The shell has an execution environment, which consists of the
following:
• open files inherited by the shell at invocation, as
modified by redirections supplied to the exec builtin
• the current working directory as set by cd, pushd, or
popd, or inherited by the shell at invocation
• the file creation mode mask as set by umask or inherited
from the shell's parent
• current traps set by trap
• shell parameters that are set by variable assignment or
with set or inherited from the shell's parent in the
environment
• shell functions defined during execution or inherited from
the shell's parent in the environment
• options enabled at invocation (either by default or with
command-line arguments) or by set
• options enabled by shopt
• shell aliases defined with alias
• various process IDs, including those of background jobs,
the value of $$, and the value of PPID
When a simple command other than a builtin or shell function is
to be executed, it is invoked in a separate execution environment
that consists of the following. Unless otherwise noted, the
values are inherited from the shell.
• the shell's open files, plus any modifications and
additions specified by redirections to the command
• the current working directory
• the file creation mode mask
• shell variables and functions marked for export, along
with variables exported for the command, passed in the
environment
• traps caught by the shell are reset to the values
inherited from the shell's parent, and traps ignored by
the shell are ignored
A command invoked in this separate environment cannot affect the
shell's execution environment.
Command substitution, commands grouped with parentheses, and
asynchronous commands are invoked in a subshell environment that
is a duplicate of the shell environment, except that traps caught
by the shell are reset to the values that the shell inherited
from its parent at invocation. Builtin commands that are invoked
as part of a pipeline are also executed in a subshell
environment. Changes made to the subshell environment cannot
affect the shell's execution environment.
Subshells spawned to execute command substitutions inherit the
value of the -e option from the parent shell. When not in posix
mode, bash clears the -e option in such subshells.
If a command is followed by a & and job control is not active,
the default standard input for the command is the empty file
/dev/null. Otherwise, the invoked command inherits the file
descriptors of the calling shell as modified by redirections.
