To know how to use sed , people should understand regular expressions ( for short). A regular expression is a pattern that is matched against a subject string from left to right. Most characters are : they stand for themselves in a pattern, and match the corresponding characters in the subject. As a trivial example, the pattern
In most scripts, pattern space is initialized to the content of each line (see How sed works). So, it is a useful simplification to think of ^#include as matching only lines where ‘ #include ’ is the first thing on line—if there are spaces before, for example, the match fails. This simplification is valid as long as the original content of pattern space is not modified, for example with an s command.
A leading ^ reverses the meaning of list , so that it matches any single character not in list . To include ] in the list, make it the first character (after the ^ if needed), to include — in the list, make it the first or last; to include ^ put it after the first character.
The characters $ , * , . , [ , and \ are normally not special within list . For example, [\*] matches either ‘ \ ’ or ‘ * ’, because the \ is not special here. However, strings like [.ch.] , [=a=] , and [:space:] are special within list and represent collating symbols, equivalence classes, and character classes, respectively, and [ is therefore special within list when it is followed by . , = , or : . Also, when not in POSIXLY_CORRECT mode, special escapes like \n and \t are recognized within list . See Escapes. regexp1 \| regexp2 Matches either regexp1 or regexp2 . Use parentheses to use complex alternative regular expressions. The matching process tries each alternative in turn, from left to right, and the first one that succeeds is used. It is a GNU extension. regexp1 regexp2 Matches the concatenation of regexp1 and regexp2 . Concatenation binds more tightly than \| , ^ , and $ , but less tightly than the other regular expression operators. \ digit Matches the digit -th \(. \) parenthesized subexpression in the regular expression. This is called a . Subexpressions are implicity numbered by counting occurrences of \( left-to-right. \n Matches the newline character. \ char Matches char , where char is one of $ , * , . , [ , \ , or ^ . Note that the only C-like backslash sequences that you can portably assume to be interpreted are \n and \\ ; in particular \t is not portable, and matches a ‘ t ’ under most implementations of sed , rather than a tab character.
Note that the regular expression matcher is greedy, i.e., matches are attempted from left to right and, if two or more matches are possible starting at the same character, it selects the longest.
Examples: ‘ abcdef ’ Matches ‘ abcdef ’. ‘ a*b ’ Matches zero or more ‘ a ’s followed by a single ‘ b ’. For example, ‘ b ’ or ‘ aaaaab ’. ‘ a\?b ’ Matches ‘ b ’ or ‘ ab ’. ‘ a\+b\+ ’ Matches one or more ‘ a ’s followed by one or more ‘ b ’s: ‘ ab ’ is the shortest possible match, but other examples are ‘ aaaab ’ or ‘ abbbbb ’ or ‘ aaaaaabbbbbbb ’. ‘ .* ’ ‘ .\+ ’ These two both match all the characters in a string; however, the first matches every string (including the empty string), while the second matches only strings containing at least one character. ‘ ^main.*(.*) ’ This matches a string starting with ‘ main ’, followed by an opening and closing parenthesis. The ‘ n ’, ‘ ( ’ and ‘ ) ’ need not be adjacent. ‘ ^# ’ This matches a string beginning with ‘ # ’. ‘ \\$ ’ This matches a string ending with a single backslash. The regexp contains two backslashes for escaping. ‘ \$ ’ Instead, this matches a string consisting of a single dollar sign, because it is escaped. ‘ [a-zA-Z0-9] ’ In the C locale, this matches any ASCII letters or digits. ‘ [^ tab ]\+ ’ (Here tab stands for a single tab character.) This matches a string of one or more characters, none of which is a space or a tab. Usually this means a word. ‘ ^\(.*\)\n\1$ ’ This matches a string consisting of two equal substrings separated by a newline. ‘ .\<9\>A$ ’ This matches nine characters followed by an ‘ A ’. ‘ ^.\<15\>A ’ This matches the start of a string that contains 16 characters, the last of which is an ‘ A ’.
