From 403bcd7509498638b6e34d38eb29e1c3620917c9 Mon Sep 17 00:00:00 2001
From: David Luevano Alvarado Ya lo único que queda es cambiar un poco el formato del blog (y de la página en general), porque en un momento de desesperación puse todo el texto en justificado y pues no se ve chido siempre, entonces queda corregir eso. Y aunque me tomó más tiempo del que quisiera, así nomás quedó, diría un cierto personaje. El Por último, también quité las extensiones ssg
modificado está en mis dotfiles (o directamente aquí)..html
de las URLs, porque se veía bien pitero, pero igual los links con .html
al final redirigen a su link sin .html
, así que no hay rollo alguno.
A shell it’s an interface between the user and the kernel. While the kernel it’s the layer that interacts between the shell and the hardware. And you access the shell either via a terminal, or executing a shell script. Note that if you’re using a GUI environment, you need a terminal emulator to actually use a terminal (most Linux distros come with everything needed, so no need to worry).
-When using a terminal a blank screen with some text and a cursor that shows you where to type will appear and depending on the shell being used (sh
, dash
, ksh
, bash
, zsh
, fish
, etc.) the prompt will be different. The most common one being of the form user@host:~$
, which tells that the user
is using host
machine and the current working directory is ~
(can be /any/path/
too), and lastly, the $
shows the current privileges of the shell/user using the shell (a $
for normal user and #
for root access).
To clear the screen use command clear
or simply do Ctrl + l
(most terminals let you do this) and to cancel or create a new prompt do Ctrl + c
, this also cancels any running program that’s using the terminal (typing q
when a program is running also stops the process, sometimes).
Also there are POSIX (portable operating system interface) compliant shells like sh
, dash
, ksh
, etc., that have a standard syntax and are portable to any Unix system. Non POSIX compliant shells (or not necessary fully POSIX compliant) are bash
, zsh
, fish
, etc., that provide a more modern syntax but lack speed on executing scripts.
A list of common commands or programs with a short description (for more, do man command
or command -h
or command --help
):
man
: an interface to the system reference manuals.pwd
: print name of current/working directory.cd
: change the working directory.ls
: list directory contents.echo
: display a line of text. Also, see escape sequences (Bash Prompt HOWTO: Chapter 2. Bash and Bash Prompts: 2.5. Bash Prompt Escape Sequences).mkdir
: make directories.touch
: change file timestamps (if no file exists, creates a new blank one).cat
: concatenate files and print on the standard output.mv
: move (rename) files.rm
: remove files or directories.rmdir
: remove empty directories.cp
: copy files and directories.ln
: make links between files (hard or soft, also known as symbolic).umask
: get or set the file mode creation mask.chmod
: change file mode bits (change file permissions).chown
: change file owner and group.wc
: print newline, word, and byte counts for each file.file
: determine file type.sort
: sort lines of text files.cut
: remove sections from each line of files.dd
: convert and copy a file (mostly used to make bootable USBs).compress
: compress data.gzip
, gunzip
, zcat
: compress or expand files.uname
: print system information.cal
: display a calendar.date
: print or set the system date and time.read
: read from standard input into shell variables (also used to read from a file).tr
: translate or delete characters.readonly
: set the readonly attribute for variables.set
: set or unset options and positional parameters.unset
: unset values and attributes of variables and functions.expr
: evaluate expressions.tput
, reset
: initialize a terminal or query terminfo database (used for more complex terminal output).grep
, egrep
, fgrep
: print lines that match patterns (usually used to find text in a file or some text).sleep
: delay for a specified amount of time.break
: exit from for, while, or until loop.continue
: continue for, while, or until loop.logname
: print user’s login name.write
: send a message to another user.mesg
: display (or do not display) messages from other users.return
: return from a function or dot script.exit
: cause the sell to exit.And some special “commands” or “operators” (for more: gnu: 3.6 Redirections):
-|
(pipe): used between two commands and the output from the command from the left serves as input to the command from the right.>
: redirects output to a file, overwriting the file (or creating a new file).>>
: redirects output to a file, appending to the file (or creating a new file).A shell script is nothing more but a file that contains commands in it; they’re executed in the same order they are present in the file. A shell script file is usually terminated with a .sh
extension, independently of the shell being used, but it’s not 100% necessary as in Unix systems, an extension mean nothing, other than distinction (visually) between files. Then one can just have an extension-less file as a script. The script must have execution permissions (chmod +x file
), unless shell script
is executed in the terminal, where shell
could be sh
, bash
, etc. Comments are created by prepending #
to whatever the text should be a comment.
