Some anotations

Variables and Arrays

Enviroment variables

    printenv

    env

Set and Unset variables in local/session shell

    TESTE="nada"

    echo $TESTE

    unset TESTE

    echo $TESTE

Exporting variables to enviroment variables

    HELLO="Hello, world!"

    export HELLO

    env | grep HELLO

    printenv | grep HELLO

Special variables

Variable	Positional Parameters
$0	Parameter number 0 (command or function name)
$1	Parameter number 1 (from command line or function)
...	Parameter number N ...
$9	Parameter number 9 (from command line or function)
${10}	Parameter number 10 (from command line or function)
...	Parameter number NN ...
$#	Total number of command line or function parameters
$*	All parameters as a single string
$@	All parameters such as multiple protected strings
Variable	Miscellaneous
$$	PID number of the current process (from the script itself)
$!	PID number of the last background job
$_	Last argument of the last command executed
$?	Return code from the last command executed

Variable expansion

Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed like this:

    $(command)

or

    ${command}

Syntax	Conditional Expansion
${var:-text}	If var is not defined, return 'text'
${var:=text}	If var is not set, set it to 'text'
${var:?text}	If var is not defined, returns error 'text'
${var:+text}	If var is defined, it returns 'text', otherwise it returns empty
Syntax	String Expansion
${var}	Same as $var, but unambiguous
${#var}	Returns the length of the string
${!var}	Execute the contents of $var (equals 'eval $$var')
${!text*}	Returns variable names starting with 'text'
${var:N}	Returns text from position 'N'
${var:N:tam}	Returns 'tam' characters from position 'N'
${var#text}	Cut 'text' from beginning of string
${var##text}	Cut 'text' from beginning of string (*greedy)
${var%text}	Cut 'text' from end of string
${var%%text}	Cut 'text' from end of string (*greedy)
${var/text/new}	Replace 'text' with 'new', once
${var//text/new}	Replace 'text' with 'new', always
${var/#text/new}	If string starts with 'text', replace 'text' with 'new'
${var/%text/new}	If string ends with 'text', replace 'text' with 'new'
${var^}	Convert the first character to uppercase
${var^^}	Convert all characters to uppercase
${var,}	Convert the first character to lowercase
${var,,}	Convert all characters to lowercase
${var~}	Invert case of first character
${var~~}	Inverts all characters

Functions

Variables in functions

If you create a variable inside a function, even calling it outside the function, the value of the variable will be displayed, as it will be treated as a global variable. In order for the value of a variable to be displayed only within the function, we need to use the 'local' command in the variable declaration.

Constant Variable

    declare -r myConstant="This is a constant"

External files

It works like include, import from another languages

    source external_file.sh

Conditions

Test

    test 1 = 1; echo $?

or

    [ 1=1 ]; echo $?

Numerical Comparison	Numeric Comparison
-lt	Is less than (LessThan)
-gt	Is greater than (GreaterThan)
-le	LessEqual (LessEqual)
-ge	GreaterEqual
-eq	Equals (EQual)
-ne	Is Different (NotEqual)
String Comparison
=	is equal
!=	It's different
-n	Is not null
-z	Is null
Logical Operators
!	logical NOT (NOT)
-a	And logical (AND)
it	Logical OR (OR)
Tests in files
-b	It is a block device
-c	It's a character device
-d	Is a directory
-e	File exists
-f	It's a regular file
-g	SGID bit is on
-G	The file's group is that of the current user
-k	Sticky-bit is on
-L	The file is a symbolic link
-O	The file owner is the current user
-p	The file is a named pipe
-r	File has read permission
-s	File size is greater than zero
-S	The file is a socket
-t	The file descriptor N is a terminal
-u	SUID bit is on
-w	File is writable
-x	File has execute permission
-nt	The file is newer (NewerThan)
-ot	The file is older (OlderThan)
-ef	The file is the same (EqualFile)

IF ELSE

Unlike other languages, 'if' tests a command and not a condition. But the 'test' command tests conditions, so just use it in conjunction with if.

LOOPS

• CONTINUE, BREAK, EXIT

I/O

Math in Shell Scripts

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