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This article is part of the on-going series.

In our previous sed articles we learned — , , , , and .

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

I. Sed Substitution Delimiter

As we discussed in our previous post, we can use the different delimiters such as @ % | ; : in sed substitute command.

Let us first create path.txt file that will be used in all the examples mentioned below.

$ cat path.txt/usr/kbos/bin:/usr/local/bin:/usr/jbin:/usr/bin:/usr/sas/bin/usr/local/sbin:/sbin:/bin/:/usr/sbin:/usr/bin:/opt/omni/bin:/opt/omni/lbin:/opt/omni/sbin:/root/bin

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

When you substitute a path name which has ‘/’, 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/usr/kbos/bin:/usr/local/bin:/usr/jbin/:/usr/bin:/usr/sas/bin/usr/local/sbin:/sbin:/bin/:/usr/sbin:/usr/bin:/opt/omni/bin:/opt/tools/bin:/opt/omni/sbin:/root/bin

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/usr/kbos/bin:/usr/local/bin:/usr/jbin/:/usr/bin:/usr/sas/bin/usr/local/sbin:/sbin:/bin/:/usr/sbin:/usr/bin:/opt/omni/bin:/opt/tools/bin:/opt/omni/sbin:/root/bin

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/usr/kbos/bin:/usr/local/bin:/usr/jbin/:/usr/bin/local:/usr/sas/bin/usr/local/sbin:/sbin:/bin/:/usr/sbin:/usr/bin/local:/opt/omni/bin:/opt/omni/lbin:/opt/omni/sbin:/root/bin

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 occurrance 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/usr/kbos/bin/usr/local/sbin/opt/omni/lbin

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

Example 2: Multigrouping

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/usr/kbos/bin:/usr/local/bin:/usr/jbin:/usr/bin:/usr/sas/bin/usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin:/opt/omni/bin:/root/bin:/opt/omni/sbin:/opt/omni/lbin

In the above command $ specifies substitution to happen only for the last line.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/passwdrootbindaemonadmlpsyncshutdown

Example 4: Parenthesize first character of each word

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

$ 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 file called numbers which has list of numbers. The below sed command example is used to commify the numbers till thousands.

第一个匹配行首, 行首可以什么都没有,也可以是一个非0-9和小数点的字符

比如 +12345678 第一个匹配+ 第二个匹配12345 第三个匹配678

或者 12345678 第一个匹配空 第二个匹配12345 第三个匹配678

或者 1.234567  第一个匹配空,第二个匹配1 第三个匹配不起来,所以整体不匹配 不会打逗号 

$ cat  numbers1234121213434123$sed 's/\(^\|[^0-9.]\)\([0-9]\+\)\([0-9]\{3\}\)/\1\2,\3/g' numbers1,23412,1213,434123

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