Arrays and Flow Control

Arrays

We define arrays using parentheses:

domains=("google.com" "freyhm.com" "facebook.com")

To access an element by index:

echo "${domains[0]}"   # google.com

The ${ } with the index in square brackets is the syntax for element access — different from regular variable access. We quote the whole expansion so its value stays one argument.

Arithmetic

Basic operators: +, -, *, /, % (modulus). We use $(( )) for arithmetic expressions:

result=$((var1 + var2))

Parameter expansion can also give us the length of a variable’s value:

echo ${#variable}

tee

tee reads from stdin and writes to stdout and to a file at the same time. Useful when we want to see output on screen while also saving it to a file:

./script.sh | tee output.txt

Loops

Bash has several loop forms. The three common list/condition forms below all use do / done. for — iterates over a list:

for var in 1 2 3 4
do
  echo "$var"
done

while — runs while the condition is true:

while [ "$var" -eq 1 ]
do
  do_something
done

until — runs while the condition is false (the opposite of while):

until [ "$counter" -eq 10 ]
do
  ((counter++))
done

case

case is for branching without chaining a long list of elif statements. We match a value against patterns and run the corresponding block:

case "$expression" in
  pattern) statement ;;
esac

A real example:

case "$port" in
  80)  handle_http ;;
  443) handle_https ;;
  22)  handle_ssh ;;
  *)   : ;;   # catches anything else and does nothing
esac

;; closes each branch. esac closes the whole block. The * pattern is the catch-all — anything that didn’t match above falls here. : is a command that does nothing successfully. If unknown values should terminate the whole script instead, we can use exit in that branch.

Writeups

Manual

Windows Internals

Arrays and Flow Control

Arrays

Arithmetic

tee

Loops

case

​Arrays

​Arithmetic

​tee

​Loops

​case

Arrays

Arithmetic

tee

Loops

case