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Unix-to-Unix copy (UUCP) is the name of the uucp command, which copies files between UNIX systems, and of a wider family of programs that let UNIX computers communicate with one another. The term covers both meanings, and most confusion comes from mixing them up. In the Debian implementation, a copy is not always carried out at once: the request is queued and the transfer happens when the remote system is next called.
Two meanings of the name
The phrase is used in two related senses, and reference manuals distinguish between them.
The uucp command
In the narrow sense, UUCP is a command. The Debian trixie manual page is titled “uucp – Unix to Unix copy” and describes copying files between systems. Its source and destination operands can each be local or remote, so a single command can move a file from your machine to another one, or fetch a file from a remote system to a local directory (Debian Manpages, uucp(1)).
The broader UUCP suite
In the wider sense, UUCP is a collection of programs. Oracle’s Solaris system administration guide explains that “UUCP” refers to the complete range of files and utilities, not only the uucp command. Those utilities include file transfer tools and facilities for remote login and remote command execution (Oracle, Overview of UUCP, System Administration Guide Volume 3). The exact set of components differs between implementations, so a list taken from one vendor should not be assumed to apply to another.
How a UUCP path names a remote system
The distinctive syntax is the remote pathname. A file on another system is written as system!path, where system is a system name the local machine knows and path is the location on that machine. The exclamation mark separates the two parts.
The POSIX specification also allows a sequence of system names to describe a route, so a file can be sent to a second system through a first one. The notation is a chain of names separated by exclamation marks, ending with the path. A form such as hosta!hostb!/path/to/file expresses that route in illustrative terms; whether a given system will forward through an intermediate host depends on how its UUCP configuration is set up (POSIX Programmer’s Manual, uucp(1p)).
Why a copy can wait
The most useful behavioral fact for a reader is that submitting a UUCP copy does not necessarily move the file at that moment. The Debian manual states:
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“The copy does not take place immediately, but is queued up for the uucico (8) daemon; the daemon is started immediately unless the -r or –nouucico switch is given.”
The same manual explains that the files are copied the next time the remote system is called. In practice this means a successful uucp command has only created a job. The transfer completes when the scheduling daemon connects to the remote host.
This is a description of the Debian implementation. Other UUCP implementations may queue, schedule, or dispatch work differently, so the behavior should be checked in the manual for the system you are running.
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- Default Debian behavior: the
uucicodaemon starts immediately after the job is queued. - With
-ror--nouucico: the job is queued but the daemon is not started, so the transfer waits for a later call to the remote system.
What POSIX standardizes and what it leaves open
POSIX standardizes the command-line interface, not the network mechanics. The POSIX Programmer’s Manual calls the utility “system-to-system copy,” allows file operands on local or remote systems, and documents the system-name!pathname form. It also makes two qualifications that matter in practice:
- The utility cannot guarantee that all character encodings will be transferred correctly in every circumstance.
- The protocol used for file transfer is not defined by POSIX. The manual states: “The protocol for file transfer is unspecified by POSIX.1‐2008.”
So a UUCP transfer between two systems depends on their implementations agreeing on how the bytes move. POSIX only defines what the user types and what the command is expected to accept.
Historical context
UUCP was built for connections that were slow and often intermittent, which explains the queued design. DEC OSF/1 documentation describes UUCP as a group of programs supporting communication between UNIX systems, and records historical uses including electronic mail, network news, and distribution of public-domain software over low-speed links (DEC OSF/1 3.0, uucp_intro(7)). That account explains why the system mattered, but it does not show how widely UUCP is used today.
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The command also has a long documentary history. The 1983 AT&T Bell Laboratories UNIX Programmer’s Manual, Seventh Edition includes a UUCP(1C) entry (UNIX Programmer’s Manual, Seventh Edition, 1983), which places the tool in the early Unix toolchain.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Terms that are easy to confuse
| Term | What it refers to | Where it is documented |
|---|---|---|
| uucp | The command that copies files between systems | Debian uucp(1); POSIX uucp(1p) |
| UUCP suite | The wider set of UNIX-to-UNIX communication programs | Oracle System Administration Guide; DEC OSF/1 uucp_intro(7) |
| uucico | The daemon that carries out queued transfers in the Debian implementation | Debian uucp(1) |
| system!path | Notation for a file on a named remote system, optionally with a route | Debian uucp(1); POSIX uucp(1p) |
Limits of the definition
- The sources establish what the command and notation mean, not how many systems use UUCP now. No reliable current usage figure is available, so none is given here.
- The origin date of the system is not established by the sources reviewed for this article. The 1983 Seventh Edition manual is cited only as documentation of the command at that time.
- Queueing behavior, option names, and daemon names come from the Debian implementation. Confirm them with the manual on your own platform before relying on them.
For a working definition, treat UUCP as the copy command and its supporting suite, using system!path to name remote files and expecting that a transfer may be queued rather than immediate.
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