The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group


uudecode - decode a binary file


uudecode [-o outfile] [file]


The uudecode utility shall read a file, or standard input if no file is specified, that includes data created by the uuencode utility. The uudecode utility shall scan the input file, searching for data compatible with one of the formats specified in uuencode, and attempt to create or overwrite the file described by the data (or overridden by the -o option). The pathname shall be contained in the data or specified by the -o option. The file access permission bits and contents for the file to be produced shall be contained in that data. The mode bits of the created file (other than standard output) shall be set from the file access permission bits contained in the data; that is, other attributes of the mode, including the file mode creation mask (see umask), shall not affect the file being produced. If either of the op characters '+' and '-' (see chmod) are specified in symbolic mode, the initial mode on which those operations are based is unspecified.

If the pathname of the file resolves to an existing file and the user does not have write permission on that file, uudecode shall terminate with an error. If the pathname of the file resolves to an existing file and the user has write permission on that file, the existing file shall be overwritten and, if possible, the mode bits of the file (other than standard output) shall be set as described above; if the mode bits cannot be set, uudecode shall not treat this as an error.

If the input data was produced by uuencode on a system with a different number of bits per byte than on the target system, the results of uudecode are unspecified.


The uudecode utility shall conform to XBD Utility Syntax Guidelines.

The following option shall be supported by the implementation:

-o  outfile
A pathname of a file that shall be used instead of any pathname contained in the input data. Specifying an outfile option-argument of /dev/stdout shall indicate standard output.


The following operand shall be supported:

The pathname of a file containing the output of uuencode.


See the INPUT FILES section.


The input files shall be files containing the output of uuencode.


The following environment variables shall affect the execution of uudecode:

Provide a default value for the internationalization variables that are unset or null. (See XBD Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)
If set to a non-empty string value, override the values of all the other internationalization variables.
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments and input files).
Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
[XSI] [Option Start] Determine the location of message catalogs for the processing of LC_MESSAGES. [Option End]




If the file data header encoded by uuencode is - or /dev/stdout, or the -o /dev/stdout option overrides the file data, the standard output shall be in the same format as the file originally encoded by uuencode. Otherwise, the standard output shall not be used.


The standard error shall be used only for diagnostic messages.


The output file shall be in the same format as the file originally encoded by uuencode.




The following exit values shall be returned:

Successful completion.
An error occurred.



The following sections are informative.


The user who is invoking uudecode must have write permission on any file being created.

The output of uuencode is essentially an encoded bit stream that is not cognizant of byte boundaries. It is possible that a 9-bit byte target machine can process input from an 8-bit source, if it is aware of the requirement, but the reverse is unlikely to be satisfying. Of course, the only data that is meaningful for such a transfer between architectures is generally character data.




Input files are not necessarily text files, as stated by an early proposal. Although the uuencode output is a text file, that output could have been wrapped within another file or mail message that is not a text file.

The -o option is not historical practice, but was added at the request of WG15 so that the user could override the target pathname without having to edit the input data itself.

In early drafts, the [ -o outfile] option-argument allowed the use of - to mean standard output. The symbol - has only been used previously in POSIX.1-2008 as a standard input indicator. The standard developers did not wish to overload the meaning of - in this manner. The /dev/stdout concept exists on most modern systems. The /dev/stdout syntax does not refer to a new special file. It is just a magic cookie to specify standard output.




chmod, umask, uuencode

XBD Environment Variables, Utility Syntax Guidelines


First released in Issue 4.

Issue 6

This utility is marked as part of the User Portability Utilities option.

The -o outfile option is added, as specified in the IEEE P1003.2b draft standard.

The normative text is reworded to avoid use of the term "must" for application requirements.

IEEE Std 1003.1-2001/Cor 2-2004, item XCU/TC2/D6/35 is applied, clarifying in the DESCRIPTION that the initial mode used if either of the op characters is '+' or '-' is unspecified.

Issue 7

The uudecode utility is moved from the User Portability Utilities option to the Base. User Portability Utilities is now an option for interactive utilities.

SD5-XCU-ERN-97 is applied, updating the SYNOPSIS.

POSIX.1-2008, Technical Corrigendum 2, XCU/TC2-2008/0201 [635] is applied.

End of informative text.


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