• input can be specified by a list of file names
• if no files are mentioned, the filter reads from standard input (which may have been connected to a file)
• the filename "-" corresponds to standard input

# read from the file data1
filter data1
filter < data1

# read from the files data1 data2 data3
filter data1 data2 data3

# read from data1, then stdin, then data2
filter data1 - data2

If filter doesn't cope with named sources, use cat at the start of the pipeline