fork, source 与 exec

* 所謂環境變量其實就是那些會傳給子行程的變量。
* 環境變量只能從父行程到子行程單向繼承。換句話說：在子行程中的環境如何變更，均不會影響父行程的環境。
* 正常來說，當我們執行一個 shell script 時，其實是先產生一個 sub-shell 的子行程，然後 sub-shell 再去產生命令行的子行程。
* 所謂 source 就是讓 script 在當前 shell 內執行、而不是產生一個 sub-shell 來執行。
* exec 也是讓 script 在同一個行程上執行，但是原有行程則被結束了。

# cat 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"

# cat 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"

# ./1.sh fork
PID for 1.sh before exec/source/fork:40125
1.sh: $A is B
using fork by default...
PID for 2.sh: 40126
2.sh get $A=B from 1.sh
2.sh: $A is C
PID for 1.sh after exec/source/fork:40125
1.sh: $A is B

# ./1.sh source
PID for 1.sh before exec/source/fork:40133
1.sh: $A is B
using source...
PID for 2.sh: 40133
2.sh get $A=B from 1.sh
2.sh: $A is C
PID for 1.sh after exec/source/fork:40133
1.sh: $A is C

# ./1.sh exec
PID for 1.sh before exec/source/fork:40137
1.sh: $A is B
using exec...
PID for 2.sh: 40137
2.sh get $A=B from 1.sh
2.sh: $A is C