notes/IT/Scripting.md

Marking a script as executable

Sometimes, you may need to mark a script as an executable to run it. On Linux, this can be accomplished by chmod +x [FILE], where [FILE] is the file you want to mark as executable. To unmark a file as executable, use chmod -x

The Shebang

There are various shells with their own language syntax (sh, bash, fish, etc). Therefore, more complicated scripts will indicate a particular shell by specifying the absolute path to the interpreter as the first line, prefixed by #! like this:

#!/bin/sh
echo "Hello, World!"

#!/bin/sh means that this script can be executed by the binary located at /bin/sh, and so the reader knows this is a sh(ell) script.

Variables

In bash, variable assignment is done with the = operator. Variables are conventionally named with SCREAMING_SNAKE_CASE, and can be accessed by prefixing the variable name with a dollar sign ($):

#!/bin/bash
# There may not be any spaces used during assignment, `A = B` means something different than `A=B`
# Assignment is done with the equal sign (=) operator:
MY_VARIABLE="Hi Mom!"
echo $MY_VARIABLE

Quotes

In Bash, different styles of quotes (or a backtick) mean different things:

Quote	Description
Single Quotes (')	Enclosing characters in single quotes (') preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. Commands in single quotes will not be evaluated.
Double Quotes (")	Enclosing characters in double quotes (") preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !. The characters $ and \‘ retain their special meaning within double quotes. A double quote may be quoted within double quotes by preceding it with a backslash. When the shell is in POSIX mode (see Bash POSIX Mode), the ‘!’ has no special meaning within double quotes, even when history expansion is enabled. Within double quotes, substitution can be done using $(). <br>"Output of ls: $(ls)"<br>
Backtick (`)	While a backtick is not technically a quotation mark, it's included here. Backticks are used to substitute the output a command in a location: <br>sudo chown `id -u` /some/directory<br>

Conditionals

A basic if statement in bash looks like this:

if somecommand; then
	# The code here will be run if somecommand has an exit code of 0
fi

Note that the if statement is terminated by fi. This is fairly standard throughout bash scripting, where the blocks are closed with the reverse text used to open them.

You can also make use of else or elif for more complex conditional logic:

if somecommand; then
	# If the command succeeds, run this code
else
	# If the command fails, run this code.
fi

elif:

if [ "$1" = "hello" ]; then
  echo "hello yourself"
elif [ "$1" = "goodbye" ]; then
  echo "nice to have met you"
  echo "I hope to see you again"
else
  echo "I didn't understand that"
fi

To test one value against a variety of conditions, you can use case:

case "$1" in
hello|hi)
  echo "hello yourself"
  ;;
goodbye)
  echo "nice to have met you"
  echo "I hope to see you again"
  ;;
*)
  echo "I didn't understand that"
esac

If using test or the shorthand of test ([]), there are a variety of binary comparison operators you can make use of:

Operator	Description
-eq	Is equal to
-ne	Is not equal to
-gt	Is greater than
-ge	Is greater than or equal to
-lt	Is less than
-le	Is less than or equal to
The above list is far from complete but it contains some of the more commonly used operators

Loops

For loops

for loops are used when you have a finite collection over which you want to iterate, such as a list of files, or a list of server names:

THINGS="thingone thingtwo thingthree"
for THING in $THINGS; do
  echo "Doing something to $THING"
done

The above example iterates over a space separated list of items, but if you wanted to iterate over a range of numbers, you could do something like this:

for i in {1..10} ; do ... ; done

While loops

while loops operate on lists of unknown size. It will keep running until the test it evaluates returns false:

i=0
while [ $i -lt 10 ]; do
  echo $i
  i=$(( $i + 1))
done
echo “Done counting”

Functions

While there are a few ways to declare a function in bash, the most common syntax looks like this:

function_name () {
	echo "I was called by a function"
}

To execute a function, simply include the name of the function. To execute the above function:

function_name

By default, all variables in bash are global. To declare a local variable in a function, prefix the declaration with the local keyword.

function_name () {
	local A='A'
	echo `A`
}

To return a value from a function, use the return keyword. After calling, it'll be assigned to the global variable $?.

my_function () {
  return 55
}

my_function
# Will echo 55
echo $?

Commands

Command	Description
exit [CODE]	Make the script process exit, where [CODE] is the exit code the process will terminate with.
read [VARIABLE]	Read user input into the provided variable.
test [CONDITION]	Used for conditionals, test will return an exit code of 0 if the test evaluates to true. Refer to the manual for more info. As a shorthand, you can enclose the condition in square brackets: []