z/OS V1R9.0 UNIX System Services Command ... - Christian Grothoff

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shell environment starts as a duplicate, the value of the ENV environment variable
is not run. Changes made to a child shell environment do not affect the shell
environment.
Command substitutions (such as $command), commands within parentheses (such
as (command)), and commands to be run asynchronously (such as command&)—all
run in child shell environments. Each command in a pipeline (such as “command |
command”) runs in a child shell environment, unless the pipecurrent shell option is
in effect. If pipecurrent is set on (with set -o pipecurrent or set -P), then the last
command of the pipeline is executed in the current shell environment.
Shell commands also run in a separate environment that does not affect the shell
environment, except for certain built-in commands (for example, cd and umask)
that explicitly alter the shell environment. The environment of a shell command is
set up by the shell to include the following:
v Open files, subject to redirection.
v Working directory (see cd).
v File creation mask (see umask).
v Traps; traps caught by the shell are set to default values and traps ignored by
the shell are ignored by the command.
v Variables defined inside the shell and having the export attribute.
Built-in Commands
This topic lists the commands that are built into the shell. Such commands are built
into the shell to increase performance of shell scripts or to access the shell’s
internal data structures and variables. These internal commands are designed to
have semantics indistinguishable from external commands.
: cd exit let pwd test unset
. chmod export link r times wait
[ chown false ln read trap whence
[[ comm fc login readonly true writedown
alias command functions ls return type
autoload continue getopts mkdir rm typeset
basename cp history mv set ulimit
bg echo integer newgrp shift umask
break eval jobs print stop unalias
cat exec kill printf suspend unlink
POSIX.2 recognizes a subset of these commands as special built-ins. Syntax errors
in special built-in commands may cause a shell executing that command to
terminate, while syntax errors in regular built-in commands will not cause the shell
executing that command to terminate. If a special built-in command encountering a
syntax error does not terminate the shell, its exit value is nonzero.
Also, shell variable assignments included on shell command lines that invoke
special built-in commands remain in effect after the built-in command completes;
this is not the case with regular built-in commands or other utilities.
The special built-in commands are:
: continue exit readonly shift unset
. eval export return trap
break exec set typeset
568 z/OS V1R9.0 UNIX System Services Command Reference