Question

我想在创建新用户之前预测下一个UID。由于新的ID值将获得最大的ID值并加1，我想到了以下脚本：

biggestID=0
cat /etc/passwd | while read line
do
if test [$(echo $line | cut -d: -f3) > $biggestID]
then
biggestID=$(echo $line | cut -d: -f3)
fi
echo $biggestID
done
let biggestID=$biggestID+1
echo $biggestID

结果我得到1。这让我很困惑，我认为问题出在循环上，所以我在echo $biggestID下面添加了fi来检查它的值是否真正发生变化，结果表明循环没有问题我得到了多达1000的值。那么为什么biggestID的值在循环后返回0？

Answer 1

这是因为这一行：

cat /etc/passwd | while read line

在子shell中运行while循环，因此在子shell中设置biggestID，而不是在父shell中。

如果您将循环更改为以下内容，它将起作用：

while read line
...
done < /etc/passwd

这是因为while循环现在在与主脚本相同的shell中运行，而您只是将/etc/passwd的内容重定向到循环中。

Answer 2

您可以将程序更改为以下内容：

    module ActionController
        module ParamsWrapper

            Options.class_eval do
                def include
                    return super if @include_set

                    m = model
                    synchronize do
                        return super if @include_set
                        @include_set = true
                        unless super || exclude
                            if m.respond_to?(:attribute_names) && m.attribute_names.any?
                                self.include = m.attribute_names + nested_attributes_names_array_of(m)
                            end
                        end
                    end
                end

                private 
                    # added method. by default code was equivalent to this equaling to []
                    def nested_attributes_names_array_of model
                        model.nested_attributes_options.keys.map { |nested_attribute_name| 
                            nested_attribute_name.to_s + '_attributes' 
                        }
                    end
            end

        end
    end

newID=$(( $(cut -d: -f3 /etc/passwd | sort -n | tail -n 1 ) +1 )) echo $newID从passwd
cut -d: -f3 /etc/passwd| sort -n | tail -n 1代表命令的结果，这里是最大的id
$( ... )添加1并将结果存储在newID

Answer 3

使用awk，您可以在一个程序中完成所有计算：

awk -F: 'BEGIN {maxuid=0;} {if ($3 > maxuid) maxuid=$3;} END {print maxuid+1;}' /etc/passwd

如果您不想以awk开头，请对代码提出一些反馈。

biggestID=0
# Do not use cat .. but while .. do .. done < input (do not open subshell)
# Use read -r line (so line is taken literally)
cat /etc/passwd | while read line
do
   # Do not calculate the uid twice (in test and assignment) but store in var
   # uid=$(cut -d: -f3 <<< "${line}")
   # Use space after "[" and before "]"
   # test is not needed, if [ .. ] already implicit says so
   # (I only use test for onelines like "test -f file || errorfunction")
   if test [$(echo $line | cut -d: -f3) > $biggestID]
   then
      biggestID=$(echo $line | cut -d: -f3)
   fi
   # Next line only for debugging
   echo $biggestID
done
# You can use (( biggestID = biggestID + 1 ))
# or (when adding one)
# (( ++biggestID ))
let biggestID=$biggestID+1
# Use double quotes to get the contents literally, and curly brackets
# for a nice style (nothing strang will happen if you add _happy right after the var)
# echo "${biggestID}" 
echo $biggestID

如何从/ etc / passwd中提取最大的UID值？

3 个答案: