You don't need much bash code to implement classes or objects in bash.
Say, 100 lines.
Bash has associative arrays that can be used to implement a simple Object system with inheritance, methods and properties.
So, you would might define a class like this:
class Queue N=10 add=q_add remove=q_remove
Creating an instance of this Queue might be done like this:
class Q:Queue N=100
or
inst Q:Queue N=100
Since classes are implemented with an array, class and inst are really synonyms - sort of like in javascript.
Adding items into this queue could be done like this:
$Q add 1 2 aaa bbb "a string"
Removing items into a variable X might be done like this:
$Q remove X
And dumping structure of an object could be done like this:
$Q dump
Which would return something like this:
Q {
parent=Queue {
parent=ROOT {
this=ROOT
0=dispatch ROOT
}
class=Queue
N=10
add=q_add
remove=q_remove
0=dispatch Queue
}
class=Q
N=4
add=q_add
remove=q_remove
0=dispatch Q
1=
2=ccc ddd
3=
4=
}
Classes are created using a class function like this:
class(){
local _name="$1:" # append a : to handle case of class with no parent
printf "$FUNCNAME: %s\n" $_name
local _this _parent _p _key _val _members
_this=${_name%%:*} # get class name
_parent=${_name#*:} # get parent class name
_parent=${_parent/:/} # remove handy :
declare -g -A $_this # make class storage
[[ -n $_parent ]] && { # copy parent class members into this class
eval _members=\"\${!$_parent[*]}\" # get indices of members
for _key in $_members; do # inherit members from parent
eval _val=\"\${$_parent[$_key]}\" # get parent value
eval $_this[$_key]=\"$_val\" # set this member
done
}
shift 1
# overwrite with specific values for this object
ROOT_set $_this "$@" "0=dispatch $_this" "parent=${_parent:-ROOT}" "class=$_this"
}
NOTE: When defining a new class or instance, you can override any member value or function.
Bash associative arrays have a quirk that makes this work neatly: $Q[0]} is identical to $Q. This means that we can use array name to call a method dispatch function:
dispatch(){
local _this=$1 _method=$2 _fn
shift 2
_fn="$_this[$_method]" # reference to method name
${!_fn} $_this "$@"
}
A down side is that I can not use [0] for data so my queues (in this case) start from index=1. Alternatively I could have used associative indices like "q+0".
To get and set members you might do something like this:
# basic set and get for key-value members
ROOT_set(){ # $QOBJ set key=value
local _this=$1 _exp _key _val
shift
for _exp in "$@"; do
_key=${_exp%%=*}
_val="${_exp#*=}"
eval $_this[$_key]=\"$_val\"
done
}
ROOT_get(){ # $QOBJ get var=key
local _this=$1 _exp _var _key
shift
for _exp in "$@"; do
_var=${_exp%%=*}
_key=${_exp#*=}
eval $_var=\"\${$_this[$_key]}\"
done
}
And to dump an object structure, I made this:
NOTE: This is not required for OOP in bash, but it is nice to see how objects are made.
# dump any object
obj_dump(){ # obj_dump <object/class name>
local _this=$1 _j _val _key; local -i _tab=${2:-(${#_this}+2)} # add 2 for " {"
_tab+=2 # hanging indent from {
printf "%s {\n" $_this
eval "_key=\"\${!$_this[*]}\""
for _j in $_key; do # print all members
eval "_val=\"\${$_this[\$_j]}\""
case $_j in
# special treatment for parent
parent) printf "%*s%s=" $_tab "" $_j; ${!_val} dump $(( _tab+${#_j}+${#_val}+2 ));;
*) printf "%*s%s=%s\n" $_tab "" $_j "$_val";;
esac
done
(( _tab-=2 ))
printf "%*s}\n" $_tab ""
return 0
}
My OOP design has not considered objects within objects - except for inherited class. You could create them separately, or make a special constructor like class(). *obj_dump* would need to be modified to detect internal classes to recursively print them.
Oh! and I manually define a ROOT class to simplify class function:
declare -gA ROOT=( \
[this]=ROOT \
[0]="dispatch ROOT" \
[dump]=obj_dump \
[set]="ROOT_set" \
[get]="ROOT_get" \
)
With a few queue functions I defined some classes like this:
class Queue \
in=0 out=0 N=10 \
dump=obj_dump \
add=q_add \
empty=q_empty \
full=q_full \
peek=q_peek \
remove=q_remove
class RoughQueue:Queue \
N=100 \
shove=q_shove \
head_drop=q_head_drop
Created some Queue instances and made them work:
class Q:Queue N=1000
$Q add aaa bbb "ccc ddd"
$Q peek X
$Q remove X
printf "X=%s\n" "$X"
$Q remove X
printf "X=%s\n" "$X"
$Q remove X
printf "X=%s\n" "$X"
class R:RoughQueue N=3
$R shove aa bb cc dd ee ff gg hh ii jj
$R dump
cd ~followed bylsworks as in Bash. You can also assign output to Python variables (lists of lines . . sort of) with commands likelisting = !ls. – intuited Dec 05 '10 at 03:42