od (Unix)

od is a program for displaying ("dumping") data in various human-readable output formats. The name is an acronym for "octal dump" since it defaults to printing in the octal data format. It can also display output in a variety of other formats, including hexadecimal, decimal, and ASCII. It is useful for visualizing data that is not in a human-readable format, like the executable code of a program.

od is one of the earliest Unix programs, having appeared in version 1 AT&T Unix. It is also specified in the POSIX standards. The implementation for od used on Linux systems is usually provided by GNU Core Utilities.

Since it predates the Bourne shell, its existence causes an inconsistency in the do loop syntax. Other loops and logical blocks are opened by the name, and closed by the reversed name, e.g. if ... fi and case ... esac, but od's existence necessitates do ... done.

Example session

Normally a dump of an executable file is very long. The head program prints out the first few lines of the output. Here is an example of a dump of the "Hello world" program, piped through head.

 % od hello | head
 0000000 042577 043114 000401 000001 000000 000000 000000 000000
 0000020 000002 000003 000001 000000 101400 004004 000064 000000
 0000040 003610 000000 000000 000000 000064 000040 000006 000050
 0000060 000033 000030 000006 000000 000064 000000 100064 004004
 0000100 100064 004004 000300 000000 000300 000000 000005 000000
 0000120 000004 000000 000003 000000 000364 000000 100364 004004
 0000140 100364 004004 000023 000000 000023 000000 000004 000000
 0000160 000001 000000 000001 000000 000000 000000 100000 004004
 0000200 100000 004004 002121 000000 002121 000000 000005 000000
 0000220 010000 000000 000001 000000 002124 000000 112124 004004

Here is an example of od used to diagnose the output of echo where the user types Ctrl+V+Ctrl+I and Ctrl+V+Ctrl+C after writing "Hello" to literal insert a tab and ^C character:

 % echo "Hello    ^C" | od -cb
 0000000   H   e   l   l   o  \t 003  \n
         110 145 154 154 157 011 003 012
 0000010

See also

• Hex editor
• Hex dump

References

• od: dump files in various formats – Commands & Utilities Reference, The Single UNIX® Specification, Issue 7 from The Open Group
• od - GNU Core Utilities manpage