21. Subshells (Revision #2)

Running a shell script launches a new process, a subshell.

Definition: A subshell is a child process launched by a shell (or shell script).

A subshell is a separate instance of the command processor -- the shell that gives you the prompt at the console or in an xterm window. Just as your commands are interpreted at the command-line prompt, similarly does a script batch-process a list of commands. Each shell script running is, in effect, a subprocess (child process) of the parent shell.

A shell script can itself launch subprocesses. These subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously.

#!/bin/bash# subshell-test.sh

(
# Inside parentheses, and therefore a subshell . . .
while [ 1 ]   # Endless loop.
do
  echo "Subshell running . . ."
done
)

#  Script will run forever,
#+ or at least until terminated by a Ctl-C.

exit $?  # End of script (but will never get here).

Now, run the script:
sh subshell-test.sh

And, while the script is running, from a different xterm:
ps -ef | grep subshell-test.sh

UID       PID   PPID  C STIME TTY      TIME     CMD
500       2698  2502  0 14:26 pts/4    00:00:00 sh subshell-test.sh
500       2699  2698 21 14:26 pts/4    00:00:24 sh subshell-test.sh

          ^^^^

Analysis:
PID 2698, the script, launched PID 2699, the subshell.

Note: The "UID ..." line would be filtered out by the "grep" command,
but is shown here for illustrative purposes.

In general, an external command in a script forks off a subprocess, ¹ whereas a Bash builtin does not. For this reason, builtins execute more quickly and use fewer system resources than their external command equivalents.

Command List within Parentheses

( command1; command2; command3; ... )

A command list embedded between parentheses runs as a subshell.

Variables in a subshell are not visible outside the block of code in the subshell. They are not accessible to the parent process, to the shell that launched the subshell. These are, in effect, variables local to the child process.

Example 21-1. Variable scope in a subshell

#!/bin/bash
# subshell.sh

echo

echo "We are outside the subshell."
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
# Bash, version 3, adds the new         $BASH_SUBSHELL variable.
echo; echo

outer_variable=Outer
global_variable=
#  Define global variable for "storage" of
#+ value of subshell variable.

(
echo "We are inside the subshell."
echo "Subshell level INSIDE subshell = $BASH_SUBSHELL"
inner_variable=Inner

echo "From inside subshell, \"inner_variable\" = $inner_variable"
echo "From inside subshell, \"outer\" = $outer_variable"

global_variable="$inner_variable"   #  Will this allow "exporting"
                                   #+ a subshell variable?
)

echo; echo
echo "We are outside the subshell."
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
echo

if [ -z "$inner_variable" ]
then
  echo "inner_variable undefined in main body of shell"
else
  echo "inner_variable defined in main body of shell"
fi

echo "From main body of shell, \"inner_variable\" = $inner_variable"
#  $inner_variable will show as blank (uninitialized)
#+ because variables defined in a subshell are "local variables".
#  Is there a remedy for this?
echo "global_variable = "$global_variable""  # Why doesn't this work?

echo

# =======================================================================

# Additionally ...

echo "-----------------"; echo

var=41                                                 # Global variable.

( let "var+=1"; echo "\$var INSIDE subshell = $var" )  # 42

echo "\$var OUTSIDE subshell = $var"                   # 41
#  Variable operations inside a subshell, even to a GLOBAL variable
#+ do not affect the value of the variable outside the subshell!

exit 0

#  Question:
#  --------
#  Once having exited a subshell,
#+ is there any way to reenter that very same subshell
#+ to modify or access the subshell variables?

See also $BASHPID and Example 34-2.

Definition: The scope of a variable is the context in which it has meaning, in which it has a value that can be referenced. For example, the scope of a local variable lies only within the function, block of code, or subshell within which it is defined, while the scope of a global variable is the entire script in which it appears.

    While the $BASH_SUBSHELL internal variable indicates the nesting level of a subshell, the $SHLVL variable shows no change within a subshell.

echo " \$BASH_SUBSHELL outside subshell       = $BASH_SUBSHELL"           # 0
  ( echo " \$BASH_SUBSHELL inside subshell        = $BASH_SUBSHELL" )     # 1
  ( ( echo " \$BASH_SUBSHELL inside nested subshell = $BASH_SUBSHELL" ) ) # 2
# ^ ^                           *** nested ***                        ^ ^

echo

echo " \$SHLVL outside subshell = $SHLVL"       # 3
( echo " \$SHLVL inside subshell  = $SHLVL" )   # 3 (No change!)