# GRASS Help System

UNIX: dd, tar

GRASS operates under the UNIX operating system. Users therefore have access to the UNIX commands on their systems, as well as to GRASS commands. UNIX commands can be run at the system level (i.e., outside of GRASS) and can also be run while the user is running GRASS. This capability is extremely useful.

The UNIX commands "dd" and "tar" are useful for extracting data from other storage media (e.g., magnetic tapes). These commands can also be used to load GRASS (and other) data onto other storage media for export to other systems.

"dd" copies a user-named input file to a user-named output location. The user can copy files from (or to) tape by specifying a tape device as the input (or output) file name. The user can specify input and output block sizes, and perform file conversions between ASCII and EBDIC formats, fixed and variable length records, and upper and lower case, and perform other functions.

The user should use the UNIX "dd" command to extract elevation data from magnetic tape. This process is detailed in the paper "DTED and DEM Elevation Data Extraction" available from the GRASS Information Center at USACERL.

The UNIX "tar" command "archives and extracts multiple files onto a single tar-file archive, called a tarfile. A tarfile is usually a magnetic tape, but it can be any file." The user specifies the file or directory names that specify which files are to be archived or extracted.

If the user specifies a directory name, tar recursively extracts or archives all of the files and subdirectories of that directory. The user can specify a specific blocking factor to use when extracting or archiving data. "tar" will automatically determine the tape blocking factor when extracting data. "tar" can also follow symbolic links. One useful function performed by tar is to list the contents of a tarfile. (Reference: "UNIX Reference Manual", Section 1)

Users can read about the specific syntax of "dd" and "tar" in the "UNIX Reference Manual". On most systems, the manual is available on-line. Users can read about a specific UNIX command by typing "man command_name", where "command_name" is the command the user wishes to read about. For example, the user can read about the UNIX "dd" command by typing "man dd".

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Automatically created on: Thu Oct 14 02:12:26 2004