Course of Raku / Objects, I/O, and exceptions / Input and output / Running external programs 🆕

The environment

Every program is started with a set of environment variables — named values like HOME, PATH, or USER that the operating system provides. In Raku they are available in the dynamic hash %*ENV:

say %*ENV<HOME>; # the home directory, e.g. /home/anna

You read an environment variable by subscripting %*ENV with its name. Because it is an ordinary hash, you can also check whether a variable is set, or change it for programs you launch:

%*ENV<GREETING> = 'Hello';
say %*ENV<GREETING>; # Hello

Setting a key in %*ENV adds it to the environment that any program you then start with run or shell will inherit, which is how you pass configuration down to a child program:

%*ENV<GREETING> = 'Hello';

my $proc = run 'sh', '-c', 'echo $GREETING', :out;
say $proc.out.slurp(:close).chomp; # Hello

The variable is set in your program’s environment before the child is launched, so the shell that run starts already has GREETING in its own environment and can echo it back. Any variable you set this way reaches every program you start afterwards.

The * twigil tells you %*ENV is a dynamic variable, like $*OUT from the part on input and output. A few other starred variables describe the running program’s world — @*ARGS holds the command-line arguments, and $*PROGRAM-NAME is the script’s own name. Together they let a program understand the context it was launched in.

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