Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
exit 0
bash$ echo $WHATEVER
bash$
Sure enough, back at the command prompt, $WHATEVER remains unset.
Setting and manipulating variables in a subshell, then attempting to use those same variables outside the scope
of the subshell will result an unpleasant surprise.
Example 32−1. Subshell Pitfalls
#!/bin/bash
# Pitfalls of variables in a subshell.
outer_variable=outer
echo
echo "outer_variable = $outer_variable"
echo
(
# Begin subshell
echo "outer_variable inside subshell = $outer_variable"
inner_variable=inner # Set
echo "inner_variable inside subshell = $inner_variable"
outer_variable=inner # Will value change globally?
echo "outer_variable inside subshell = $outer_variable"
# End subshell
)
echo
echo "inner_variable outside subshell = $inner_variable" # Unset.
echo "outer_variable outside subshell = $outer_variable" # Unchanged.
echo
exit 0
Piping echooutput to a read may produce unexpected results. In this scenario, the read acts as if it were
running in a subshell. Instead, use the set command (as in Example 11−14).
Example 32−2. Piping the output of echo to a read
#!/bin/bash
# badread.sh:
# Attempting to use 'echo and 'read'
#+ to assign variables non−interactively.
a=aaa
b=bbb
c=ccc
Chapter 32. Gotchas 330