to keep configuration files up to date from data stored in backends like environment variables, Consul (https://www.consul.io/), Etcd (https://github.com/coreos/etcd), Redis (https://redis.io/), and others. You can also reload applications to pick up changes during runtime without restarting the container. With Confd, we can separate our configuration management from infrastructure code.
-onetime
confd supports two modes of operation daemon and onetime. In daemon mode confd polls a backend for changes and updates destination configuration files if necessary.
-backend consul
we pick consul (https://www.consul.io/) as the backend for this case. confd supports the following backends: etcd, consul, vault, env (environment variables), redis, zookeeper, dynamodb, rancher, ssm (AWS Simple Systems Manager Parameter Store), etc.
Consul has multiple components, but as a whole, it is a tool for discovering and configuring services in your infrastructure. It provides several key features: Service Discovery, Health Checking, KV Store (key/value storage), Multi Datacenter.
--node consul:8500
In general, a node is a basic unit used in computer science. Nodes are devices or data points on a larger network. Devices such as a personal computer, cell phone, or printer are nodes. When defining nodes on the internet, a node is anything that has an IP address. Nodes are individual parts of a larger data structure, such as linked lists and tree data structures. Nodes contain data and also may link to other nodes. Links between nodes are often implemented by pointers.
??? --node consul:8500 simply points to the Consul service running on exposed port number 8500.
;
The ; may be replaced with one or more newlines wherever it appears. The semicolon is needed only when the end of line is missing.
do
This is the part of until construction.
echo "Waiting for the initial confd configuration"
echo is a built-in command in the bash and C shells that writes its arguments to standard output.
sleep 5
Delay for 5 seconds.
done
This is the part of until construction.
Read more:
http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_09_03.html
https://github.com/kelseyhightower/confd
https://github.com/kelseyhightower/confd/blob/master/docs/quick-start-guide.md
https://www.consul.io/docs/index.html
http://www.linfo.org/echo.html
https://man.cx/sleep

  • In Bash scripting, what's the meaning of $!?
    The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed. $ and ! are a few of these paramters:
    $ - Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell.
    ! - Expands to the process ID of the most recently executed background (asynchronous) command.
    So, $! contains the process ID of the most recently executed background pipeline.
    Read more:
    https://unix.stackexchange.com/questions/85021/in-bash-scripting-whats-the-meaning-of
    https://linux.die.net/man/1/bash

  • What does it mean when a command ends with &, for example: php-fpm &?
    If a command is terminated by the control operator &, the shell executes the command in the background in a subshell. The shell does not wait for the command to finish, and the return status is 0. Commands separated by a ; are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.
    Read more:
    https://linux.die.net/man/1/bash

  • How to create the new folder on Ubuntu 17.10 (hotkey)?
    Ctrl + Shift + N
    Read more:
    https://ubuntuforums.org/showthread.php?t=1559684

  • In what order ssh obtains configuration data?

    1. command-line options
    2. user's configuration file (~/.ssh/config)
    3. system-wide configuration file (/etc/ssh/ssh_config)

Read more:
https://man.cx/ssh_config

  • How can I check the keywoards I may use for my ssh config file (e.g. ~/.ssh/config)?
    man ssh_config
    Read more:
    https://man.cx/man
    https://man.cx/ssh_config

  • Explain ufw allow 4500/udp
    ufw − Uncomplicated Firewall (UFW) is a program for managing a netfilter firewall.
    allow - add allow rule. To any address on this host.
    4500/udp - port 4500 of udp.
    Read more:
    https://man.cx/ufw
    https://en.wikipedia.org/wiki/Uncomplicated_Firewall
    https://wiki.ubuntu.com/UncomplicatedFirewall?action=show&redirect=UbuntuFirewall

  • Explain ssh -p 7822 uname@nlss1.a2hosting.com -L 3306:localhost:3306 -N -v -v
    SSH (ssh) into nlss1.a2hosting.com server by using port 7822 (-p 7822) and username uname. Forward connection via port 3306 (-L 3306:localhost:3306).
    -N - Do not execute a remote command. This is useful for just forwarding ports.
    -v -v - Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple −v options increase the verbosity. The maximum is 3.
    Read more:
    https://man.cx/ssh