It’s common practice to have the first line as a she-bang (#!
), which is just a comment telling the interpreter which shell to execute the script with (usable when having the script in your PATH so you only call the name of the script like any other command/program). A she-bang has the syntax #!/path/to/shell some_other_options
, the most common she-bangs being: #!/bin/sh
, #!/bin/bash
, #!/usr/bin/python
, etc.
Also, some people argue that you shouldn’t use absolute paths, since not all Unix operating systems have the same directory structure, or not all programs are going to be installed in the same folder. So a portable she-bang can be made by prepending /usr/bin/env
and the specify the program to run, for example: #!/usr/bin/env bash
.
Like always… the basic “Hello, world!” script:
-#!/bin/sh
-echo "Hello, world!"
-
-Three ways of executing this script (assuming the file name is hw
):
sh hw
../hw
. Requires the file to have execute permissions.hw
. Requires the file to have execute permissions. Requires the file to be in your PATH.Variables are case sensitive, meaning that my_var
and MY_VAR
are different and a variable name can only contain letters and numbers (a-z
, A-Z
and 0-9
) or the underscore character _
. Can’t contain a space. Variables are called by prepending $
to the variable name.
Like in most programming languages, there are some reserved words like if
, select
, then
, until
, while
, etc., that can’t be used as variables or as values of variables. For more: D.2 Index of Shell Reserved Words.
There is no need to specify a variable type. Anything surrounded by "
will be treated as text. You can use booleans, numbers, text and arrays (the implementation of arrays depends on the shell being used). Make a variable readonly by calling readonly variable_name
. Basic syntax:
var="my var"
.var=123
.var=true
and var=false
.bash
is the shell):var[0]=value1
, var[...]=...
, var[n]=valuen
, etc.var=(value1 ... valuen)
${var[index]}
and all values with ${var[*]}
or ${var[@]}
.There are special variables (for more. tutorialspoint: Unix / Linux - Special Variables):
-$
: represents the process ID number, or PID, of the current shell.0
: the filename of the current script.n
: where n
can be any whole number, correspond to arguments passed to the script (command arg1 arg2 arg3 argn
).#:
number of arguments supplied to the script.*:
all the arguments are double quoted.@:
all the arguments are individually double quoted.?:
exit status of the last command executed.!:
process number of the last background command.When calling a script, you can pass optional (or required) positional arguments like: command arg1 arg2 arg3 argn
.
Note that a variable can also take the output of another command, one common way to do this is using $(command)
or `command`
, for example: var="$(echo 'this is a command being executed inside the definition of a variable')"
which, since the echo
command is being run, var="this is a command being executed inside the definition of a variable"
, which doesn’t seem like much, but there could be any command inside $()
or `command`
. Note that this is not special to defining variables, could also be used as arguments of another command.
This is used by the shell to determine how to do word splitting (how to recognize word boundaries). The default value for IFS
consists of whitespace characters (space, tab and newline). This value can ve overridden by setting the variable IFS
to something like, for example, :
.
Any command being run has an exit status, either 0
or 1
, if the command has been executed successfully or otherwise (an error), respectively.
if
statementPretty similar to other programming languages, evaluates an expression to a true
or false
and executes code as specified. if
statements can be nested, and follow normal rules of logical operations. Basic syntax is:
#!/bin/sh
-if expression
-then
-do_something
-elif another_expression
-then
-do_another_thing
-else
-do_something_else
-fi
-
-The expression is usually wrapped around []
or [[]]
, the first being POSIX compliant and the second bash
-specific (and other shells).
Also, some operators to compare things use ==
for “equals” and >
for “greater than”, for example; while in a POSIX compliant shell, =
for “equals” and -gt
for “greater than” has to be used. For more operators: tutorialspoint: Unix / Linux - Shell Basic Operators (this also covers logical operators and file test operators).
