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.

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