  • Explain: tag=${1:-latest} line found in a bash script

  • Explain ACCESS_KEY=$1 line found in a bash script

  • Explain VBoxManage convertdd dd-image.raw sda.vdi --format VDI --variant Fixed
    Read more:
    https://www.agix.com.au/use-a-dd-image-as-a-disk-for-a-virtualbox-guest/

  • Explain man hier
    Outputs description of the filesystem hierarchy.
    Read more:
    https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard
    https://man.cx/hier

  • What is the difference between apt and apt-get on Ubuntu OS?
    They are very similar command line tools available in Trusty. apt-get and apt-cache's most commonly used commands are available in apt.
    apt-get may be considered as lower-level and "back-end", and support other APT-based tools. apt is designed for end-users (human) and its output may be changed between versions.
    Currently the apt binary supports the following commands:

    • list: which is similar to dpkg list and can be used with flags like --installed or --upgradable.
    • search: works just like apt-cache search but sorted alphabetically.
    • show: works like apt-cache show but hide some details that people are less likely to care about (like the hashes). The full record is still available via apt-cache show of course.
    • update: just like the regular apt-get update with color output enabled.
    • install,remove: adds progress output during the dpkg run.
    • upgrade: the same as apt-get upgrade --with-new-pkgs.*
    • full-upgrade: a more meaningful name for dist-upgrade.
    • edit-sources: edit sources.list using $EDITOR. May be worth mentioning that starting in 16.04 by default apt does not save the downloaded packages while apt-get does.

  • Explain sudo dpkg -i libgstreamer*.deb
    sudo - execute a command as another user (super user).
    dpkg - package manager for Debian. dpkg is a tool to install, build, remove and manage Debian packages. The primary and more user-friendly front-end for dpkg is aptitude. dpkg itself is controlled entirely via command line parameters, which consist of exactly one action and zero or more options. The action-parameter tells dpkg what to do and options control the behavior of the action in some way.
    -i, −−install - Install the package. If −−recursive or −R option is specified, package−file must refer to adirectory instead.
    libgstreamer*.deb - package−file (matching the given pattern) to install.
    Installation consists of the following steps:

    1. Extract the control files of the new package.
    2. If another version of the same package was installed before the new installation, execute prerm script of the old package.
    3. Run preinst script, if provided by the package.
    4. Unpack the new files, and at the same time back up the old files, so that if something goes wrong, they can be restored.
    5. If another version of the same package was installed before the new installation, execute the postrm script of the old package. Note that this script is executed after the preinst script of the new package, because new files are written at the same time old files are removed.
    6. Configure the package. See −−configure for detailed information about how this is done.
      Read more:
      https://man.cx/sudo
      https://man.cx/dpkg

  • What is the Alt + F2 combination for?
    Run an application by typing its name in the box which appears.
    Read more:
    https://help.ubuntu.com/community/KeyboardShortcuts

  • What is Magic SysRq key?
    The magic SysRq key is a key combination understood by the Linux kernel, which allows the user to perform various low-level commands regardless of the system's state. It is often used to recover from freezes, or to reboot a computer without corrupting the filesystem. Its effect is similar to the computer's hardware reset button (or power switch) but with many more options and much more control.
    Read more:
    https://en.wikipedia.org/wiki/Magic_SysRq_key

  • Explain docker rm -f angular || true
    We try to remove the Docker container. Since the container may not exist, we are using pipes to force true and prevent failure regardless of the scenario.
    docker - a computer program that performs operating-system-level virtualization also known as containerization.
    rm - remove one or more containers.
    -f or --force - force the removal of a running container (uses SIGKILL).
    angular - the container name.
    || - if the command docker rm -f angular has a non-zero exit status, the double pipe OR (||) kicks in, and tries to execute the command.
    true - the command to execute in case the command docker rm -f angular has a non-zero exit status. true − do nothing, successfully. Exit with a status code indicating success.
    Read more:
    https://www.thepolyglotdeveloper.com/2017/04/continuous-deployment-of-web-application-containers-with-jenkins-and-docker/
    https://docs.docker.com/engine/reference/commandline/rm/
    https://stackoverflow.com/questions/41625521/bash-which-or-operator-to-use-pipe-v-double-pipe
    https://man.cx/true

  • Explain the following:

$ sed ': loop; n; n; n; n; n; n; n; n; n; a GO
n; b loop' < bigfile.sql > bigfile2.sql

That inserts a GO every 10 lines (the number of times n appears in the sed script). Read more:
https://stackoverflow.com/questions/24665757/sqlcmd-scripting-error

