Command line options and help


devio has only two command line options:

-h [subject]
Output help on subject subject.  With no argument outputs a short help message.  By default devio is built with full help, however this increases the size of the devio program considerable.  If devio has been compiled without help only the short help message will be available.
-v {commands}
Perform write verification.  Instead of writing devio checks the current device contents to ensure that they match what would have been written.  If a difference is detected devio exits immediately with a failure error code.


devio has extensive built in help, about one third of the executable size is help strings.  The help within the program should be up to date, this manual may not be.  Even though there is a lot of help text it is still fairly brief; use the built-in help to verify command syntax once you understand the commands.

To get help on a specific command use:

devio -h 'cc'

Where cc are the two characters of the command.   The letters of the command should be quoted (with single quotes) to avoid shell interpretation of the command.

devio also recognises a number of keywords which give overview information.  These keywords may be abbreviated where unambiguous.

devio -h commands
Lists all the commands and some basic help information.  Commands are grouped into classes followed by a keyword in square brackets; [keyword].  To get more help give keyword as an argument to -h.
devio -h additional
This explains the syntax of the 'commands' help.
devio -h conditional
Documentation of conditional execution, both commands and elements of expressions can be executed conditionally and within loops.
devio -h exit
How to cause an immediate exit from a devio script.
devio -h io
General help for the IO commands.
devio -h write
Information about commands which write bytes to the output.
devio -h messages
Information about commands which write to standard output.
devio -h assign
How to assign to variables within devio scripts.
devio -h string
Output the syntax of a string command argument.
devio -h number
Output the syntax of numbers (for commands which take numeric arguments) as well as general help for numeric expressions.
devio -h operator
Output help for the operators available for use in numeric expressions.

Exit codes

The devio program exits with a status (exit) code which indicates whether a failed write within devio has modified an output file.  This may be important for error recovery and to inform the user of a script how to recover from the problem.  Exit codes are as follows:

exit code 0
Success exit - the program script was parsed and executed without errors, any write operations have been synced to the underlying file system.
exit code 1
Failure exit - an error occured (and an error message will have been produced), however no write was made to any output device.
exit code 3
Failure exit, an error occured after a successful write to an output device.

If the exit code is non-zero no attempt has been made to flush output data - in particular even with exit code 3 no fdatasync(2) has been attempted.

Exit from devio can also be accomplished from the script using the !! and !? commands.  In this case all writes are completed and the fdatasync(2) call is executed.  It is recommended that error codes 1 and 3 are not used with these commands to avoid confusion.

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