how to find which process is using a file?
You can view the process using the file or filesystem using lsof & grep command.
lsof lists all files in use in the system.
if you want to find the application which is using the file sag.pdf use
$lsof |grep sag.pdf
and you will get the output as
ld-linux. 4220 nandam 30r REG 3,1 869300 136163 /home/nandam/tldp books/sag.pdf
you can also use lsof with grep to find the files used by a process
like
$lsof|grep bash