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.shecho “We are outside the subshell.“echo “Subshell level OUTSIDE subshell = $BASH_SUBSHELL"echo; echoouter_variable=Outerglobal_variable=(echo “We are inside the subshell.“echo “Subshell level INSIDE subshell = $BASH_SUBSHELL"inner_variable=Innerglobal_variable="$inner_variable"echo “From inside subshell, "inner_variable" = $inner_variable"echo “From inside subshell, "outer" = $outer_variable”)echo; echoecho “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 “—————–”; echovar=41 # Global variable.( let “var+=1”; echo “$var INSIDE subshell = $var” ) # 42echo “$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 $6}' /etc/passwddo [ -d “$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waitecho “finished"I/O重定向到子shell使用管道操作符，例如ls -al | (command)