In this chapter, we will discuss in detail about regular expressions with SED in Unix.
A regular expression is a string that can be used to describe several sequences of characters. Regular expressions are used by several different Unix commands, including ed, sed, awk, grep, and to a more limited extent, vi.
Here SED stands for stream editor. This stream-oriented editor was created exclusively for executing scripts. Thus, all the input you feed into it passes through and goes to STDOUT and it does not change the input file.
Invoking sed
Before we start, let us ensure we have a local copy of /etc/passwd text file to work with sed.
As mentioned previously, sed can be invoked by sending data through a pipe to it as follows −
The cat command dumps the contents of /etc/passwd to sed through the pipe into sed’s pattern space. The pattern space is the internal work buffer that sed uses for its operations.
The sed General Syntax
Following is the general syntax for sed −
Here, pattern is a regular expression, and action is one of the commands given in the following table. If pattern is omitted, action is performed for every line as we have seen above.
The slash character (/) that surrounds the pattern are required because they are used as delimiters.
Sr.No.
Range & Description
1
Prints the line
Deletes the line
Substitutes the first occurrence of pattern1 with pattern2
Deleting All Lines with sed
We will now understand how to delete all lines with sed. Invoke sed again; but the sed is now supposed to use the editing command delete line, denoted by the single letter d −
Instead of invoking sed by sending a file to it through a pipe, the sed can be instructed to read the data from a file, as in the following example.
The following command does exactly the same as in the previous example, without the cat command −
The sed Addresses
The sed also supports addresses. Addresses are either particular locations in a file or a range where a particular editing command should be applied. When the sed encounters no addresses, it performs its operations on every line in the file.
The following command adds a basic address to the sed command you’ve been using −
Notice that the number 1 is added before the delete edit command. This instructs the sed to perform the editing command on the first line of the file. In this example, the sed will delete the first line of /etc/password and print the rest of the file.
The sed Address Ranges
We will now understand how to work with the sed address ranges. So what if you want to remove more than one line from a file? You can specify an address range with sed as follows −
The above command will be applied on all the lines starting from 1 through 5. This deletes the first five lines.
Try out the following address ranges −
Sr.No.
Range & Description
1
Lines starting from the 4 th till the 10 th are deleted
Only 10 th line is deleted, because the sed does not work in reverse direction
This matches line 4 in the file, deletes that line, continues to delete the next five lines, and then ceases its deletion and prints the rest
This deletes everything except starting from 2 nd till 5 th line
This deletes the first line, steps over the next three lines, and then deletes the fourth line. Sed continues to apply this pattern until the end of the file.
This tells sed to delete the second line, step over the next line, delete the next line, and repeat until the end of the file is reached
Lines starting from 4 th till 10 th are printed
This generates the syntax error
This would also generate syntax error
Note − While using the p action, you should use the -n option to avoid repetition of line printing. Check the difference in between the following two commands −
The Substitution Command
The substitution command, denoted by s, will substitute any string that you specify with any other string that you specify.
To substitute one string with another, the sed needs to have the information on where the first string ends and the substitution string begins. For this, we proceed with bookending the two strings with the forward slash (/) character.
The following command substitutes the first occurrence on a line of the string root with the string amrood.
It is very important to note that sed substitutes only the first occurrence on a line. If the string root occurs more than once on a line only the first match will be replaced.
For the sed to perform a global substitution, add the letter g to the end of the command as follows −
Substitution Flags
There are a number of other useful flags that can be passed in addition to the g flag, and you can specify more than one at a time.
Sr.No.
Flag & Description
1
Replaces all matches, not just the first match
Replaces only NUMBER th match
If substitution was made, then prints the pattern space
If substitution was made, then writes result to FILENAME
Matches in a case-insensitive manner
In addition to the normal behavior of the special regular expression characters ^ and $, this flag causes ^ to match the empty string after a newline and $ to match the empty string before a newline
Using an Alternative String Separator
Suppose you have to do a substitution on a string that includes the forward slash character. In this case, you can specify a different separator by providing the designated character after the s.