A common good alternative to multilevel if
statements, enables you to match several values against one variable. Basic syntax is:
case $var in
- pattern1)
- do_something1
- ;;
- pattern2)
- subpattern1)
- do_subsomething1
- ;;
- subpattern2)
- do_subsomething2
- ;;
- *)
- pattern3|pattern4|...|patternN)
- do_something3
- ;;
- patternM)
- do_somethingM
- ;;
- *)
- do_something_default
- ;;
-esac
-
-Where the *
pattern is not necessary but serves the same purpose as a “default” case.
Loops enable execution of a set of commands repeatedly. Loops, naturally, can be nested. expression
here (in the basic syntax examples) work the same as mentioned in the “if
statement” section. For more: tutorialspoint: Unix / Linux - Shell Loop Types.
Similar than other programming languages, there are loop controls to interrupt or continue a loop:
-* `break` statement.
-* `continue` statement.
-
-These statements accept an argument that specify from which loop to exit/continue.
-while
loopEnables to execute a set of commands repeatedly until some condition occurs. Basic syntax:
-#!/bin/sh
-while expression
-do
- do_something
-done
-
-until
loopSimilar to the while
loop, the difference is that the while
loop is executed as long as a condition is true, but the until
loop… until a condition is true. Basic syntax (similar to while
loop):
#!/bin/sh
-until expression
-do
- do_something
-done
-
-for
loopOperates on lists of items. It repeats a set of commands for every item in a list. Basic syntax:
-#!/bin/sh
-for var in word1 word2 ... wordN
-do
- do_something_with_var
-done
-
-Where var
is the current value (word1
, word2
, etc.) in the loop and the expression after for
can refer to an array, or the output of a command that outputs a list of things, etc.
select
loopProvides an easy way to create a numbered menu from which users can select options. Basic syntax (similar to for
loop):
select var in word1 word2 ... wordN
-do
- do_something_with_var
-done
-
-Meta characters are used to execute several commands on a single line (depending on what it’s needed). The most used meta characters to accomplish this are semi-colon ;
, double ampersand &&
and double “pipe” ||
.
;
: is used to finish one command (similar to some programming languages), after the command on the left of ;
is finished (whatever the exit code is), the command on the right will be executed.&&
: similar to ;
, but only if the command on the left exits with code 0
(success).||
: similar to &&
, but for exit code 1
(error).Enable to break down the overall functionality of a script into smaller, logical subsections, which can then be called upon to perform their individual tasks when needed (like in any other programming language…). For more: tutorialspoint: Unix / Linux - Shell Functions. Basic syntax:
-#!/bin/sh
-function_name () {
- do_something
-}
-
-Functions can also take arguments and can access their individual arguments (each function will have a different “storage” for their arguments). Functions can also be nested. Here exit
will not only will finish the function code, but also the shell script that called it, instead use return
plus an exit code to just exit the function.
A list of basic commands and small explanation (note that options are started with either -
or --
, depending on the program, but most of the time -
is used for letter options and --
for word options, -l
vs --list
for example):
pwd
: “print working directory”, full absolute path to the current directory.cd
: “change directory”, followed by the absolute or relative path of the directory to change to./
, while a relative path is started with ./
or just the name of the folder...
(two dots) to go up one directory./home/username
is ~
(tilde).ls
: “list” files and directories in current directory, or specify a directory from which to show the list after typing ls
. Has many options, the most common ones being:l
: use long listing format.r
or reverse
: reverse order while sorting.s
: sort by file size, largest first.a
or all
: do not ignore entries starting with .
.mkdir
: “make directory”, create a new directory with specified name.touch
: create new (empty) files.cp
: “copy” files or directories (using option r
for recursive). Requires file/directory to copy and destination, separated by space.mv
: “move” files or directories, also requires file/directory to copy and destination, separated by space. This is also used to rename files/directories.rm
: “remove”, followed by a file to remove it.rmdir
: “remove empty directory”, followed by a directory to remove it. If the directory is not empty, use rm -r
on the directory (“remove recursive”).su
: “switch user”, by default to root user, but another one can be specified.sudo
: “switch user, do”, similar to su
, but only to execute a command as root or the specified user.clear
: clear the terminal window, a (common) keyboard shortcut is Ctrl + l
.find
: search for files/directories matching a pattern or all contents of a directory (using .
