fork ( /directory/script.sh)
fork是最普通的, 就是直接在腳本里面用/directory/script.sh來調用script.sh這個腳本.
運行的時候開一個sub-shell執行調用的腳本，sub-shell執行的時候, parent-shell還在。
sub-shell執行完畢後返回parent-shell. sub-shell從parent-shell繼承環境變量.可是sub-shell中的環境變量不會帶回parent-shell
exec (exec /directory/script.sh)
exec與fork不一樣，不須要新開一個sub-shell來執行被調用的腳本. 被調用的腳本與父腳本在同一個shell內執行。可是使用exec調用一個新腳本之後, 父腳本中exec行以後的內容就不會再執行了。這是exec和source的區別
source (source /directory/script.sh)
與fork的區別是不新開一個sub-shell來執行被調用的腳本，而是在同一個shell中執行. 因此被調用的腳本中聲明的變量和環境變量, 均可以在主腳本中獲得和使用.
能夠經過下面這兩個腳原本體會三種調用方式的不一樣:
1.sh
#!/bin/bash
A=B
echo "PID for 1.sh before exec/source/fork:$$"
export A
echo "1.sh: \$A is $A"
case $1 in
exec)
echo "using exec…"
exec ./2.sh ;;
source)
echo "using source…"
. ./2.sh ;;
*)
echo "using fork by default…"
./2.sh ;;
esac
echo "PID for 1.sh after exec/source/fork:$$"
echo "1.sh: \$A is $A"
2.sh
#!/bin/bash
echo "PID for 2.sh: $$"
echo "2.sh get \$A=$A from 1.sh"
A=C
export A
echo "2.sh: \$A is $A"
執行狀況：
$ ./1.sh
PID for 1.sh before exec/source/fork:5845364
1.sh: $A is B
using fork by default…
PID for 2.sh: 5242940
2.sh get $A=B from 1.sh
2.sh: $A is C
PID for 1.sh after exec/source/fork:5845364
1.sh: $A is B
$ ./1.sh exec
PID for 1.sh before exec/source/fork:5562668
1.sh: $A is B
using exec…
PID for 2.sh: 5562668
2.sh get $A=B from 1.sh
2.sh: $A is C
$ ./1.sh source
PID for 1.sh before exec/source/fork:5156894
1.sh: $A is B
using source…
PID for 2.sh: 5156894
2.sh get $A=B from 1.sh
2.sh: $A is C
PID for 1.sh after exec/source/fork:5156894
1.sh: $A is C
$