Is installation in . /bin but not /bin an expected behavior when use bash script?

[AuroraTea]

When use

curl --proto '=https' --tlsv1.2 -sSf https://raw.githubusercontent.com/nektos/act/master/install.sh | sudo bash

The act was installed in ./bin but not /bin

I see that the script does say to install on . /bin, but I don't understand why that's the case.

[jsoref]

The help says so:

act/install.sh

Line 12 in f75a2d8

-b sets bindir or installation directory, Defaults to ./bin

The code here does the work:

act/install.sh

Line 29 in f75a2d8

BINDIR=${BINDIR:-./bin}

You can see that GitHub's highlighter understands a bit of shell:

BINDIR=${BINDIR:-./bin} 

the ${var_name:-fallback} will use $var_name if available or fallback if not.

Such questions are probably best resolved by reading shell documentation, e.g. https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html