如果eval不能安全使用,我应该使用什么?
消毒数据看起来容易一目了然。假设我们抛出一个选项列表,bash已经提供了一个很好的方法来清理单个元素,另一种方法是将整个数组作为单个字符串进行清理:
- function println
- {
- # Send each element as a separate argument,starting with the second element.
- # Arguments to printf:
- # 1 -> "$1\n"
- # 2 -> "$2"
- # 3 -> "$3"
- # 4 -> "$4"
- # etc.
- printf "$1\n" "${@:2}"
- }
- function error
- {
- # Send the first element as one argument,and the rest of the elements as a combined argument.
- # Arguments to println:
- # 1 -> '\e[31mError (%d): %s\e[m'
- # 2 -> "$1"
- # 3 -> "${*:2}"
- println '\e[31mError (%d): %s\e[m' "$1" "${*:2}"
- exit "$1"
- }
- # This...
- error 1234 Something went wrong.
- # And this...
- error 1234 'Something went wrong.'
- # Result in the same output (as long as $IFS has not been modified).
现在我们想要添加一个选项来将输出作为参数重定向到println。当然,我们可以在每次调用时重定向println的输出,但是为了举例,我们不会这样做。我们需要使用eval,因为变量不能用于重定向输出。
- function println
- {
- eval printf "$2\n" "${@:3}" $1
- }
- function error
- {
- println '>&2' '\e[31mError (%d): %s\e[m' "$1" "${*:2}"
- exit $1
- }
- error 1234 Something went wrong.
看起来不错,对吧?问题是,eval解析两次命令行(在任何shell)。在解析的第一遍,去除一层引用。删除引号后,将执行一些变量内容。
我们可以通过让变量扩展发生在eval中来解决这个问题。我们要做的是单引号的一切,留下双引号,它们是。一个例外:我们必须在eval之前扩展重定向,所以必须保持在引号之外:
- function println
- {
- eval 'printf "$2\n" "${@:3}"' $1
- }
- function error
- {
- println '&2' '\e[31mError (%d): %s\e[m' "$1" "${*:2}"
- exit $1
- }
- error 1234 Something went wrong.
这应该工作。它也是安全的,只要println中的$ 1不会脏。
现在持续只是一个时刻:我使用相同的无引号的语法,我们最初使用sudo的时间!为什么它在那里工作,而不是在这里?为什么我们必须单引号? sudo有点更现代:它知道在引号中包含它接收的每个参数,虽然这是一个过度简化。 eval简单地连接一切。
不幸的是,没有用于处理像sudo这样的参数的eval的替换,因为eval是一个shell内置的;这是重要的,因为它在执行时需要周围代码的环境和范围,而不是像函数那样创建一个新的栈和范围。
eval替代
具体的用例通常有可行的替代eval。这里有一个方便的列表。命令表示通常发送给eval的内容;在任何你喜欢的替代。
无操作
在bash中的无操作中的简单冒号:
:
创建子shell
- ( command ) # Standard notation
执行命令的输出
不要依赖外部命令。你应该总是控制返回值。把这些放在自己的线上:
- $(command) # Preferred
- `command` # Old: should be avoided,and often considered deprecated
- # Nesting:
- $(command1 "$(command2)")
- `command "\`command\`"` # Careful: \ only escapes $ and \ with old style,and
- # special case \` results in nesting.
基于变量的重定向
- exec 3<&0 # Redirect from stdin
- exec 3>&1 # Redirect to stdout
- exec 3>&2 # Redirect to stderr
- exec 3> /dev/null # Don't save output anywhere
- exec 3> file.txt # Redirect to file
- exec 3> "$var" # Redirect to file stored in $var--only works for files!
- exec 3<&0 4>&1 # Input and output!
- func arg1 arg2 3>&2
- command <&3 # Redirect stdin
- command >&3 # Redirect stdout
- command 2>&3 # Redirect stderr
- command &>&3 # Redirect stdout and stderr
- command 2>&1 >&3 # idem,but for older bash versions
- command >&3 2>&1 # Redirect stdout to &3,and stderr to stdout: order matters
- command <&3 >&4 # Input and output!
可变间接
情况:
- VAR='1 2 3'
- REF=VAR
坏:
- eval "echo \"\$$REF\""
为什么?如果REF包含双引号,这将打破并打开代码来利用。它可以清除REF,但它是浪费时间,当你有这样:
- echo "${!REF}"
没错,bash的版本2内置了变量间接。如果你想做更复杂的事情,它会比eval更棘手:
- # Add to scenario:
- VAR_2='4 5 6'
- # We could use:
- local ref="${REF}_2"
- echo "${!ref}"
- # Or:
- ref="${REF}_2" echo "${!ref}"
- # Versus the bash < 2 method,which might be simpler to those accustomed to eval:
- eval "echo \"\$${REF}_2\""
无论如何,新的方法更直观,虽然它可能不像经验的编程谁是用于eval。
关联数组
关联数组本质上在bash 4中实现。一个警告:它们必须使用declare创建。
- declare -A VAR # Local
- declare -gA VAR # Global
- # Use spaces between parentheses and contents; I've heard reports of subtle bugs
- # on some versions when they are omitted having to do with spaces in keys.
- declare -A VAR=( ['']='a' [0]='1' ['duck']='quack' )
- VAR+=( ['alpha']='beta' [2]=3 ) # Combine arrays
- VAR['cow']='moo' # Set a single element
- unset VAR['cow'] # Unset a single element
- unset VAR # Unset an entire array
- unset VAR[@] # Unset an entire array
- unset VAR[*] # Unset each element with a key corresponding to a file in the
- # current directory; if * doesn't expand,unset the entire array
- local KEYS=( "${!VAR[@]}" ) # Get all of the keys in VAR
在旧版本的bash中,您可以使用变量间接:
- VAR=( ) # This will store our keys.
- # Store a value with a simple key.
- # You will need to declare it in a global scope to make it global prior to bash 4.
- # In bash 4,use the -g option.
- declare "VAR_$key"="$value"
- VAR+="$key"
- # Or,if your version is lacking +=
- VAR=( "$VAR[@]" "$key" )
- # Recover a simple value.
- local var_key="VAR_$key" # The name of the variable that holds the value
- local var_value="${!var_key}" # The actual value--requires bash 2
- # For < bash 2,eval is required for this method. Safe as long as $key is not dirty.
- local var_value="`eval echo -n \"\$$var_value\""
- # If you don't need to enumerate the indices quickly,and you're on bash 2+,this
- # can be cut down to one line per operation:
- declare "VAR_$key"="$value" # Store
- echo "`var_key="VAR_$key" echo -n "${!var_key}"`" # Retrieve
- # If you're using more complex values,you'll need to hash your keys:
- function mkkey
- {
- local key="`mkpasswd -5R0 "$1" 00000000`"
- echo -n "${key##*$}"
- }
- local var_key="VAR_`mkkey "$key"`"
- # ...