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Program Invocation ​

The syntax for starting a program is the same as the syntax for function calls. In Elk, function calls and program invocations are the exact same thing syntactically. Read more about how this works in Functions. It is also possible to specify a path in order to run a program. Characters in paths can be escaped using a backslash.

Program invocations always return a pipe object. Iterating over a Pipe yields one line at a time.

elk
echo("hello world")
echo hello world
cat("file") | grep("line")
cat file | grep line

./someScript.sh
../someOtherScript.sh(cat file)
echo("hello world")
echo hello world
cat("file") | grep("line")
cat file | grep line

./someScript.sh
../someOtherScript.sh(cat file)

INFO

Keep in mind that all arguments that are given to a program invocation are converted into string values.

Exec ​

The exec function is used to invoke a program by passing a string value as the name. This is useful when invoking programs with names containing special characters, in order to avoid having to escape all the characters using backslashes.

elk
exec "../some script|with&&a weirdname" arg1 arg2
exec "../some script|with&&a weirdname" arg1 arg2

Standard Output Redirection ​

In Elk, it is not necessary to be explicit about when the standard output of a process should be redirected. Instead, redirection happens automatically when the value of a program invocation expression is used in some way. In short, this means that standard output is captured automatically unless the program invocation expression is a free-standing line in the global scope or in a block. If it is the last line of a block, redirection only happens if the value of that block is used.

elk
# redirection happens, the output of `echo` is captured
# and therefore not printed to the terminal
let hello = echo hello world
assert(hello == "hello world")

# no redirection, gets printed to the terminal
echo hello world

fn files(path) {
    ls(path)
}

 # the result of `ls` is saved to the variable and
 # therefore prevented from being printed to the terminal
let a = files("/bin")

# the result of `ls` is not used, meaning standard output
# is not captured and instead printed to the terminal
files("/bin")
# redirection happens, the output of `echo` is captured
# and therefore not printed to the terminal
let hello = echo hello world
assert(hello == "hello world")

# no redirection, gets printed to the terminal
echo hello world

fn files(path) {
    ls(path)
}

 # the result of `ls` is saved to the variable and
 # therefore prevented from being printed to the terminal
let a = files("/bin")

# the result of `ls` is not used, meaning standard output
# is not captured and instead printed to the terminal
files("/bin")