198 lines
5.2 KiB
Plaintext
198 lines
5.2 KiB
Plaintext
Scripting Documentation
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-----------------------
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This document provides a brief introduction to the script facilities of the shell.
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Command line arguments
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----------------------
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Variables can be passed to a script from the command line and accessed with
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$num where num is the number of the shell script variable. The number of
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command line variables is given by the $# variable.
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Comments
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--------
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Text following a # on a line is deemed a comment.
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Data types
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----------
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At the moment, the shell only supports primitive types such as integers, booleans, and strings.
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Variables
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---------
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Variables can be assigned to with a simple assignment construct.
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For instance, a = 'bc' assigns the string 'bc' to the variable a.
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Variables can be referenced by preceding the name with $ (e.q. echo $a)
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or with ${name} to separate the reference from surround text such as
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b = "surrounding${var}text". References to variables that have not been
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previously assigned are considered an error. An error message is printed
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and script execution is halted.
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Operators
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---------
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integer: *, / , %, +, -
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string: . (concatenation)
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boolean: ||, &&, !
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comparison: <, <=, ==, >=, >.
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The integer operations coerce the
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operand types to ints. Therefore, "5" + "9" will call
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Int32.ParseInt() on both of these arguments. If one of the operands
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to an integer operator is boolean than true is converted to 1
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and false is converted to 0.
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String operators will call ToString on the value for an operand.
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Boolean operators: coerce operand types to booleans. In the case
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of integers, an integer equal to 0 is false while all other values
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are equivalent to true. For strings, a value is only coercible
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if it is either one of Boolean.TrueString (true) or Boolean.FalseString (false)
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(case-insensitive).
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Comparison:
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Currently, comparison operators coerce both operands to the type of the left operand
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using the methods described thus far.
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Stricter and more sensible typing may be implemented in the future.
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Expressions
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-----------
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Expressions can be constructed from variables, any of the basic primitive
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types and the set of operators.
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Control Flow
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------------
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Language currently supports if, if-else, and while. The conditions to these constructs
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are arbitrary expressions that successfully evaluate to a boolean value. Bracing of the statements
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for these constructs is required to eliminate ambiguity.
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Commands
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--------
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Unfortunately, commands currently consist of a sequence of alpha-numeric words, a string
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literal, interpolation, or integer ending in a new line. Therefore and non-alpha-numeric characters (i.e. %)
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must be either placed in a string or interpolation. Hopefully, this will eventually change.
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Exit and Exit codes
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-----------
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The exit status of the last command is given by the $? variable.
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The built-in exit command takes optionally one integer argument indicating
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the exit code of the script execution. If no argument is specified, the status
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of the last executed command is returned. If no argument is given and no
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commands have been executed, then 0 is returned. This exit builtin differs from
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that of the shell prompt. That version performs a shutdown while this
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exits the script.
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Grammar
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-------
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SCRIPT => STMT_LIST
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STMT_LIST => STMT
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| STMT STMT_LIST
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STMT => while( E ) { STMT_LIST }
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| if( E ) { STMT_LIST }
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| if( E ) { STMT_LIST } else { STMT_LIST }
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| var = E newline
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E => integer_literal | string_literal | bool_literal | variable_reference | interpolation
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| E + E | E - E | E * E | E % E |
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| E < E | E <= E | E == E | E >= E | E > E
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| E . E
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| E && E | E || E | !E
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COMMAND_LINE=> COMMAND_LIST newline
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COMMAND_LIST => COM
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| COM COMMAND_LIST
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COM => word
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| variable_reference
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| integer_literal
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| string_literal
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| interpolation
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Example (test script from Shell.sg )
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------------------------------------
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echo $2 #echo second argument
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decho $2
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echo number of arguments '=' $#
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decho number of arguments '=' $#
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echo last command exit code '=' $?
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decho last command exit code '=' $?
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if (true) {
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variable = false
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if ($variable) {
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var2 = true
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echo broken conditional
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decho broken conditional
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}
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else {
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var2 = true
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output1 = "bet${variable}ween"
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echo $output1
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decho $output1
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}
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#some comments #
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#
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decho var2 is $var2
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add = -2 + 4
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echo '-2 + 4 =' $add
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decho '-2 + 4 =' $add
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mod = 6 % 5
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echo '6 % 5 =' $mod
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decho '6 % 5 =' $mod
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div = 10 / 5
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echo '10 / 5 =' $div
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decho '10 / 5 =' $div
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mult = 2 * 5
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echo '2 * 5=' $mult
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decho '2 * 5=' $mult
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var1 = 5
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var2 = 6
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if($var1 < $var2){
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decho $var1 is less than $var2
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two = 1
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power = 10
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echo starting loop
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decho starting loop
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while($power > 0){
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two = $(two) * 2
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power = $power - 1
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}
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output2 = "2^10 = $two"
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echo $output2
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decho $output2
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}
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if ($var1 > $var2) {
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echo $var2 is less than $var1
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decho $var2 is less than $var1
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}
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else {
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output3 = 'var1' . " is " . $var1
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echo $output3
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decho $output3
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}
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var = "test"
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if ("test" == $var) {
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echo string compare success
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decho string compare success
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}
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} |