变量扩展

按优先级顺序：保持跟你所发现的一致；引用你的变量；推荐用 ${var} 而不是 $var ，详细解释如下。 这些仅仅是指南，因为作为强制规定似乎饱受争议。

以下按照优先顺序列出。

1. 与现存代码中你所发现的保持一致。

2. 引用变量参阅下面一节，引用。

3. 除非绝对必要或者为了避免深深的困惑，否则不要用大括号将单个字符的 shell 特殊变量或定位变量括起来。推荐将其他所有变量用大括号括起来。

# Section of recommended cases.

# Preferred style for 'special' variables:
echo "Positional: $1" "$5" "$3"
echo "Specials: !=$!, -=$-, _=$_. ?=$?, #=$# *=$* @=$@ \$=$$ ..."

# Braces necessary:
echo "many parameters: ${10}"

# Braces avoiding confusion:
# Output is "a0b0c0"
set -- a b c
echo "${1}0${2}0${3}0"

# Preferred style for other variables:
echo "PATH=${PATH}, PWD=${PWD}, mine=${some_var}"
while read f; do
  echo "file=${f}"
done < <(ls -l /tmp)

# Section of discouraged cases

# Unquoted vars, unbraced vars, brace-quoted single letter
# shell specials.
echo a=$avar "b=$bvar" "PID=${$}" "${1}"

# Confusing use: this is expanded as "${1}0${2}0${3}0",
# not "${10}${20}${30}
set -- a b c
echo "$10$20$30"