overgrown/docs/dj.1

.\" Copyright (c) 2024 DTB <trinity@trinity.moe>
.\" Copyright (c) 2024 Emma Tebibyte <emma@tebibyte.media>
.\"
.\" This work is licensed under CC BY-SA 4.0. To see a copy of this license,
.\" visit <http://creativecommons.org/licenses/by-sa/4.0/>.
.\"
.TH DJ 1
.SH NAME
dj \(en disk jockey
.\"
.SH SYNOPSIS

dj
.RB ( -AdHnq )
.RB ( -a
.RB [ byte ])
.RB ( -c
.RB [ count ])

.RB ( -i
[\fBinput file\fP])
.RB ( -b
[\fBinput block size\fP])
.RB ( -s
[\fBinput offset\fP])

.RB ( -o
[\fBoutput file\fP])
.RB ( -B
[\fBoutput block size\fP])
.RB ( -S
[\fBoutput offset\fP])
.\"
.SH DESCRIPTION

Perform precise read and write operations on files. This utility is useful for
reading and writing binary data to and from disks, hence the name.

This manual page uses the terms \(lqskip\(rq and \(lqseek\(rq to refer to moving
to a specified byte by index in the input and output of the program
respectively. This language is inherited from the
.BR dd (1p)
utility and is used here to decrease ambiguity.

When seeking or skipping to a byte, writing or reading starts at the byte
immediately subsequent to the specified byte.
.\"
.SH OPTIONS

.IP \fB-i\fP
Takes a file path as an argument and opens it for use as an input.
.IP \fB-b\fP
Takes a numeric argument as the size in bytes of the input buffer, with the
default being 1024.
.IP \fB-s\fP
Takes a numeric argument as the number of bytes to skip into the input
before starting to read. If the standard input is used, bytes read to this point
are discarded.
.IP \fB-o\fP
Takes a file path as an argument and opens it for use as an output.
.IP \fB-B\fP
Does the same as
.B -b
but for the output buffer.
.IP \fB-S\fP
Seeks a number of bytes through the output before starting to write from
the input. If the output is a stream, null characters are printed.
.IP \fB-a\fP
Accepts a single literal byte with which input buffer is padded in the event
of an incomplete read from the input file.
.IP \fB-A\fP
Specifying this option pads the input buffer with null bytes in the event of an
incomplete read. Equivalent to specifying
.B -a
with a null byte instead of a character.
.IP \fB-c\fP
Specifies a number of reads to make. The default is zero, in which case the
input is read until a partial or empty read is made.
.IP \fB-d\fP
Prints invocation information before program execution as described in the
DIAGNOSTICS section below. Each invocation increments the debug level of the
program.
.IP \fB-H\fP
Prints diagnostics messages in a human-readable manner as described in the
DIAGNOSTICS section below.
.IP \fB-n\fP
Retries failed reads once more before exiting.
.IP \fB-q\fP
Suppresses error messages which print when a read or write is partial or
empty. Each invocation decrements the debug level of the program.
.\"
.SH STANDARD INPUT

The standard input shall be used as an input if no inputs are specified or if
one or more of the input files is \(lq-\(rq.
.\"
.SH DIAGNOSTICS

On a partial or empty read, a diagnostic message is printed (unless the
.B -q
option is specified) and the program exits (unless the
.B -n
option is specified).

By default, statistics are printed for input and output to the standard error in
the following format:

.RS
{records read} {ASCII unit separator} {partial records read}
{ASCII record separator} {records written} {ASCII unit separator}
{partial records written} {ASCII group separator} {bytes read}
{ASCII record separator} {bytes written} {ASCII file separator}
.RE

This format for diagnostic output is designed to be machine-parseable for
convenience. For a more human-readable format, the
.B -H
option may be specified. In this event, the following format is used instead:

.RS
{records read} '+' {partial records read} '>' {records written}
'+' {partial records written} ';' {bytes read} '>' {bytes written}
{ASCII line feed}
.RE

If the
.B -d
option is specified, debug output will be printed at the beginning of
execution. This debug information contains information regarding how the program
was invoked. The following example is the result of running the program with
.B -d
as the only argument:

.RS
argv0=dj
in=<stdin>      ibs=1024        skip=0  align=ff       count=0
out=<stdout>    obs=1024        seek=0  debug= 3       noerror=0
.RE

In non-recoverable errors that don\(cqt pertain to the read-write cycle, a
diagnostic message is printed and the program exits with the appropriate
.BR sysexits.h (3)
status.
.\"
.SH BUGS

If
.B -n
is specified along with the
.B -c
option and a count, actual byte output may be lower than expected (the product
of the count and the input block size). If the
.B -a
or
.B -A
options are used, this could make data written nonsensical.
.\"
.SH CAVEATS

Existing files are not truncated on ouput and are instead overwritten.

Many lowercase options have capitalized variants and vice-versa which can be
confusing. Capitalized options tend to affect output or are more intense
versions of lowercase options.
.\"
.SH RATIONALE

This program was based on the
.BR dd (1p)
utility as specified in POSIX. While character conversion may have been the
original intent of
.BR dd (1p),
it is irrelevant to its modern use. Because of this, this program eschews
character conversion and adds typical option formatting, allowing seeks to be
specified in bytes rather than in blocks, allowing arbitrary bytes as padding,
and printing in a format that\(cqs easy for machines to parse.
.\"
.SH COPYRIGHT

Copyright \(co 2023 DTB. License AGPLv3+: GNU AGPL version 3 or later
<https://gnu.org/licenses/agpl.html>.
.\"
.SH SEE ALSO
.BR dd (1p)