qpdldecode

qpdldecode [file...]

file...
    One or more input files to decode. If no files are specified, qpdldecode reads the quoted-printable encoded data from standard input.

The qpdldecode command is a utility designed to decode data that has been encoded using the quoted-printable transfer encoding. Quoted-printable is primarily used in the context of email and MIME (Multipurpose Internet Mail Extensions) to represent non-ASCII characters (bytes with values from 128 to 255) and certain ASCII characters (like '=', '?', '_') in a way that is safe for various mail transport agents and older systems that might not handle 8-bit data correctly.

It works by replacing such characters with an equals sign ('=') followed by a two-digit hexadecimal representation of the character's byte value. For example, the character 'é' (0xE9 in Latin-1) would be encoded as '=E9'. Newline characters are typically handled by explicit carriage return and line feed, or by 'soft' line breaks indicated by '=<' followed by a line ending.

qpdldecode reads the quoted-printable encoded data either from specified input files or, if no files are provided, from standard input. It then converts this encoded data back into its original binary or plain text form and writes the decoded output to standard output. This makes it a crucial tool for parsing and displaying email messages or other MIME parts that utilize quoted-printable encoding.

Character Set Interpretation: qpdldecode decodes byte sequences. The actual character interpretation (e.g., UTF-8, Latin-1) depends on the MIME `charset` parameter, which qpdldecode itself does not interpret. It simply outputs the raw decoded bytes.
Error Handling: Malformed quoted-printable sequences might lead to decoding errors or unexpected output, depending on the specific implementation.
Availability: qpdldecode is typically part of the mmencode or mpack package and may not be universally installed on all Linux distributions by default.

To decode a file named encoded.txt containing quoted-printable data:
qpdldecode encoded.txt > decoded.txt

To decode data piped from another command:
cat encoded.txt | qpdldecode > decoded.txt

Or directly using echo for a simple string:
echo 'Hello=2C world=21' | qpdldecode

The quoted-printable encoding, and consequently tools like qpdldecode, emerged as part of the MIME (Multipurpose Internet Mail Extensions) standard, primarily defined in RFCs like RFC 2045, RFC 2046, and RFC 2047, published in the mid-1990s. MIME was developed to allow email to contain rich content beyond plain ASCII text, including non-ASCII characters, binary data, and multimedia.

qpdldecode is typically bundled with mmencode (often part of the mpack suite of utilities), which provides a comprehensive set of tools for handling MIME encodings. Its development closely mirrors the adoption of MIME in email and web technologies, making it an essential, though often behind-the-scenes, component for parsing and rendering complex message formats.

mmencode(1), mpack(1), base64(1), uuencode(1), uudecode(1)