一次又一次,我使用 eval
在 Stack Overflow 上看到 Bash 答案并且答案被抨击,双关语,因为使用了这种“邪恶”的结构。为什么是 eval
这么邪恶?
如 eval
不能安全使用,应该用什么代替?
最佳答案
这个问题远不止表面上的问题。我们将从显而易见的开始:eval
有可能执行“脏”数据。脏数据是任何没有被重写为安全使用情况-XYZ 的数据;在我们的例子中,它是任何没有被格式化以便可以安全评估的字符串。
乍一看,清理数据似乎很容易。假设我们抛出一个选项列表,bash 已经提供了一种很好的方法来清理单个元素,以及另一种将整个数组清理为单个字符串的方法:
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).
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.
eval
中来解决这个问题。 .我们所要做的就是单引号所有内容,保留双引号。一个异常(exception):我们必须在 eval
之前扩展重定向,因此必须保留在引号之外: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.
$1
也是安全的在 println
从不脏。sudo
相同的未加引号语法。所有的时间!为什么它在那里工作,而不是在这里?为什么我们必须用单引号引用所有内容? sudo
更现代一点:它知道将收到的每个参数用引号括起来,尽管这过于简单化了。 eval
简单地连接一切。eval
没有替代品。处理类似 sudo
的参数确实如此,如 eval
是一个内置的shell;这很重要,因为它在执行时会占用周围代码的环境和作用域,而不是像函数那样创建新的堆栈和作用域。eval
.这是一个方便的列表。 command
代表您通常会发送到 eval
的内容;随意替换。:
( command ) # Standard notation
$(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.
&3
(或任何高于 &2
)到您的目标: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 $var--only works for files!
exec 3<&0 4>&1 # Input and output!
func arg1 arg2 3>&2
&3
:command <&3 # Redirect stdin
command >&3 # Redirect stdout
command 2>&3 # Redirect stderr
command &>&3 # Redirect stdout and stderr
command 2>&1 >&3 # idem, but for older bash versions
command >&3 2>&1 # Redirect stdout to &3, and stderr to stdout: order matters
command <&3 >&4 # Input and output!
VAR='1 2 3'
REF=VAR
eval "echo \"\$$REF\""
echo "${!REF}"
eval
有点棘手。如果你想做一些更复杂的事情:# Add to scenario:
VAR_2='4 5 6'
# We could use:
local ref="${REF}_2"
echo "${!ref}"
# Versus the bash < 2 method, which might be simpler to those accustomed to eval:
eval "echo \"\$${REF}_2\""
eval
的有经验的编程人员来说似乎不是这样。 .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=( "${!VAR[@]}" ) # Get all of the keys in VAR
VAR=( ) # This will store our keys.
# Store a value with a simple key.
# You will need to declare it in a global scope to make it global prior to bash 4.
# In bash 4, use the -g option.
declare "VAR_$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"`"
# ...
https://stackoverflow.com/questions/17529220/