Shell scripts '' () {} [] "" [[]] `` different uses

• [[]] In the action script in one extension to match a regular expression [[= ~]] For example:

  [root@localhost data]# touch f1.sh
  [root@localhost data]# file=f1.sh ;[[ $file =~ \.sh$ ]]
  [root@localhost data]# echo $?
  0
  [root@localhost data]# file=f1.shsh ;[[ $file =~ \.sh$ ]]
  [root@localhost data]# echo $?
  1

• [[]] Role in the script is to match the two wildcard Usage: [[==]] or [[=]!] For example:

  [root@localhost script]# file=f1.shsh
  [root@localhost script]# [[ "$file" == f ]]
  [root@localhost script]# echo $?
  0
  [root@localhost script]# [[ "$file" == fx ]]
  [root@localhost script]# echo $?
  1

• echo $ _ represents the last parameter of a command.
• Shell scripts () action to the inside of the sub-shell command execution inside, quit after finished

  [root@localhost script]# ( u=1;sleep 1000 )

See FIG Process

In contrast {;} is only performed in the current shell.