Linux Bash变量无法保留换行符的问题排查与解决

我在Debian Stable Linux系统中工作，想要修改apt search zypper的输出格式，把每组非空的两行合并成一行。

原命令输出：

$ apt search zypper
Sorting... Done
Full Text Search... Done
libzypp-bin/stable 17.25.7-1 amd64
  openSUSE/SLES package management system library (library tools)

libzypp-common/stable,stable 17.25.7-1 all
  openSUSE/SLES package management system library (common files)

libzypp-config/stable,stable 17.25.7-1 all
  openSUSE/SLES package management system library (configuration)

libzypp-dev/stable 17.25.7-1 amd64
  openSUSE/SLES package management system library (development files)

libzypp-doc/stable,stable 17.25.7-1 all
  openSUSE/SLES package management system library (documentation)

libzypp1722/stable 17.25.7-1 amd64
  openSUSE/SLES package management system (library)

zypper/stable 1.14.42-1 amd64
  command line software manager using libzypp

zypper-common/stable,stable 1.14.42-1 all
  command line software manager using libzypp (common files)

zypper-doc/stable,stable 1.14.42-1 all
  command line software manager using libzypp (documentation)

我用了以下sed命令处理，直接运行时输出符合预期：

$ apt search zypper | sed ':a;N;s/\n\(\.\)/ \1/;ta'

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

Sorting... Full Text Search... libzypp-bin/stable 17.25.7-1 amd64   openSUSE/SLES package management system library (library tools)

libzypp-common/stable,stable 17.25.7-1 all   openSUSE/SLES package management system library (common files)

libzypp-config/stable,stable 17.25.7-1 all   openSUSE/SLES package management system library (configuration)

libzypp-dev/stable 17.25.7-1 amd64   openSUSE/SLES package management system library (development files)

libzypp-doc/stable,stable 17.25.7-1 all   openSUSE/SLES package management system library (documentation)

libzypp1722/stable 17.25.7-1 amd64   openSUSE/SLES package management system (library)

zypper/stable 1.14.42-1 amd64   command line software manager using libzypp

zypper-common/stable,stable 1.14.42-1 all   command line software manager using libzypp (common files)

zypper-doc/stable,stable 1.14.42-1 all   command line software manager using libzypp (documentation)

但把输出存入变量再用echo $var输出时，所有行被合并成一长行：

var=`apt search zypper | sed ':a;N;s/\n\(\.\)/ \1/;ta'`
echo $var 

输出结果：

Sorting... Full Text Search... libzypp-bin/stable 17.25.7-1 amd64 openSUSE/SLES package management system library (library tools) libzypp-common/stable,stable 17.25.7-1 all openSUSE/SLES package management system library (common files) libzypp-config/stable,stable 17.25.7-1 all openSUSE/SLES package management system library (configuration) libzypp-dev/stable 17.25.7-1 amd64 openSUSE/SLES package management system library (development files) libzypp-doc/stable,stable 17.25.7-1 all openSUSE/SLES package management system library (documentation) libzypp1722/stable 17.25.7-1 amd64 openSUSE/SLES package management system (library) zypper/stable 1.14.42-1 amd64 command line software manager using libzypp zypper-common/stable,stable 1.14.42-1 all command line software manager using libzypp (common files) zypper-doc/stable,stable 1.14.42-1 all command line software manager using libzypp (documentation)

需要保留换行符维持各行独立，请问问题出在哪？怎么解决？

问题原因 #

Bash中，当你执行echo $var时，未加引号的变量展开会触发分词操作：变量内的所有空白字符（包括换行、空格、制表符）都会被替换成单个空格，同时还会处理通配符匹配，这就是换行消失的原因。

解决办法 #

1. 用双引号包裹变量输出 #

给变量加上双引号，Bash会保留变量内的换行符和原始空格结构：

var=$(apt search zypper | sed ':a;N;s/\n\(\.\)/ \1/;ta')
echo "$var"

注：推荐用$(...)替代反引号`...`，前者可读性更强，支持嵌套使用。

2. 直接输出，跳过变量存储 #

如果不需要后续复用处理后的内容，直接执行管道命令即可，避免变量展开带来的问题：

apt search zypper | sed ':a;N;s/\n\(\.\)/ \1/;ta'

额外优化：改进sed命令 #

原sed命令会把Sorting... Done和Full Text Search... Done合并成一行，如果你想保留这两行的独立换行，可以调整sed逻辑，只合并以空格开头的包描述行：

apt search zypper | sed '/^[[:space:]]/{N;s/\n[[:space:]]+/ /;}'

这个命令会匹配以空白字符开头的行，将其与上一行合并，替换换行和开头的空格为单个空格，结果更贴合原始输出的结构。

内容的提问来源于stack exchange，提问作者rnso