programming and related languages. Occurrences of $1 are replaced by the first actual argument provided by the user when the shell script is run. $2 is replaced by the second argument, and so on up to $9.
You may have arrived at this entry by following a URL like "http://foldoc.org?$1", which is actually a template used to generate pointers to FOLDOC definitions by replacing "$1" with the term to be defined, e.g. in a wiki interwiki map.