Sed

Sed stands for Stream Editor.

Basic sed syntax:

sed [options] {sed-commands} {input-file}

Sed reads one line at a time from the {input-file} and executes the {sed-commands} on that particular line

The input 并非必须是文件

echo "Some string" | sed '' 当然也是支持的.

Option: -n

通常sed是按照行来处理文本的, 然后打印处理后的结果. 然而并不是符合匹配的行被打印, 所有的行都会被打印. 例如sed 's/t/T/'会输出所有的行, 并且替换其中某些行的t.

这样的结果不是我们想见的(大多数情况下), 所以, 我们可以添加-n option 来 禁止自动打印所有内容, 例如sed -n 's/t/T/' 不会输出任何结果.

如果想要将匹配的结果单独打印, 则sed为我们提供了p命令. 例如, sed -n 's/t/T/p' 只会打印替换后的行.

Option: -i

As we know, sed doesn’t modify the input files by default. Sed writes the output to standard output. When you want to store that in a file, you redirect it to a file (or use the w command.

如果要修改到源文件, 我们以前只能这样做:

sed 's/John/Johnny/' employee.txt > new-employee.txt
mv new-employee.txt employee.txt

Don’t do sed 's/John/Johnny/' employee.txt > employee.txt

Because whell shell see > employee.txt in the command line, it opens the file employee.txt for writing, wiping off all its previous contents.

但现在, 有了-i, 我们可以选择直接修改input file.

sed -i 's/John/Johnny/' employee.txt

‼️ 直接修改源文件是很危险的, 你也可以使用-ibak来做备份.

sed -ibak 's/John/Johnny/' employee.txt

Option: -z

原本sed是按行处理的, 也就是\n结尾. 使用-z 选项后, 转换为以\0 即0x00来结尾. 借用-z 一次跨行对整个文本进行处理.

Commands

g

p

w

i (ignore case)

Command: s (substitute)

The most powerful command in the stream editor is substitute.

sed '[address-range|pattern-range] s/old/new/[substitute-flags]' inputfile

• 匹配范围字段是可选的. 未指定情况下默认是all lines. 指定范围Example: sed '/Sales/s/Manager/Director/' employee.txt 仅替换包含Sales字段的行.

Sed 替换分隔符

When there is a slash/ in the original-string or the replacement-string, we need to escape it using \. For this example create a path.txt file which contains a directory path as shown below.

sed 's/\/usr\/local\/bin/\/usr\/bin/' path.txt

Substitution Grouping

Sed中可以使用正则表达式中匹配分组. A group is opened with \( and closed with \).

Example: 输出每个行第一个,前的所有内容.

 sed 's/\([^,]*\).*/\1/g' employee.txt

[^,]* means zero or more non-comma.

In [], a , means just a comma. And the leading ^ means “anything but..”.

Power of & - Get Matched Pattern

& replaces it with whatever text matched the original-string or the regular-expression.

$ echo 1234 | sed 's/123/<&>/'
<123>4

Sed 处理Bash变量

一旦你的pattern中包含变量, 整个pattern必须由双引号包围.

对于某些变量里含有forward slash(/), 需要换一个pattern delimiter. Sed 允许替换为任意的字符, 需要做的工作针对不同的使用情况有些许区别.

Scenario #1: s command, 直接替换即可,

sed -i "s:$pushed_dir:REPLACE:g" input_file

Scenario #2: For patterns used in addresses, 需要首先对新的delimiter使用\:进行escaped(转义)

sed -i "\:$pushed_dir:d" input_file

See: Using different delimiters in sed « \1 (backreference.org)