为何Bash中sed会移除字符串首尾空格？如何避免？

问题场景 #

有一个批量替换文件名字符的Bash脚本，运行时发现sed命令会意外移除文件名的首尾空格，但在终端单独执行sed命令时却不会出现这个问题。以下是最小可复现示例（MRE）：

脚本代码 #

#!/usr/bin/env bash

declare -A char_map

char_map["1"]="-"
char_map["2"]="+"

filelist=$(echo -e "./mydir/ number1number2withspaceoutside \n./mydir/number1 number2 with space inside")

while IFS= read current_file
do
    [[ -z "$current_file" ]] && continue

    path=$(dirname "$current_file")
    filename=$(basename "$current_file")
    fixed_filename="$filename"

    for bad in "${!char_map[@]}"
    do
        echo "before sed: '$fixed_filename'"
        fixed_filename=$(echo $fixed_filename | sed s/$bad/${char_map[$bad]}/g)
        echo "after sed: '$fixed_filename'"
        echo "--------------------"
    done

    if [ "$fixed_filename" == "$filename" ]
    then
        echo "nothing bad found: $current_file"
        continue
    fi
    
    echo "moving '$current_file' to '$path/$fixed_filename'"
    echo "================================================================================================"

done < <(echo "$filelist")

脚本输出 #

before sed: ' number1number2withspaceoutside '
after sed: 'number1number+withspaceoutside'
--------------------
before sed: 'number1number+withspaceoutside'
after sed: 'number-number+withspaceoutside'
--------------------
moving './mydir/ number1number2withspaceoutside ' to './mydir/number-number+withspaceoutside'
================================================================================================
before sed: 'number1 number2 with space inside'
after sed: 'number1 number+ with space inside'
--------------------
before sed: 'number1 number+ with space inside'
after sed: 'number- number+ with space inside'
--------------------
moving './mydir/number1 number2 with space inside' to './mydir/number- number+ with space inside'
================================================================================================

终端单独测试 #

执行命令：

echo " bla " | sed s/l/-/g

输出：

' b-a '

原因分析 #

问题根本不在sed，而是Bash的单词拆分（Word Splitting）机制：
脚本中执行echo $fixed_filename时，变量fixed_filename没有被双引号包裹。Bash在处理未加引号的变量展开时，会把所有连续的空白字符（空格、换行、制表符等）当作单词分隔符，同时自动忽略字符串首尾的空白。这就导致传给sed的内容已经丢失了首尾空格，sed处理后的结果自然也没有首尾空格。

而终端单独测试时，echo " bla "给字符串加了双引号，Bash不会进行单词拆分，首尾空格被完整保留，所以sed处理后也能保留首尾空格。

解决方法 #

只需要给echo命令中的变量加上双引号，同时为了避免sed替换表达式中的特殊字符问题，最好也给sed的命令加上双引号：

修改脚本中的sed执行行：

fixed_filename=$(echo "$fixed_filename" | sed "s/$bad/${char_map[$bad]}/g")

修改后的脚本输出示例 #

第一个文件的处理会变成：

before sed: ' number1number2withspaceoutside '
after sed: ' number1number+withspaceoutside '
--------------------
before sed: ' number1number+withspaceoutside '
after sed: ' number-number+withspaceoutside '
--------------------
moving './mydir/ number1number2withspaceoutside ' to './mydir/ number-number+withspaceoutside '

首尾空格被完整保留，符合预期。

内容的提问来源于stack exchange，提问作者Ovski