1. Replacing string

原文地址：https://www.systemcodegeeks.com/shell-scripting/bash/linux-sed-examples/?ref=dzone

Sed is basically a stream editor used for modifying files in unix or linux. It provides a nifty way to perform operations on files which can be passed around through pipes. Most people never learn its real power,they just simply use sed to replace text. You can do many things apart from replacing text with sed.

As mentioned earlier,sed is an editor that allows us to use pattern matching to search and replace within a file,or an input stream. This works by using Regular Expressions. By default,the results of any edits we make to a source file,or input stream,will be sent to STDOUT,the standard output. The original file will be unaffected by the edits.

Also the sed command can be incredibly useful when bootstrapping a new server,or machine,as it can easily be scripted. A common use case for sed is to script the editing of configuration files on a new server instance to facilitate the further setup of the needed environment for that machine.

In this article I will describe the capabilities of sed with examples. Consider the below file as input to our examples:

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Sed command is mostly used to replace the text in a file. The below sed command replaces the word “java” with “guava” in the file only for the first occurrence in each line:


Here the “s” specifies the substitution operation. The “/” are delimiters. The “java” is the search pattern and the “guava” is the replacement string.
By default,the sed command replaces the first occurrence of the pattern in each line and it won’t replace next occurences.
2. Replacing the nth occurrence of a pattern in a line
Use the /1,/2 etc flags to replace the first,second occurrence of a pattern in a line. The below command replaces the second occurrence of the word “java” with “guava” in a line:
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3. Replacing all the occurrences of a pattern in a line
The substitute flag /g (global replacement) specifies the sed command to replace all the occurrences of the string in the line:
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8. Replacing on a line which matches a pattern

You can specify a pattern to the sed command to match in a line. If the pattern match occurs,then the sed command looks only for the string to be replaced and if it finds it,then it replaces the string. Here the sed command first looks for the lines which have the pattern “java” and then replaces the word “java” with “guava”.

13. Using \1,\2 and so on to \9

The first pair of parenthesis specified in the pattern represents the \1,the second represents the \2 and so on. The \1,\2 can be used in the replacement string to make changes to the source string. As an example,if you want to replace the word “java” in a line with twice as the word like “javajava” use the sed command as below: