Null Utility

Documentation

Sorry for the limited documentation here. Basically null is a combination of /dev/null, tee, md5sum, with the addition of some other features.

Arguments

Here are more details on some of the less obvious flags.

• [-d size] or --dot-blocks show a dot each X bytes of input

  With this size, you can have null output a period ('.') to standard error for every X bytes read in. You can specify the size as 20k or 100m.

• [-f output-file] or --output-file output file(s) to write input

  You can write any input bytes into an output file by using this option. To handle multiple files, specify multiple -f options.

• [-F] or --flush-output flush output to files

  This will cause null to call fflush on each of the output streams after it writes to them.

• [-m] or --md5 run input bytes through md5

  This will display the md5 signature for the input data. If you are transporting data across a stream, it is useful to use the -r and -w options. See below.

• [-n] or --non-block don't block on input

  Set the input file-descriptor to be non-blocking. Not sure if this really accomplishes anything.

• [-p] or --pass-input pass input data to output

  This will write the input to the standard output.

• [-r] or --read-pagination read pagination data

  Null can add basic pagination information into the stream. Network transmissions often block the input and output to fixed sizes and add \0 characters at the end as padding. With pagination, these extraneous characters will be removed and so MD5 calculations (with -m) will be valid. This should be used to read the output of null with a -w flag specified.

• [-t size] or --throttle-size throttle output to X bytes / sec

  This will throttle the output of null to a specific size (10k or 1m) per second. This is useful if you don't want to overflow a network connection for instance.

• [-w] or --write-pagination write paginate data

  Like -r but this should be used to write output to a null with a -r flag specified.

Examples

To write output to multiple log files:

tar -cvzf - /usr | \
	null -f /backup/usr.tar.gz -f /backup2/usr.tar.gz

To transmit some files between two systems with pagination and while reporting the md5 signature of the data. On the remote host:

nc -l -p 5000 | null -r -p -m | tar -xf -

On the local host: tar -cf - . | null -w -p -m | nc remote-hostname 5000

To see a dot ('.') for every megabyte of byte we are getting out of our backup program.

tar -cf - . | null -d 1m -p -f /dev/nrst0

To limit the transfer of a hierarchy to a remote system to 100kB/sec and print a dot for every 10kB.

tar -cf - . | null -d 10k -t 100k -p | nc remote-hostname 5000

Repository

The newest versions of the library are available via: https://github.com/j256/null

Enjoy. Gray Watson