shell中赋值与替换

#Method 1: assignment

VAR="unassigned"

if [ -z "$VAR" ]

then

        echo "$VAR"

else 

        VAR="assigned"

        echo "$VAR"

fi

#Method 2: assignment

[ -z "$VAR" ]&&(VAR="unassigned";echo "$VAR")||(VAR="assigned";echo "$VAR")

#Method 3: assignment

[ "$VAR" ]||(VAR="assigned";echo "$VAR")

#Method 4: assignment

[ ! "$VAR" ]&&(VAR="assigned";echo "$VAR")

# replacement 1

# = 没有定义时，才替换赋值;定义了就显示原值

#username1=""

echo "${username1=$LOGNAME}"

echo "$username1"

# replacement 2

# :=定义为非空值时，才不替换赋值;否则替换

#username2="i"

echo "${username2:=$LOGNAME}"

echo "$username2"

# replacement 3

# :-定义为非空值时，才不替换不赋值;否则替换不赋值

#username3=""

echo "${username3:-$LOGNAME}"

echo "$username3"

# replacement 4

# -未定义时，才替换但不会赋值

#username4=""

echo "${username4-$LOGNAME}"

echo "$username4"

# replacement 5

# :? 非真正值，脚本自行退出

username5="tt"

echo "${username5:?$LOGNAME}"

# replacement 6

# ? 非定义变量，脚本自行退出

username6=""

echo "${username6?$LOGNAME}"

# replacement 7

# :+ 非真正值，不替换 

username7="tt"

echo "${username7:+$LOGNAME}"

echo "$username7"

# replacement 8

# + 定义就替换，但不赋值

username8=""

echo "${username8+$LOGNAME}"

echo $username8