tr -cd [:digit:]tr -cd '[:digit:]'From the shell's point of view, unquoted [:digit:] is a
glob equivalent to [digit:] that matches any single
character filename from the group, such as d or
t, in the current directory.
If someone starts learning D and creates a directory named
d to hold the source code, the glob will be expanded and
the script will end up executing tr -cd d instead, which is
clearly unintended.
Quoting the argument prevents this, and will pass it correctly as the
literal string [:digit:] no matter which files exist in the
current directory.
None
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.