According to bash reference manual[1]: ($_, an underscore.) At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, expands to the last argument to the previous command, after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file.
Now consider following example (with bash-completion package installed): $ cd /tmp/ $ touch rpmall.txt rpmshort.txt $ mkdir testdir $ cp rpmall.txt rpmshort.txt $_ # Use tab completion to complete filenames cp: target '_filedir' is not a directory Last command fails because tab completing 'cp' command modifies value of '$_'. Shall value of '$_' be modified if a command gets executed in background ? [1] https://www.gnu.org/software/bash/manual/html_node/Special-Parameters.html#Special-Parameters -- -- Siteshwar Vashisht