).grep
: comes from the ed
command “g/re/p”, for searching plain-text for lines that match a regular expression (regex).top
: a task manager program, shows currently running commands and gives important info such as PID (process ID), user who is running that command, command name, cpu and ram usage, etc.. Some useful commands to manage programs running are:pgrep
: get the PID of a running process, or a list in chronological order.kill
or pkill
: kill a running process either by PID or by name.killall
: similar to pkill
.ssh
: “secure shell” is a remote login client used to connect into a remote machine and executing commands remotely, basically taking control of the remote machine. Widely used when managing servers.ftp
or sftp
: “(secure) file transfer protocol” used to transfer files from one machine to another one (usually a server). It’s recommended to use sftp
instead of ftp
because anyone can look through the packages if it’s not secured (encrypted).And in general, to see the options supported by almost any command, use command -h
or command --help
, for a quick explanation. IMPORTANT: Most programs have man (manual) pages; to access them do man command
, this is a very powerful tool to use.
Commands can be redirected to other commands (the output), which is powerful to create mini scripts or to achieve a goal in a single command. Most of the time the redirection can be done with the special characters >
, <
and most powerful, the |
(pipe). Also, some commands accept an option to execute another command, but this depends on a command to command basis (exec
option for find
, for example).
Most terminal programs accept Ctrl-c
or just q
to exit the program.
When listing files with ls -l
, an output with file attributes (permissions) and ownership is shown, such as drwxr-xr-x 2 user group 4096 Jul 5 21:03 Desktop
, where the first part are the attributes, and user
and group
the ownership info (all other info is irrelevant for now).
File attributes (drwxr-xr-x
in the example above) are specified by 10 (sometimes 11) characters, and can be break into 4 parts (or 5):
d
for directories or just -
for files. There is l
too, which is for symlinks.+
character that specifies whether an alternate access method applies to the file. When the character is a space, there is no alterante access method.Each of the three permission triads (rwx
) can be:
-
or r
, for the first character, if the file can be read or directory’s content can be shown.-
or w
, for the second character, if the file can be modified or the directory’s content can be modified (create new files or folders or rename existing files or folders).-
or x
, for the third character, if the file can be executed or the directory can be accessed with cd
. Other characters can be present, like s
, S
, t
and T
(for more: Arch Linux Wiki: File permissions and attributes).To change attributes or ownership use chmod
and chown
, respectively.
Special type of linux process (think of a program or set of programs that run in the background waiting to be used, or doing essential tasks). There are many ways to manage (start, stop, restart, enable, disable, etc.) services, the most common way (if using systemd
) is to just use systemctl
. Basic usage of systemctl
is systemctl verb service
, where verb
could be start
, enable
, stop
, disable
, restart
, etc. Also, to get a general system status run systemctl status
or just systemctl
for a list of running units (a unit is an instance of a service, or a mount point or even a device or a socket). For more: Arch Linux Wiki: systemd.
systemd
also provides a way to do tasks based on a timer, where you can schedule from the second to the year. One could also use cron
(using crontab
with option e
) to do this. These timers provide support for calendar time events, monotonic time events, and can be run asynchronously.
Most mainstream linux distributions come with a Graphic User Interface (GUI) to manage users and groups on the system. For a Command-Line Interface (CLI) just use useradd
(with passwd
to create a password for a given user) and groupadd
. Also, other useful commands are usermod
, userdel
, groups
, gpasswd
, groupdel
and more, each used for a basic management of users/groups like modification, deletion, listing (of all existing users/groups), etc.. For more: Arch Linux Wiki: Users and groups.
Located at /etc/hosts
, serves as a translator from hostname (web addresses or URLs) into IP addresses (think of DNS records), meaning that any URL can be overridden to make it point to whatever IP address it’s specified (only locally on the machine affected). The syntax of the file is pretty simple: first column for IP, second for hostname (URL) and third+ for aliases.
