bash-scripting

https://www.udemy.com/course/bash-scripting/learn/lecture/5739688#overview

Script Order & Checklist

Shebang (#!)
Comments/introduction to the script
Global variables
Functions
Main script content
Exit status

Cheatsheet

Testing Files

-d FILE checks if the file is a directory

-e FILE checks if the file exists

-f FILE checks if the file exists and is a regular file (i.e. not a symlink)

-r FILE checks if the file is readable

-s FILE checks if the file exists and is not empty

-w FILE checks if the file is writable

-x FILE checks if the file is executable

String Operators

-z STRING checks if the string is empty

-n STRING checks if the string is not empty

STRING1 = STRING2 checks equality between two strings

STRING1 != STRING2 checks inequality between two strings

Arithmetic Operators

NUM1 -eq NUM2 checks if the two numbers are equal

NUM1 -ne NUM2 checks if the two numbers are not equal

NUM1 -lt NUM2 checks if number one is less than number two

NUM1 -le NUM2 checks if number one is less than or equal to number two

NUM1 -gt NUM2 checks if number one is greater than number two

NUM1 -ge NUM2 checks if number one is greater than or equal to number two

Logical Operators

&& is AND

|| is OR

Control Statements

if [ condition ] then
    # do this
elif [ another condition ] then
    # do that
else
    # do something else
fi

for VARIABLE_NAME in ITEM_1 ITEM_N do
    # do this
    # do that
    # do something else
done

while [ condition ]
do
    # do something
    # do something else
done

case "$VAR" in
    pattern_1)
        # do something
        ;; # terminates the case
   pattern_n)
        # do something else
        ;;
esac

Script Parameters

Scripts will often accept parameters. These can be accessed using $X where X is the parameter number. $0 is a special parameter and is reserved for the name of the shell or shell script. $@ is another special parameter. This parameter is an array of all of the parameters passed to the script.

User Input

read -p "PROMPT MESSAGE" VAR will prompt the user for input using the "PROMPT MESSAGE" and store it in $VAR

Exit Status

Every command returns an exit status. This return code can range from 0 to 255. 0 means the command executed successfully whilst any other return code means an error occurred - which can be useful for error checking.

$? is a variable that contains the return code of the previously executed command.

return NUM explicitly defines the return code

Functions

Functions must be defined BEFORE they're called and can be defined in a couple of ways:

function function-name() {
    # do something
}

function-name() {
    # do something else
}

Functions can be called simply by referencing them by name:

function hello() {
    echo "Hello $1"
}

hello Kiel

Variable Scope

By default, all variables are global. Variables must be declared before they can be used.

Local variables can be created using the local keyword: local LOCAL_VAR="VALUE". This keyword can only be used inside functions.

Wildcards

A wildcard is a character or string used for pattern matching.

* matches zero or more characters

? matches exactly one character

[abc] matches any of the included characters

! excludes characters

- defines a range of characters

[[:alpha:]] matches upper and lowercase characters

[[:alnum:]] matches upper and lowercase characters and integer numbers

[[:digit:]] matches any integer number

[[:lower:]] matches lowercase letters

[[:upper:]] matches uppercase letters

[[:space:]] matches whitespace

\ escapes a character

Logging

# create user.notice message
logger "Message"

# create local0.info message
logger -p local0.info "Message"

# tagging
logger -t your_tag -p local1.debug "Message"

# include the PID
logger -i "Message"

# log to console
logger -s "Message"

Debugging

Add -x to the end of the shebang to print commands and their arguments as they execute. You can also specify specific sections of your script to debug using:

# begin debugging
set -x

# do something
# do something else

# stop debugging
set +x

Modify the value of the PS4 variable to help with debugging:

# print the script name and line number when the -x option is used
PS4='+ ${BASH_SOURCE}:${LINENO}'

Add -e to terminate the script when a command returns a non-zero exit status.

Add -v to print commands as they are written.

zaheercena/bash-scripting