`sed` - Search and Replace¶

sed is a command that stands for "stream editor."

Conventions¶

path: The actual path. Example: /var/www/html/
filename: The actual filename. Example: index.php

Using `sed`¶

Using sed for search and replace is my personal preference because you can use a delimiter of your choice, which makes replacing things such as web links with “/” in them very handy. The default examples for in place editing using sed show things like this example:

sed -i 's/search_for/replace_with/g' /path/filename

But what if you are searching for strings that will contain "/" in them? If the forward slash was the only option available as a delimiter? You would have to escape each forward slash before you could use it in the search. That is where sed excels over other tools, because the delimiter is changeable on-the-fly (no need to specify that you are changing it anywhere). As stated, if you are looking for things with "/" in them, you can do that by changing the delimiter to "|". Here's an example of looking for a link by using this method:

sed -i 's|search_for/with_slash|replace_string|g' /path/filename

You can use any single-byte character for the delimiter with the exception of backslash, newline, and "s". For instance, this works too:

sed -i 'sasearch_forawith_slashareplace_stringag' /path/filename where "a" is the delimiter, and the search and replace still works. For safety, you can specify a backup while searching and replacing, which is handy for making sure the changes you are making with sed are what you really want. This gives you a recovery option from the backup file:

sed -i.bak s|search_for|replacea_with|g /path/filename

Which will create an unedited version of filename called filename.bak

You can also use full quotes instead of single quotes:

sed -i "s|search_for/with_slash|replace_string|g" /path/filename

Options Explained¶

Option	Explanation
i	edit file in place
i.ext	create a backup with whatever the extension is (ext here)
s	specifies search
g	specifies to replace globally, or all occurrences

Multiple files¶

Unfortunately, sed does not have an in-line looping option like perl. To loop through multiple files, you need to combine your sed command within a script. Here's an example of doing that.

First, generate a list of files that your script will use. Do this at the command line with:

find /var/www/html -name "*.php" > phpfiles.txt

Next, create a script to use this phpfiles.txt:

#!/bin/bash

for file in `cat phpfiles.txt`
do
        sed -i.bak 's|search_for/with_slash|replace_string|g' $file
done

The script loops through all of the files created in phpfiles.txt, creates a backup of each file, and executes the search and replace string globally. When you have verified that the changes are what you want, you can delete all of the backup files.

Conclusion¶

sed is a powerful tool and works very well for search and replace functions, particularly where the delimiter needs to be flexible.

Author: Steven Spencer

sed - Search and Replace¶