举例
- 变量可重入追加
#在CFLAGS变量后追加localCFLAGS变量值
CFLAGS="$CFLAGS $localCFLAGS" #若原来变量为空，则会带有前导空格，此外还有重复调用会多次冗余追加
CFLAGS="${CFLAGS+$CFLAGS }$localCFLAGS" #解决结果前导空格，但还是会冗余追加
CFLAGS="${oldCFLAGS=${CFLAGS+$CFLAGS }}$localCFLAGS" #解决前导空格和冗余追加，但产生新全局变量保存原值
#CFLAGS=originalCFLAGS
# localCFLAGS=/opt/xx/include && CFLAGS="$CFLAGS $localCFLAGS" && echo CFLAGS=\"$CFLAGS\"
## Simple, but if CFLAGS is null, then the outputted CFLAGS has a leading space.
#localCFLAGS=/opt/xx/include && CFLAGS="${CFLAGS+$CFLAGS }$localCFLAGS" && echo CFLAGS=\"$CFLAGS\"
## Strip the outputted CFLAGS a leading space if CFLAGS is null.
## But, if executed more than once in a same shell, CFLAGS will include multiple localCFLAGS-es
localCFLAGS=/opt/xx/include && CFLAGS="${oldCFLAGS=${CFLAGS+$CFLAGS }}$localCFLAGS" && echo CFLAGS=\"$CFLAGS\"
## Solve both the above problems to CFLAGS(a leading space if null and a repeatedly appended value if reentrant)
## But add a global variable to save old value in the shell.