These commands serve the sole purpose of showing information about the network and stuff related to it:
-ping
: gives information about latency to a given ip/domain.ifconfig
: gives similar information to ipconfig
on windows, general info of physical network devices with their addresses and properties. An alternative could be ip addr
, depending on the linux distribution being used and programs installed.tcpdump
: “transmission control protocol dump” gives information on all “packets” being sent and received through the network.netstat
: “network statistics” general statistics about network devices usage, display connections to the machine and more.traceroute
: shows the route that the packets go through (how the packets jump from one server to another one) when trying to access an IP (or, for example, a website).nmap
: “network mapper” explore network available hosts, opened ports, reverse DNS names, can guess the operating system of the device, it’s type, MAC address and more.Any collection of related information, such as a phone book, a shopping list, Facebook’s user base, etc.. It can be stored in different ways: on paper, on a computer, in your mind, etc..
-A special software program that helps users create and maintain a database that makes it easy to manage large amounts of information, handles security, backups and can connect to programming languages for automation.
-The four main operations that a DBMS will do: create, read, update and delete.
-When we want to create a RDB we need a Relational Database Management System (RDBMS) that uses Structured Query Language (SQL) which is a standardized language for interacting with RDBMS and it’s used to perform CRUD operations (and other administrative tasks).
-Anything that’s not relational, stores data in anything but static tables. Could be a document (JSON, XML, etc.), graph (relational nodes), key-value hash (strings, json, etc.), etc.
-NRDB also require a Non-Relational Database Management System (NRDBMS) to maintain a database. But it doesn’t have a standardized language for performing CRUD and administrative operations like how RDB have.
-A DB query is a request that is made to the (R/NR)DBMS for a specific information. A google search is a query, for example.
-A table is composed of columns, rows and a primary key. The primary key is unique and identifies one specific row. Columns and rows are trivial, a column identifies a field and has a specific data type (name, email, birth) and a row identifies a table entry (person that contains a name, email and birth).
-Also, there are foreign keys, it’s purpose is to relate to another database table; this foreign key is unique in it’s own table, but can be repeated where you use it as a foreign key.
-It’s possible to use the same table keys as foreign keys to make relations inside the same table.
-It’s actually a hybrid language, basically 4 types of languages in one:
-A set of instructions given to the RDBMS (written in SQL) that tell the RDBMS what information you want it to retrieve. Instead of getting the whole database, retrieve only a bit of information that you need.
-Also, SQL keywords can be either lower or upper case, but it’s convention to use upper case. And queries are ended by a semi-colon.
-Just some SQL data types (for more: MySQL 8.0 Reference Manual: Chapter 11 Data Types, the notation is DATATYPE(SIZE(,SIZE))
:
INT
: integer numbers.DECIMAL(M,N)
: decimal numbers.VARCHAR(N)
: string of text of length N.BLOB
: Binary Large Object, stores large data.DATE
: YYYY-MM-DD.TIMESTAMP
: YYYY-MM-DD HH:MM:SS.To create a table, the basic syntax is CREATE TABLE tablename (column1 datatype constraint, column2 datatype constraint, ...)
, where a constraint could be (for more: MySQL 8.0 Reference Manual: 13.1.20 CREATE TABLE Statement):
NOT NULL
: can’t have a NULL
value.UNIQUE
: all values are unique.PRIMARY KEY
: uniquely identifies each row.FOREIGN KEY
: uniquely identifies a row in another table.CHECK expresion
: satisfy a special condition (expresion
).DEFAULT value
: if no value is specified use value value
.INDEX
: to create and retrieve data from the database very quickly.Get the table structure with DESCRIBE tablename
and delete it with DROP TABLE tablename
. Add columns to the table with ALTER TABLE tablename ADD column DATATYPE(N,M)
, similar syntax to delete a specific column ALTER TABLE tablename DRORP COLUMN column
.
Add entries to the table with INSERT INTO tablename VALUES(value1, value2, ...)
where all the fields must be specified, or INSERT INTO tablename(column1, column2) VALUES(value1, value2)
to just add some fields to the new entry. While at it, (all) the table content can be fetched with SELECT * FROM tablename
.
