DIFF

Section: User Commands (1)
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NAME

diff - Public Domain diff (context diff) program

SYNOPSIS

diff [ -b -c -i -e ] file1 file2

DESCRIPTION

Diff compares two files, showing what must be changed to make them identical. Either file1 or file2 (but not both) may refer to directories. If that is the case, a file in the directory whose name is the same as the other file argument will be used. The standard input may be used for one of the files by replacing the argument by "-". Except for the standard input, both files must be on disk devices.

OPTIONS

-b: Remove trailing whitespace (blanks and tabs) and compress all other strings of whitespace to a single blank.
-c: Print some context -- matching lines before and after the non-match section. Mark non-matched sections with "|".
-i: Ignore lower/upper case distinctions.
-e: Output is in an "editor script" format which is compatible with the Unix 'ed' editor.

All information needed to compare the files is maintained in main memory. This means that very large files (or fairly large files with many differences) will cause the program to abort with an "out of space" message. Main memory requirements (in words) are approximately:

2 * (length of file1 + length of file2)

+ 3 * (number of changes)

(Where "length" is the number of lines of data in each file.)

The algorithm reads each file twice, once to build hash tables and once to check for fortuitous matches (two lines that are in fact different, but which have the same hash value). CPU time requirements include sorting the hash tables and randomly searching memory tables for equivalence classes. For example, on a time-shared VAX-11/780, comparing two 1000 line files required about 30 seconds (elapsed clock time) and about 10,000 bytes of working storage. About 90 per-cent of the time was taken up by file I/O.

DIAGNOSTICS

: Warning, bad option 'x'
The option is ignored.
: Usage ...
Two input files were not specified.
: Can't open input file "filename".
Can't continue.
: Out of space
The program ran out of memory while comparing the two files.
: Can't read line nnn at xxx in file[A/B]
This indicates an I/O error when seeking to the specific line. It should not happen.
: Spurious match, output is not optimal.
Two lines that were different yielded the same hash value. This is harmless except that the difference output is not the minimum set of differences between the two files. For example, instead of the output: lines 1 to 5 were changed to ...

the program will print lines 1 to 3 were changed to ...

lines 4 to 5 were changed to ...
: The program uses a CRC16 hash code.
The likelihood of this error is quite small.

AUTHOR

The diff algorithm was developed by J. W. Hunt and M. D. McIlroy, using a central algorithm defined by H. S. Stone. It was published in:

Hunt, J. W., and McIlroy, M. D.,
An Algorithm for Differential File Comparison,
Computing Science Technical Report #41,
Bell Laboratories, Murray Hill, NJ  07974

BUGS

On RSX and DECUS C on VMS systems, diff may fail if the both files are not "variable-length, implied carriage control" format. The scopy program can be used to convert files to this format if problems arise.

When compiled under VAX C, diff handles STREAM_LF files properly (in addition to the canonical variable-length implied carriage control files). Other variations should work, but have not been tested.

When compiled under VAX C, diff is quite slow for unknown reasons which ought to be investigated. On the other hand, it has access to effectively unlimited memory.

Output in a form suitable for ed - the -e option - seems rather pointless; the analogue on DEC systems is SLP (SUMSLP on VMS). It would be simple to provide SLP-compatible output. The question is, why bother - since the various DEC file comparison utilities already produce it.

This document was created by man2html, using the manual pages.
Time: 00:26:36 GMT, January 03, 2023