(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)
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