請看http://www.tldp.org/LDP/abs/html/parameter-substitution.html舉例
${parameter-default}, ${parameter:-default}
如果未設置參數,請使用默認值。通話結束後,參數仍未設置。
這兩種形式幾乎相同。額外的:
僅在聲明參數時纔有所作爲,但爲空。
unset EGGS
echo 1 ${EGGS-spam} # 1 spam
echo 2 ${EGGS:-spam} # 2 spam
EGGS=
echo 3 ${EGGS-spam} # 3
echo 4 ${EGGS:-spam} # 4 spam
EGGS=cheese
echo 5 ${EGGS-spam} # 5 cheese
echo 6 ${EGGS:-spam} # 6 cheese
${parameter=default}, ${parameter:=default}
如果未設置參數,將參數值設置爲default。
兩種形式幾乎相同。的:使得僅在參數已被宣佈爲差爲空
# sets variable without needing to reassign
# colons suppress attempting to run the string
unset EGGS
: ${EGGS=spam}
echo 1 $EGGS # 1 spam
unset EGGS
: ${EGGS:=spam}
echo 2 $EGGS # 2 spam
EGGS=
: ${EGGS=spam}
echo 3 $EGGS # 3 (set, but blank -> leaves alone)
EGGS=
: ${EGGS:=spam}
echo 4 $EGGS # 4 spam
EGGS=cheese
: ${EGGS:=spam}
echo 5 $EGGS # 5 cheese
EGGS=cheese
: ${EGGS=spam}
echo 6 $EGGS # 6 cheese
${parameter+alt_value}, ${parameter:+alt_value}
如果參數設置,使用alt_value,否則使用空字符串。通話結束後,參數值不變。
兩種形式幾乎相同。 The:僅在聲明參數時纔有所作用,且爲空
unset EGGS
echo 1 ${EGGS+spam} # 1
echo 2 ${EGGS:+spam} # 2
EGGS=
echo 3 ${EGGS+spam} # 3 spam
echo 4 ${EGGS:+spam} # 4
EGGS=cheese
echo 5 ${EGGS+spam} # 5 spam
echo 6 ${EGGS:+spam} # 6 spam
引用文本時,包含指向源的鏈接很有幫助。 – 2015-06-11 15:19:20