shell的${}使用

${param:-default}   If  param is null, then set it to the value of  default .

${#param} Gives the length of  param
${param%word} From the end, removes the smallest part of  param that
matches  word and returns the rest
${param%%word} From the end, removes the longest part of  param that
matches  word and returns the rest
${param#word} From the beginning, removes the smallest part of  param that
matches  word and returns the rest
${param##word} From the beginning, removes the longest part of  param that
matches  word and returns the rest

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unset foo

echo ${foo:-bar}

output:bar

-----------------------------------

foo=fud

echo ${foo:-bar}

output:fud

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foo=/usr/bin/X11/startx
echo ${foo#*/}

echo ${foo##*/}

output: usr/bin/X11/startx
            startx

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bar=/usr/local/etc/local/networks

echo ${bar%local*}

echo ${bar%%local*}

output: /usr/local/etc
            /usr