6 特性及错误

6.1 命令替换

使用 $(command) 而不是反引号。

嵌套的反引号要求用反斜杠转义内部的反引号。而 $(command) 形式嵌套时不需要改变，而且更易于阅读。

例如：

# This is preferred:
var="$(command "$(command1)")"

# This is not:
var="`command \`command1\``"

6.2 test，[和[[

推荐使用 [[ ... ]] ，而不是 [ , test , 和 /usr/bin/ [ 。

因为在 [[ 和 ]] 之间不会有路径名称扩展或单词分割发生，所以使用 [[ ... ]] 能够减少错误。而且 [[ ... ]] 允许正则表达式匹配，而 [ ... ] 不允许。

# This ensures the string on the left is made up of characters in the
# alnum character class followed by the string name.
# Note that the RHS should not be quoted here.
# For the gory details, see
# E14 at http://tiswww.case.edu/php/chet/bash/FAQ
if [[ "filename" =~ ^[[:alnum:]]+name ]]; then
  echo "Match"
fi

# This matches the exact pattern "f*" (Does not match in this case)
if [[ "filename" == "f*" ]]; then
  echo "Match"
fi

# This gives a "too many arguments" error as f* is expanded to the
# contents of the current directory
if [ "filename" == f* ]; then
  echo "Match"
fi

6.3 测试字符串

尽可能使用引用，而不是过滤字符串。

Bash足以在测试中处理空字符串。所以，请使用空（非空）字符串测试，而不是过滤字符，使得代码更易于阅读。

# Do this:
if [[ "${my_var}" = "some_string" ]]; then
  do_something
fi

# -z (string length is zero) and -n (string length is not zero) are
# preferred over testing for an empty string
if [[ -z "${my_var}" ]]; then
  do_something
fi

# This is OK (ensure quotes on the empty side), but not preferred:
if [[ "${my_var}" = "" ]]; then
  do_something
fi

# Not this:
if [[ "${my_var}X" = "some_stringX" ]]; then
  do_something
fi

为了避免对你测试的目的产生困惑，请明确使用-z或者-n

# Use this
if [[ -n "${my_var}" ]]; then
  do_something
fi

# Instead of this as errors can occur if ${my_var} expands to a test
# flag
if [[ "${my_var}" ]]; then
  do_something
fi

6.4 文件名的通配符扩展

当进行文件名的通配符扩展时，请使用明确的路径。

因为文件名可能以 - 开头，所以使用扩展通配符 ./* 比 * 来得安全得多。

# Here's the contents of the directory:
# -f  -r  somedir  somefile

# This deletes almost everything in the directory by force
psa@bilby$ rm -v *
removed directory: `somedir'
removed `somefile'

# As opposed to:
psa@bilby$ rm -v ./*
removed `./-f'
removed `./-r'
rm: cannot remove `./somedir': Is a directory
removed `./somefile'

6.5 Eval

应该避免使用eval。

当用于给变量赋值时，Eval解析输入，并且能够设置变量，但无法检查这些变量是什么。

# What does this set?
# Did it succeed? In part or whole?
eval $(set_my_variables)

# What happens if one of the returned values has a space in it?
variable="$(eval some_function)"

6.6 管道导向while循环

请使用过程替换或者for循环，而不是管道导向while循环。在while循环中被修改的变量是不能传递给父shell的，因为循环命令是在一个子shell中运行的。

管道导向while循环中的隐式子shell使得追踪bug变得很困难。

last_line='NULL'
your_command | while read line; do
  last_line="${line}"
done

# This will output 'NULL'
echo "${last_line}"

如果你确定输入中不包含空格或者特殊符号（通常意味着不是用户输入的），那么可以使用一个for循环。

total=0
# Only do this if there are no spaces in return values.
for value in $(command); do
  total+="${value}"
done

使用过程替换允许重定向输出，但是请将命令放入一个显式的子shell中，而不是bash为while循环创建的隐式子shell。

total=0
last_file=
while read count filename; do
  total+="${count}"
  last_file="${filename}"
done < <(your_command | uniq -c)

# This will output the second field of the last line of output from
# the command.
echo "Total = ${total}"
echo "Last one = ${last_file}"

当不需要传递复杂的结果给父shell时可以使用while循环。这通常需要一些更复杂的"解析"。请注意简单的例子使用如awk这类工具可能更容易完成。当你特别不希望改变父shell的范围变量时这可能也是有用的。

# Trivial implementation of awk expression:
#   awk '$3 == "nfs" { print $2 " maps to " $1 }' /proc/mounts
cat /proc/mounts | while read src dest type opts rest; do
  if [[ ${type} == "nfs" ]]; then
    echo "NFS ${dest} maps to ${src}"
  fi
done