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IO(1) UNIX User's Manual IO(1)

NAME
io - control and monitor input/output (io) flow intelli-
gently

SYNOPSIS
io [-mfa] [file ...]

OPTIONS
-m request monitoring of flow

-f force overwrites of files without user interaction
(useful in shell scripts)

-a appends text the named file rather than overwriting it
(only useful when output is being directed to a file)

DESCRIPTION
_▶08◀I_▶08◀o is a program that initiates, continues, and terminates
input and output. It makes intelligent decisions about what
it should do based on where it is called in a pipeline
(beginning, middle, or end), whether it is given file argu-
ments, and whether flow monitoring is requested.

_▶08◀I_▶08◀o can replace shell file redirection used in shells (with
<, > and >>), the simplest uses of _▶08◀c_▶08◀a_▶08◀t, the pipefitting pro-
gram _▶08◀t_▶08◀e_▶08◀e, all with the ability to monitor the progress of
the processes in a pipeline.

EXAMPLES
Some uses of _▶08◀i_▶08◀o are best shown by example:

The following form simply prints the named files to the
standard output:
io file1 file2 file3 ...

The following form redirects the output from the pipe to the
named file.
program1 | program2 | io file
If the output file from _▶08◀i_▶08◀o exists, _▶08◀i_▶08◀o makes a temporary file
before changing it to avoid the possibility of persons
over-writing their input files. Thus, it is possible to
sort a file onto itself with _▶08◀i_▶08◀o:
sort foo | io foo
is both legal and safe. _▶08◀I_▶08◀o will ask you if you want to
safely overwrite any files that already exist, for which you
should respond with a 'y' or a 'n'. To avoid having to
interact with _▶08◀i_▶08◀o, see the -f flag which forces the safe
overwrite.

The following form simply continues the flow from standard
input to standard output:
program1 | io | program2

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IO(1) UNIX User's Manual IO(1)

When _▶08◀i_▶08◀o is used in the middle of a pipeline, it automati-
cally monitors the flow of characters through it by printing
a character about every 1000 bytes. This character changes
from instance to instance of _▶08◀i_▶08◀o so that several flow meters
can supply discriminable information. The character selec-
tion is by process identification number, so that successive
programs would have different flow characters. For example,
the following shell command would set up three meters each
with different flow characters:
io foo | io | io | io -m bar

The automatic monitoring of flow is not used if _▶08◀i_▶08◀o is used
in the middle of a pipeline and is called with an argument
file name. For example:
program | io fubar | program
In this case, the input to _▶08◀i_▶08◀o is copied into the file fubar,
and another copy is continued along the pipeline. This is
similar to the standard UNIX _▶08◀t_▶08◀e_▶08◀e program. The monitoring
could be turned on if the -m flag were used:
program | io -m fubar | program

_▶08◀I_▶08◀o can replace the redirection symbols '<' (used to redirect
the standard input), and '>' (used to redirect the standard
output), used in the standard UNIX command shells, _▶08◀s_▶08◀h and
_▶08◀c_▶08◀s_▶08◀h.
io file | program
has the same effect as:
program < file
but is a bit slower. And,
program | io file
is the same as
program > file
except that it is safer. _▶08◀I_▶08◀o can be used to append text to a
file (emulating the shell >> redirection) by using the -a
flag:
program | io -a file
is the exact same as
program >> file

If _▶08◀i_▶08◀o is called with the -m option, it will monitor the flow
of bytes through it. For example, the following command
sets up a bunch of flow meters.
io -m file | io | io -m file
When the flow meter is at the beginning of a pipeline, its
meter prints the percent of its input so far read. Its
speed depends on how fast programs down the pipeline are
reading the output from whatever version of _▶08◀i_▶08◀o is used.
Note that the -m flag is not needed in the middle of the
pipeline as its only use in that context could be as a flow
meter.

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IO(1) UNIX User's Manual IO(1)

COMMENTS
The conceptual model of UNIX for piping programs implies
that we move data from some file, through several programs,
and into another file, which -- conceptually at least, may
be the same file as the data have come from. Thus, concep-
tually, the flow through a pipe should look like this:
file => program1 ==> program2 ==> ... ==> programN ==> file
Within the existing UNIX shells, this pipe must be specified
as
program1 < file | program2 | ... | programN > temp
mv temp file
This specification of the pipe presents a system image of
UNIX that conflicts with the underlying mental model of most
users. First, the specification of "file" and "program1"
are out of order; second, one must distinguish among "<" ">"
and "|". The program _▶08◀i_▶08◀o is designed to allow a format more
consistent with the underlying conceptualization. Along the
way, it eliminates the need for distinguishing between <, >,
and | and offers some new properties. Using _▶08◀i_▶08◀o, the example
pipe can be written
io file | program1 | program2 | ... | programN | io file

DIAGNOSTICS
The program has a lot of knowledge about how it is being
used. It knows whether it is reading from piped input or
from the terminal keyboard (tty). It also knows whether it
is writing to a pipe or to the tty. It knows to check argu-
ment files to see if ones to be read can be read, and if a
file to be written exists, and if so, to make a temporary
version of it before over-writing it. It also knows that
depending on the combination of its information, that some
combinations are not meaningful, and even dangerous. It
will complain about all of these, and in most cases, break
the pipeline it is in to the best of its ability.

SEE ALSO
sh(1), csh(1), cat(1), tee(1), more(1)

AUTHORS
Gary Perlman and Don Norman

BUGS
Hitting the BREAK or DEL keys while _▶08◀i_▶08◀o is overwriting a file
can have bad effects.

With several flow meters going at the same time, it is dif-
ficult to figure what is going on. _▶08◀I_▶08◀o is better suited to a

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IO(1) UNIX User's Manual IO(1)

system with better screen management.

KEYWORDS
input/output flow control/monitor

Printed 5/30/85 March 5, 1985 4

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