murex Shell Docs

Command Reference: let

Evaluate a mathematical function and assign to variable

Description

let evaluates a mathematical function and then assigns it to a locally scoped variable (like set)

Usage

let var_name=evaluation

let var_name++

let var_name--

Examples

» let: age=18
» $age
18

» let: age++
» $age
19

» let: under18=age<18
» $under18
false

» let: under21 = age < 21
» $under21
true

Detail

Variables

There are two ways you can use variables with the math functions. Either by string interpolation like you would normally with any other function, or directly by name.

String interpolation:

» set abc=123
» = $abc==123
true

Directly by name:

» set abc=123
» = abc==123
false

To understand the difference between the two, you must first understand how string interpolation works; which is where the parser tokenised the parameters like so

command line: = $abc==123
token 1: command (name: "=")
token 2: parameter 1, string (content: "")
token 3: parameter 1, variable (name: "abc")
token 4: parameter 1, string (content: "==123")

Then when the command line gets executed, the parameters are compiled on demand similarly to this crude pseudo-code

command: "="
parameters 1: concatenate("", GetValue(abc), "==123")
output: "=" "123==123"

Thus the actual command getting run is literally 123==123 due to the variable being replace before the command executes.

Whereas when you call the variable by name it's up to = or let to do the variable substitution.

command line: = abc==123
token 1: command (name: "=")
token 2: parameter 1, string (content: "abc==123")

command: "="
parameters 1: concatenate("abc==123")
output: "=" "abc==123"

The main advantage (or disadvantage, depending on your perspective) of using variables this way is that their data-type is preserved.

» set str abc=123
» = abc==123
false

» set int abc=123
» = abc==123
true

Unfortunately is one of the biggest areas in murex where you'd need to be careful. The simple addition or omission of the dollar prefix, $, can change the behavior of = and let.

Strings

Because the usual murex tools for encapsulating a string (", ' and ()) are interpreted by the shell language parser, it means we need a new token for handling strings inside = and let. This is where backtick comes to our rescue.

» set str abc=123
» = abc==`123`
true

Please be mindful that if you use string interpolation then you will need to instruct = and let that your field is a string

» set str abc=123
» = `$abc`==`123`
true

Best practice recommendation

As you can see from the sections above, string interpolation offers us some conveniences when comparing variables of differing data-types, such as a str type with a number (eg num or int). However it makes for less readable code when just comparing strings. Thus the recommendation is to avoid using string interpolation except only where it really makes sense (ie use it sparingly).

Non-boolean logic

Thus far the examples given have been focused on comparisons however = and let supports all the usual arithmetic operators:

» = 10+10
20

» = 10/10
1

» = (4 * (3 + 2))
20

» = `foo`+`bar`
foobar

Read more

murex uses the govaluate package. More information can be found in it's manual.

Scoping

Variable scoping is simplified to three layers:

  1. Local variables (set, !set, let)
  2. Global variables (global, !global)
  3. Environmental variables (export, !export, unset)

Variables are looked up in that order of too. For example a the following code where set overrides both the global and environmental variable:

» set:    foobar=1
» global: foobar=2
» export: foobar=3
» out: $foobar
1

Local variables

These are defined via set and let. They're variables that are persistent across any blocks within a function. Functions will typically be blocks encapsulated like so:

function example {
    # variables scoped inside here
}

...or...

private example {
    # variables scoped inside here
}

...however dynamic autocompletes, events, unit tests and any blocks defined in config will also be triggered as functions.

Code running inside any control flow or error handing structures will be treated as part of the same part of the same scope as the parent function:

» function example {
»     try {
»         # set 'foobar' inside a `try` block
»         set: foobar=example
»     }
»     # 'foobar' exists outside of `try` because it is scoped to `function`
»     out: $foobar
» }
example

Where this behavior might catch you out is with iteration blocks which create variables, eg for, foreach and formap. Any variables created inside them are still shared with any code outside of those structures but still inside the function block.

Any local variables are only available to that function. If a variable is defined in a parent function that goes on to call child functions, then those local variables are not inherited but the child functions:

» function parent {
»     # set a local variable
»     set: foobar=example
»     child
» }
» 
» function child {
»     # returns the `global` value, "not set", because the local `set` isn't inherited
»     out: $foobar
» }
» 
» global: $foobar="not set"
» parent
not set

It's also worth remembering that any variable defined using set in the shells FID (ie in the interactive shell) is localised to structures running in the interactive, REPL, shell and are not inherited by any called functions.

Global variables

Where global differs from set is that the variables defined with global will be scoped at the global shell level (please note this is not the same as environmental variables!) so will cascade down through all scoped code-blocks including those running in other threads.

Environmental variables

Exported variables (defined via export) are system environmental variables. Inside murex environmental variables behave much like global variables however their real purpose is passing data to external processes. For example env is an external process on Linux (eg /usr/bin/env on ArchLinux):

» export foo=bar
» env -> grep foo
foo=bar

Function Names

As a security feature function names cannot include variables. This is done to reduce the risk of code executing by mistake due to executables being hidden behind variable names.

Instead murex will assume you want the output of the variable printed:

» out "Hello, world!" -> set hw
» $hw
Hello, world!

On the rare occasions you want to force variables to be expanded inside a function name, then call that function via exec:

» set cmd=grep
» ls -> exec: $cmd main.go
main.go

This only works for external executables. There is currently no way to call aliases, functions nor builtins from a variable and even the above exec trick is considered bad form because it reduces the readability of your shell scripts.

Usage Inside Quotation Marks

Like with Bash, Perl and PHP: murex will expand the variable when it is used inside a double quotes but will escape the variable name when used inside single quotes:

» out "$foo"
bar

» out '$foo'
$foo

» out ($foo)
bar

See Also

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