Multiple unary operators in an if statement

if [ -f "file1" -a -f "file2" -a "file3" ]; then
   #some code
fi

If you use Bash's double-bracket, you can do this:

if [[ -f "$input_file" && -f "$output_file" && -f "$log_file" ]]

which I find cleaner to read than other options shown here (but that's subjective). However, it has other advantages.

And, as ghostdog74 shows, you should always quote variables containing filenames.

You can see the [ ... ] operator only as a shortcut for test .... The options are used th same way.

So in your case you could either write the ghostdog74 way or :

if [ -f $input_file ] && [ -f $output_file ] && [ -f $log_file ]
then
### Some Code here
fi

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