  • Explain which sqlcmd
    Returns the pathnames of the files (or links) which would be executed in the current environment, had its arguments been given as commands in a strictly POSIX-conformant shell. It does this by searching the PATH for executable files matching the names of the arguments. It does not canonicalize path names.
    Command prompt (Windows OS) alternative would be where <command-name>.
    Read more:
    https://man.cx/which
    https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/where_1

  • What is the difference between semicolon (;) and double ampersand (&&)? For example:
    echo "Hello " ; echo "world"
    and
    echo "Hello " && echo "world"
    The ansswer is... echo "Hello " ; echo "world" means run echo "world" no matter what the exit status of the previous command echo "Hello" is i.e. echo "world" will run irrespective of success or failure of the command echo "Hello".
    Whereas in case of echo "Hello " && echo "world", echo "world" will only run if the first command (echo "Hello") is a success (i.e. exit status 0).
    The following commands give an example of how the shell handles commands chaining using the different operators:

$ false ; echo "OK"
OK
$ true ; echo "OK"
OK
$ false && echo "OK"
$ true && echo "OK"
OK
$ false || echo "OK"
OK
$ true || echo "OK"
$

Read more:
https://unix.stackexchange.com/questions/187145/whats-the-difference-between-semicolon-and-double-ampersand

Read more:
https://www.php.net/manual/en/language.types.integer.php
https://unix.stackexchange.com/questions/103413/is-there-any-difference-between-mode-value-0777-and-777
https://superuser.com/questions/592309/whats-the-difference-between-chmod-777-and-chmod-7777
https://ss64.com/bash/chmod.html
https://ss64.com/bash/sticky.html
https://en.wikipedia.org/wiki/Setuid https://en.wikipedia.org/wiki/Sticky_bit

  • Explain du -cha --max-depth=1 / | grep -E "M|G"
    TBD

  • Explain sudo find / -size +10G -ls
    TBD

  • Explain scp -o ProxyJump=paulius@bastion.example.com mytestfile.txt paulius@10.140.16.201:~
    Sends file mytestfile.txt from the current working dirrectory to 10.140.16.201 server (as seen from bastion server) via the middle server bastion.example.com. TBD

  • Explain scp -o ProxyJump=paulius@bastion.example.com.com paulius@10.140.16.201:/srv/www/example.com/logs/dev.log /home/paulius/dev/scp-tests/backup_logs/dev.log
    Copy file /srv/www/example.com/logs/dev.log of 10.140.16.201 server (as seen from bastion server) to local /home/paulius/dev/scp-tests/backup_logs/dev.log via the middle server bastion.example.com
    TBD

  • Explain scp -r -o ProxyJump=paulius@bastion.example.com paulius@10.140.16.201:/srv/www/example.com/logs /home/paulius/dev/scp-tests
    Copy directory /srv/www/example.com/logs and it's content of 10.140.16.201 server (as seen from bastion server) to local /home/paulius/dev/scp-tests/logs directory via the middle server bastion.example.com
    TBD

  • Explain sudo du -sh /var
    Typically, you would want to display the space occupied by the directory in a human-readable format. For example, to get the total size of the /var directory, you would run the mentioned command. The command starts with sudo because most of the files and directories inside the /var directory are owned by the root user and are not readable by the regular users. If you omit sudo the du command will print "du: cannot read directory". s - Display only the total size of the specified directory, do not display file size totals for subdirectories. h - Print sizes in a human-readable format (h). /var - The path to the directory you want to get the size.
    Read more: https://linuxize.com/post/how-get-size-of-file-directory-linux/

  • Explain sudo du -shc /var/*
    What if you want to display the disk usage of the first-level subdirectories? You have two options. The first one is to use the asterisk symbol (*) as shown above, which means "match everything that doesn’t start with a period (.)". The -c option tells du to print a grand total of all sizes. For the rest, the following applies: The command starts with sudo because most of the files and directories inside the /var directory are owned by the root user and are not readable by the regular users. If you omit sudo the du command will print "du: cannot read directory". s - Display only the total size of the specified directory, do not display file size totals for subdirectories. h - Print sizes in a human-readable format (h). /var - The path to the directory you want to get the size.
    Read more: https://linuxize.com/post/how-get-size-of-file-directory-linux/

