is a separate program and not something internal to bash
. The command line you were given uses an obsolete syntax, and specifies a sort beginning with field 3 and extending up to but not including field 25, where the fields are numbered from 0. The current manual page for sort
no longer describes this syntax, though the sort
command itself does still accept it. You can find a page that includes the obsolete syntax here
The modern equivalent would be
sort -k4,25 file1 >file2
The field numbers here start from 1, and the sort is inclusive
of the starting and ending field numbers.