Basic Updating of entries with UPDATE tablename SET expression1 WHERE expression2
, where expression1
could be column = value2
and expression2
could be column = value1
, meaning that the value of column
will be changed from value1
to value2
. Note that the expressions are not limited by column = value
, and that the column
has to be the same, it would be any expression. Also, this is really extensive as SET
can set multiple variables and WHERE
take more than one condition by chaining conditions with AND
, OR
and NOT
keywords, for example.
When an entry needs to be updated somehow based on a modification on a foreign key. If two tables are related to each other, if something is deleted on one end, update the other end in some way.
-For example on creation of a table, on the specification of a foreign key: CREATE TABLE tablename (..., FOREIGN KEY(column) REFERENCES othertable(othertablecolumn) ON DELETE something)
. That something could be SET NULL
, CASCADE
, etc..
Instead of doing SELECT * FROM tablename
, which gets all the data from a table, more complex SELECT
queries can be implemented, such as SELECT column FROM tablename
to only get all data from one column of the table. Append LIMIT N
to limit the query to N
entries. Append WHERE condition
to meet a custom condition.
Other statements that can be used in conjunction with SELECT
are ORDER BY column ASC|DESC
, SELECT DISTINCT
, MIN(column)
, MAX(column)
, COUNT(column)
, AVG(column)
, SUM(column)
, LIKE
and more. For more, visit MySQL 8.0 Reference Manual: 13.2.10 SELECT Statement.
MySQL uses regular expressions (regex) like pattern matching, some wildcards that can be used with the LIKE
statement are:
%
: zero or more characters._
: a single character.[]
: any single character within the brackets.^
: any character not in the brackets.-
: a range of characters.An extended regex can be used with the statement REGEX_LIKE(expression)
; REGEXP
and RLIKE
are synonyms for REGEX_LIKE
. For more: MySQL 8.0 Reference Manual: 3.3.4.7 Pattern Matching.
A specialized SQL operator that is used to combine multiple SELECT
statements into one. The basic syntax is SELECT ... UNION SELECT ...
, where ...
is a whole SELECT
statement; there can be any amount of unions. There are some rules that apply when doing unions, such as having the same amount of columns on both statements and being of the same data type.
Used to combine rows from two or more tables based on a related column between them. Basic syntax is SELECT table1.column1, ..., table2.column1, ... FROM table(1|2) JOIN table(1|2) ON table1.common_column = table2.common_column
, where the table specified in the FROM
statement is called the “left” table, where the one in the JOIN
statement is the “right” table. For more: MySQL 8.0 Reference Manual: 13.2.10.2 JOIN Clause.
There are different types of SQL JOINs:
-(INNER) JOIN
: returns records that have matching values in both tables.LEFT (OUTER) JOIN
: returns all records from the left table, and the matched records from the right table.RIGHT (OUTER) JOIN
: returns all records from the right table, and the matched records from the left table.FULL (OUTER) JOIN
: returns all records when there is a match in either left or right table.- - -
-A query composed of multiple select statements to get a specific piece of information. This is self explanatory, you do a SELECT
query somewhere inside another one, for example SELECT ... IN (SELECT ...)
, where the nesting is occurring inside the parenthesis after the IN
statement.
A nesting isn’t constrained to the IN
statement, it can appear anywhere, for example in a WHERE
statement: SELECT ... WHERE something = (SELECT ...)
.
A block of SQL code that will define a certain action that will happen when a specific operation is performed on the database. It is recommended to change the DELIMITER
temporarily from semi-colon to something else (since we need to use semi-colon to end the trigger) while the trigger is created. The basic syntax is CREATE TRIGGER trigername triggertime triggerevent ON tablename FOR EACH ROW triggerorder triggerbody
. For more: MySQL 8.0 Reference Manual: 13.1.22 CREATE TRIGGER Statement and MySQL 8.0 Reference Manual: 25.3.1 Trigger Syntax and Examples.
When designing a database it’s important to define a database schema which is just a definition of all the different tables and their attributes that are going to live inside the database. So, basically, an ERD diagram is a diagram that consists of text, symbols and shapes that are combined to create a relationship model.
-The diagram consists of:
-