Bash脚本编程之subshell

(command1;command2;command3;...)会启动子shell。子shell可以访问父shell的变量，对父shell变量的改动只在子shell中有效；子shell中定义的变量是局部变量，外部不能访问：

#!/bin/bash
# subshell.sh

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

outer_variable=Outer
global_variable=

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

inner_variable=Inner
global_variable="$inner_variable"

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

echo; echo
echo "We are outside the subshell."
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
echo "From main body of shell, \"inner_variable\" = $inner_variable"
#  $inner_variable will show as blank (uninitialized)
#+ because variables defined in a subshell are "local variables".
echo "global_variable = "$global_variable""

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

在子shell中对目录的改变不会影响父shell：

#!/bin/bash
# allprofs.sh: Print all user profiles.

FILE=.bashrc  #  File containing user profile.

for home in `awk -F: '{print $6}' /etc/passwd`
do
  [ -d "$home" ] || continue    # If no home directory, go to next.
  [ -r "$home" ] || continue    # If not readable, go to next.
  (cd $home; [ -e $FILE ] && cat $FILE)
done

#  When script terminates, there is no need to 'cd' back to original directory,
#+ because 'cd $home' takes place in a subshell.

exit 0

程序可以在不同的子shell中并行执行：

#!/bin/bash
# subshell.sh

# 在后台运行以确保并行执行
(ping -c 10 127.0.0.1 > /dev/null) &
(ping -c 20 127.0.0.1 > /dev/null) &

# 等同于：
# ping -c 10 127.0.0.1 > /dev/null &
# ping -c 20 127.0.0.1 > /dev/null &

# 通过ps可以发现两条子命令都是当前脚本启动的子shell，拥有不同的进程ID

# 直到子shell执行完成才执行后续命令
wait

echo "finished"

I/O重定向到子shell使用管道操作符，例如ls -al | (command)