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   __                      __
  / /_______  ___     ____/ /
 / __/ / /  \/ _ \   / __/  \
/ /_/ / / / / ___/__/_  / / /
\__/\  / __/\__/ /_/___/_/_/
____/ /_/
\____/    STANDALONE LIBRARY est. 2020

TYPE.SH - Simple Type Match/Cast for Shell-Scripts

This standalone library provides rudimentary type checking/casting functionality, entirely relying on shell builtins.

Types

As far as the shell is concerned the only type is the character string. It can handle integer arithmetic and comparison, so there is a notion of integers. However even integers are stored in strings.

The library adds a set of types with more constraints:

Type Description Matches (regex, case insensitive)
uint unsigned integer `^(0x[0-9a-f]+
int integer `^[-+]?(0x[0-9a-f]+
bool boolean `^(0
empty empty string ^$
argname argument number ^[0-9]$
varname variable name `^([a-z]
funcname function name ^[0-9]*[][a-z_.:][][a-z0-9_.:]*$

Conformity to a type can be checked using the match functions:

Function Cost Complexity
type:match:empty() O(1) single test call
type:match:bool() O(1) single glob pattern match
type:match:uint() O(n) recursive glob pattern match
type:match:int() O(n) recursive glob pattern match
type:match:argname() O(1) single glob pattern match
type:match:varname() O(n) recursive glob pattern match
type:match:funcname() O(n) recursive glob pattern match

The type:match() function allows checking whether a value matches one of a set of types:

if ! type:match empty,bool "$1"; then
    …
fi

Type Detection

The type:which() function provides provides the name of the first matched type from a list of given types.

if ! type:which type empty,bool,uint,int,varname "$1"; then
    echo "invalid value: $1" >&2
    return 1
fi
case "$type" in
empty)
    echo "empty value" >&2
    return 1
;;
bool)
    …
;;
…
esac

Type Casting

The type:cast[int]() function assigns matching inputs as decimal integer values:

type:cast[int] x empty,bool
echo "$x" # 0
type:cast[int] x empty,bool YES
echo "$x" # 1
type:cast[int] x empty,bool no
echo "$x" # 0
type:cast[int] x empty,bool True
echo "$x" # 1

Only the following types can be cast to int:

  • empty
  • bool
  • uint
  • int

The conversion is done for the first matching type. Independent of the matched type, for a given value the assigned value is always the same. E.g. 1 is a valid bool, int and uint and the assigned value for all three is 1. Thus the order of the given types does not affect the outcome of the type conversion.

The return value of type:cast[int] functions is the same as that of type:match functions, so the same uses for control flow are possible:

if ! type:cast[int] verbose bool,int "$VERBOSITY"; then
    echo "Invalid VERBOSITY level, assume 0!" >&2
    verbose=0
fi

Individual cast functions can be called directly:

x=1
type:cast[int]:empty x || type:cast[int]:bool x
echo "$x" # 0
type:cast[int]:empty x YES || type:cast[int]:bool x YES
echo "$x" # 1
type:cast[int]:empty x no || type:cast[int]:bool x no
echo "$x" # 0
type:cast[int]:empty x True || type:cast[int]:bool x True
echo "$x" # 1

If no type matches, the destination variable remains unchanged:

x=nope
type:cast[int] x bool,int NaN
echo "$x" # nope