  • Explain sort myfile.json | awk '{$1=$1;print}' | uniq --count | sort -n
    Count line occurences in the file and order all from the most occuring line to the least occuring line. This wai you may find duplicates, etc.
    Read more: https://stackoverflow.com/questions/6712437/find-duplicate-lines-in-a-file-and-count-how-many-time-each-line-was-duplicated

  • Explain echo " lol " | xargs
    Xargs will do the trimming. It's one command/program, no parameters, returns the trimmed string, easy as that! Note: this doesn't remove all internal spaces so "foo bar" stays the same; it does NOT become "foobar". However, multiple spaces will be condensed to single spaces, so "foo bar" will become "foo bar". In addition it doesn't remove end of lines characters.
    Read more: https://stackoverflow.com/questions/369758/how-to-trim-whitespace-from-a-bash-variable

  • Explain sudo apt-mark showmanual
    Lists manually installed packages.

  • Explain apt-mark hold
    To protect a package from being upgraded with apt upgrade

  • How to stop a process on Ubuntu which is using port X (e.g. 8080) and is based on the Docker (the finding on the way)

 sudo netstat -tulpn | grep :8080
> tcp        0      0 0.0.0.0:8080            0.0.0.0:*               LISTEN      3083/docker-proxy
ps -aux | egrep 3083
> root        3083  0.0  0.0 695692  2780 ?        Sl   10:17   0:00 /usr/bin/docker-proxy -proto tcp -host-ip 0.0.0.0 -host-port 8080 -container-ip 192.168.112.3 -container-port 8080
docker ps | egrep ":8080"
> a3f4f9c9ba29   adminer                                                   "entrypoint.sh docke…"   4 months ago         Up 2 hours                        0.0.0.0:8080->8080/tcp                    github_adminer_1
docker stop a3f4f9c9ba29
> a3f4f9c9ba29

  • What is the difference between man and help in bash?
    help - displays information about builtin bash commands. Does not include system-wide reference manuals.
    man - an interface to the system reference manuals. Does not include bash built ins.

  • Explain readarray -td '' a < <(awk '{ gsub(/, /,"\0"); print; }' <<<"$DB_IDS, "); unset 'a[-1]'; declare -p a;

!!! THIS IS INCOMPLETE ANSSWER AND MAY EVEN BE WRONG. !!!
                                                                           
See:
# https://stackoverflow.com/a/45201229/2026314
  
Main commands ant flags:
  
# readarray                            A synonym for mapfile. Parses a bytestream into an array variable in one shot; no messing with loops, conditionals, substitutions, or anything else. And it doesn't surreptitiously strip any whitespace from the input string.
# -t                                   Remove any trailing newline from a line read, before it is assigned to an array element.
# -d                                   Defining the separator
# a                                    Variable name
# < <                                  https://askubuntu.com/questions/983369/spaces-in-commands-with-redirection , https://stackoverflow.com/questions/2811319/difference-between-and , "In Bash, you can use the command1 <( command0 ) redirection syntax, which redirects command0's stdout and passes it to a command1 that takes a filename as a command-line argument. This is called process substitution." , - https://unix.stackexchange.com/questions/63923/pseudo-files-for-temporary-data
# gsub(regexp, replacement [, target]) gsub stands for global substitution. It replaces every occurrence of regex with the given string (sub). The third parameter is optional. If it is omitted, then $0 is used. https://www.tutorialspoint.com/awk/awk_string_functions.htm
# \0                                   Null character
# unset 'a[-1]'                        The problem here is that readarray preserved the trailing field (e.g. "[1]=$'\n'"), since the <<< redirection operator appended the LF to the input string, and therefore the trailing field was not empty (otherwise it would've been dropped). We can take care of this by explicitly unsetting the final array element after-the-fact:
# declare                              Set variable values and attributes. Declare variables and give them attributes. If no NAMEs are given, display the attributes and values of all variables.
# -p                                   display the attributes and value of each NAME
# -a                                   to make NAMEs indexed arrays (if supported)

  • Explain sudo dumpe2fs /dev/sda1 | head -n 30
    dumpe2fs prints the super block and blocks group information for the filesystem present on device. We take 30 first lines of the information by using head -n 30.
    More info: https://linux.die.net/man/8/dumpe2fs , https://youtu.be/B6kg2zeJ9do?t=1268

  • Explain stat myfilename.ext
    Gets inode information of the file named myfilename.ext
    To see the inode number of the file, you may use ls -li myfilename.ext
    More info: https://linux.die.net/man/1/stat

  • Explain lsblk
    The command prints all block devices (except RAM disks) in a tree-like format by default.

