Execute shell script:

./script.sh
bash script.sh
source script.sh

Read user input:

read

Echo text to screen:

echo

Arithmetic operations:

let
expr

Conditional statements:

if
case

Loops:

for
while

Array operations:

declare 
read -a

Check file/directory existence:

[file] -e 
[file] -d 
[file] -f

Check file permissions:

[file] -r 
[file] -w 
[file] -x

Check file type:

[file] -b 
[file] -c 
[file] -L
[file] -S