Uses Of Sed Substitution In Unix Server

March 23, 2012 / Linux Web Hosting

In this article, let us review some interesting workarounds with the “s” substitute command in sed with several practical examples.

I. Sed Substitution Delimiter

In the sed substitute command, users can use different delimiters like, @ % | ; :

We first need to create the path.txt file which is used in all the given examples

$ cat path.txt

Example 1 – sed @ delimiter: Substitute /opt/omni/lbin to /opt/tools/bin

If you substitute a path as ‘/’ and you can use @ as a delimiter instead of ‘/’. In the sed example below, in the last line of the input file, /opt/omni/lbin was changed to /opt/tools/bin.

$ sed 's@/opt/omni/lbin@/opt/tools/bin@g' path.txt

Example 2 – sed / delimiter: Substitute /opt/omni/lbin to /opt/tools/bin

When you should use ‘/’ in path name-related substitution, you have to escape ‘/’ in the substitution data as shown below. In this sed example, the delimiter ‘/’ was escaped in the REGEXP and REPLACEMENT part.

$ sed 's/\/opt\/omni\/lbin/\/opt\/tools\/bin/g' path.txt

II. Sed ‘&’ Get Matched String

The precise part of an input line on which the Regular Expression matches is represented by &, which can then be used in the replacement part.

Example 1 – sed & Usage: Substitute /usr/bin/ to /usr/bin/local

$ sed 's@/usr/bin@&/local@g' path.txt

In the above example ‘&’ in the replacement part will replace with /usr/bin which is matched pattern and add it with /local. So in the output all the occurrence of /usr/bin will be replaced with /usr/bin/local

Example 2 – sed & Usage: Match the whole line

& replaces whatever matches with the given REGEXP.

$ sed 's@^.*$@<<<&>>>@g' path.txt

In the above example regexp has “^.*$” which matches the whole line. Replacement part <<<&>>> writes the whole line with <<< and >>> in the beginning and end of the line respectively.

III. Grouping and Back-references in Sed

Grouping can be used in sed like normal regular expression. A group is opened with “\(” and closed with “\)”.Grouping can be used in combination with back-referencing.

Back-reference is the re-use of a part of a Regular Expression selected by grouping. Back-references in sed can be used in both a Regular Expression and in the replacement part of the substitute command.

Example 1: Get only the first path in each line

$ sed 's/\(\/[^:]*\).*/\1/g' path.txt

In the above example, \(\/[^:]*\) matches the path available before first: comes. \1 replaces the first matched group.

Example 2: multi grouping

In the file path.txt change the order of field in the last line of the file.

$ sed '$s@\([^:]*\):\([^:]*\):\([^:]*\)@\3:\2:\1@g' path.txt

In the above command, $ specifies substitution to happen only for the last line. The output shows that the order of the path values in the last line has been reversed.

Example 3: Get the list of usernames in /etc/passwd file

This sed example displays only the first field from the /etc/passwd file.

$sed 's/\([^:]*\).*/\1/' /etc/passwd

Example 4: Parenthesize first character of each word

This sed example prints the first character of every word in parenthesis.

$ echo "Welcome To The Geek Stuff" | sed 's/\(\b[A-Z]\)/\(\1\)/g'
(W)elcome (T)o (T)he (G)eek (S)tuff

Example 5: Commify the simple number.

Let us create a file called numbers which has a list of numbers. The below sed command example is used to commify the numbers to thousands.

$ cat  numbers

$sed 's/\(^\|[^0-9.]\)\([0-9]\+\)\([0-9]\{3\}\)/\1\2,\3/g' numbers

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