V10/man/man5/environ.5

.TH ENVIRON 5
.CT 2 proc_man
.SH NAME
environ \- user environment
.SH SYNOPSIS
.B extern char **environ;
.SH DESCRIPTION
An array of strings called the `environment' is
made available by
.IR exec (2)
when a process begins.
By convention these strings have either the form
.IB name = value ,
defining a variable, or
.IB name (){ value } ,
defining a function; see
.IR sh (1).
The following variables are used by various commands:
.TF TERMCAP
.TP
.B PATH
The sequence of directory prefixes that
.I sh,
.IR time (1),
.IR nice (1),
etc.,
apply in searching for a file known by an incomplete path name.
The prefixes are separated by 
.LR : .
.IR Login (8)
sets
.LR PATH=:/bin:/usr/bin .
.PD
.TP
.B HOME
A user's login directory, set by
.IR login (8)
from the password file
.IR passwd (5).
.TP
.B TERM
The kind of terminal for which output is to be prepared.
This information is used by commands, such as
.I nroff
or
.IR plot (1),
which may exploit special terminal capabilities.
Some customary values of
.B TERM
are
.BR 2621 
(HP),
.BR 4014 
(Tektronix),
.BR 5620 
(Teletype), and
.BR 630 
(Teletype).
See
.FR /etc/termcap ,
described in
.IR termcap (5),
for a longer list.
.TP
.B SHELL
The name of the login shell.
.PP
The environment may be queried by
.IR getenv (3)
or by the
.L set
or
.L whatis
commands of
.IR sh (1).
Names may be placed in the environment by the
.L export
command and by 
.IB name = value
arguments of
.IR sh (1).
Names may also be placed in the environment at the point of an
.IR exec (2).
It is unwise to conflict with
certain
.IR sh (1)
variables that are frequently exported by
.F .profile
files:
.BR MAIL ,
.BR PS1 ,
.BR PS2 ,
.BR IFS .
.SH SEE ALSO
.IR sh (1),
.IR printenv (1),
.IR exec (2),
.IR getenv (3),
.IR term (6)
.SH BUGS
Function definitions in the environment break some old programs,
including old shells.