  • Explain sync
    Synchronize cached writes to persistent storage.

  • Explain sudo fsck /dev/sdb1
    Checks and repairs a Linux filesystem /dev/sdb1.

  • Explain sudo tune2fs -m 1 /dev/sdb1
    Setting reserved block percentage to 1%. By default, Linux reserves 5% of the drive for the core operations (e.g. in case the drive is full, this space is in use for commands to operate on).
    tune2fs - adjust tunable filesystem parameters on ext2/ext3/ext4 filesystems.
    -m reserved-blocks-percentage Set the percentage of the filesystem which may only be allocated by privileged processes. Reserving some number of filesystem blocks for use by privileged processes is done to avoid filesystem fragmentation, and to allow system daemons, such as syslogd(8), to continue to function correctly after non-privileged processes are prevented from writing to the filesystem. Normally, the default percentage of reserved blocks is 5%.
    More info: https://youtu.be/B6kg2zeJ9do?t=2210

  • Explain sudo e4defrag -c /
    Attempts to defragment the target.
    target is a regular file, a directory, or a device that is mounted as ext4 filesystem. If target is a directory, e4defrag reduces fragmentation of all files in it. If target is a device, e4defrag gets the mount point of it and reduces fragmentation of all files in this mount point.
    e4defrag - online defragmenter for ext4 filesystem.
    -c Get a current fragmentation count and an ideal fragmentation count, and calculate fragmentation score based on them. By seeing this score, we can determine whether we should execute e4defrag to target.
    Omit -c, in case you want the defragmentation process to actually run.

  • Explain dmesg (it usually is sudo dmesg -w)
    The dmesg command lets you peer into the hidden world of the Linux startup processes. Review and monitor hardware device and driver messages from the kernel’s own ring buffer with “the fault finder’s friend.”
    -w - follows messages real time More info: https://en.wikipedia.org/wiki/Dmesg

  • Explain tcpdump port 9001 -A | strings
    TBD

  • Explain type alias
    If you're not sure whether the given command is an alias, shell built-in, file, function, or keyword, use type command to find out.
    More info: https://ostechnix.com/how-to-install-missing-man-pages-of-commands-on-ubuntu/

  • Explain sudo apt install manpages-posix
    Builtins are part of the shell. Each shell has its own set of builtins. They are not independent commands and they don't have separate man pages. Luckily, the man pages of shell builtins are available in POSIX Programmer's Manual. You need to install it to access those man pages.

  • Explain pushd "$(dirname "$0")" > /dev/null || exit 1 and the later to be used popd > /dev/null || exit 1.

Here's a breakdown of the command: 1 pushd is a command that adds the current directory to a stack and changes the current working directory to the directory specified in the argument. 2 "$(dirname "$0")" is a parameter expansion that expands to the directory name of the currently executing script. The $0 variable holds the name of the script, and the dirname command extracts the directory name. 3 > /dev/null is a redirection that discards any output from the command. It sends the output to the null device, which discards it. 4 || exit 1 is a logical OR operator that checks the exit status of the previous command. If the exit status is non-zero (indicating an error), the exit 1 command will be executed, which will terminate the script with an exit status of 1. So, the entire command will change the current working directory to the directory containing the script and suppress any output. If there is an error, the script will terminate with an exit status of 1. This is a common practice in scripts to ensure that the script can continue running as intended and handle errors gracefully.

The shell command `popd > /dev/null || exit 1` is used to change the current working directory to the previous directory that was stored in the directory stack by the pushd command.

The popd command is used to remove the top directory from the directory stack and change the current working directory to the new top directory. The > /dev/null part of the command redirects the standard output of the popd command to the null device, which discards the output and prevents it from being displayed on the console.

The || operator is used to execute the exit command if the previous command (popd in this case) fails with a non-zero exit code. The exit 1 command terminates the shell script and sets its exit code to 1, indicating that an error occurred.

Overall, this command is commonly used in shell scripts that make use of the pushd and popd commands to change directories and manage the directory stack. It ensures that if the popd command fails, the script will exit with a non-zero exit code, indicating that an error occurred.

  • ???