Declaring Variables with Type Safety in BASH

Contrary to popular belief, BASH has some type safety if you declare your variables with typeset or declare.

Rant

These two BASH builtins, typeset and declare are synonyms, but since typeset also works in Korn Shell (ksh) I use that keyword to make my code a bit more portable. Perhaps silly since I also advocate writing pure BASH and not worry about following the POSIX Utilities spec as BASH is such a much richer language than sh and it's available on all platforms I care about these days. In any case, onwards:

Integers

typeset -i age=20
printf "Age is %s\n" "${age}"
typeset -i age=foo
printf "Age is %s\n" "${age}"
typeset -i age=85
printf "Age is %s\n" "${age}"

Running this produces:

$ ./declaring-variables.sh 
Age is 20
Age is 0
Age is 85

As you can see, BASH guarantees that the variable stays an integer even if some piece of code assigns a string to it.

Lowercase string

typeset -l always_lowercase=FOO

BASH guarantees that all strings written to this variable are stored as lowercase:

$ echo "${always_lowercase}"
foo

Uppercase string

typeset -u always_uppercase=bar

BASH guarantees that all strings written to this variable are stored as uppercase:

$ echo "${always_uppercase}"
BAR

Arrays

As you probably already know, you declare arrays with declare -a or typeset -a. What you may not know, is that you can also make the variable a constant, making modifications of this variable a runtime error:

typeset -a -r COMPASS=(
  "north"
  "west"
  "south"
  "east"
)

for direction in "${COMPASS[@]}"; do
  printf "%s\n" "${direction}"
done

COMPASS=(
  "down"
  "up"
  "nowhere"
  "somewhere"
)

for direction in "${COMPASS[@]}"; do
  printf "%s\n" "${direction}"
done

Running this produces:

$ ./declaring-variables.sh 
north
west
south
east
declaring-variables.sh: line 25: COMPASS: readonly variable

I bet you didn't know BASH had these features, I for sure didn't! 😉