In the above example, we have used : as the delimiter instead of slash / because we were trying to search /root instead of the simple root.
Replacing with Empty Space
Use an empty substitution string to delete the root string from the /etc/passwd file entirely −
Address Substitution
If you want to substitute the string sh with the string quiet only on line 10, you can specify it as follows −
Similarly, to do an address range substitution, you could do something like the following −
As you can see from the output, the first five lines had the string sh changed to quiet, but the rest of the lines were left untouched.
The Matching Command
You would use the p option along with the -n option to print all the matching lines as follows −
Using Regular Expression
While matching patterns, you can use the regular expression which provides more flexibility.
Check the following example which matches all the lines starting with daemon and then deletes them −
Following is the example which deletes all the lines ending with sh −
The following table lists four special characters that are very useful in regular expressions.
Sr.No.
Character & Description
1
Matches the beginning of lines
Matches the end of lines
Matches any single character
Matches zero or more occurrences of the previous character
Matches any one of the characters given in chars, where chars is a sequence of characters. You can use the — character to indicate a range of characters.
Matching Characters
Look at a few more expressions to demonstrate the use of metacharacters. For example, the following pattern −
Sr.No.
Expression & Description
1
Matches lines that contain strings such as a+c, a-c, abc, match, and a3c
Matches the same strings along with strings such as ace, yacc, and arctic
Matches the string The and the
Matches blank lines
Matches an entire line whatever it is
Matches one or more spaces
Matches blank lines
Following table shows some frequently used sets of characters −
Sr.No.
Set & Description
1
Matches a single lowercase letter
Matches a single uppercase letter
Matches a single letter
Matches a single number
Matches a single letter or number
Character Class Keywords
Some special keywords are commonly available to regexps, especially GNU utilities that employ regexps. These are very useful for sed regular expressions as they simplify things and enhance readability.
For example, the characters a through z and the characters A through Z, constitute one such class of characters that has the keyword [[:alpha:]]
Using the alphabet character class keyword, this command prints only those lines in the /etc/syslog.conf file that start with a letter of the alphabet −
The following table is a complete list of the available character class keywords in GNU sed.
Sr.No.
Character Class & Description
1
Alphanumeric [a-z A-Z 0-9]
Alphabetic [a-z A-Z]
Blank characters (spaces or tabs)
Any visible characters (excludes whitespace)
Lowercase letters [a-z]
Printable characters (non-control characters)
Uppercase letters [A-Z]
Hex digits [0-9 a-f A-F]
Aampersand Referencing
The sed metacharacter & represents the contents of the pattern that was matched. For instance, say you have a file called phone.txt full of phone numbers, such as the following −
You want to make the area code (the first three digits) surrounded by parentheses for easier reading. To do this, you can use the ampersand replacement character −
Here in the pattern part you are matching the first 3 digits and then using & you are replacing those 3 digits with the surrounding parentheses.
Using Multiple sed Commands
You can use multiple sed commands in a single sed command as follows −
Here command1 through commandN are sed commands of the type discussed previously. These commands are applied to each of the lines in the list of files given by files.
Using the same mechanism, we can write the above phone number example as follows −
Note − In the above example, instead of repeating the character class keyword [[:digit:]] three times, we replaced it with \ , which means the preceding regular expression is matched three times. We have also used \ to give line break and this has to be removed before the command is run.
Back References
The ampersand metacharacter is useful, but even more useful is the ability to define specific regions in regular expressions. These special regions can be used as reference in your replacement strings. By defining specific parts of a regular expression, you can then refer back to those parts with a special reference character.
To do back references, you have to first define a region and then refer back to that region. To define a region, you insert backslashed parentheses around each region of interest. The first region that you surround with backslashes is then referenced by \1, the second region by \2, and so on.
Assuming phone.txt has the following text −
Try the following command −
Note − In the above example, each regular expression inside the parenthesis would be back referenced by \1, \2 and so on. We have used \ to give line break here. This should be removed before running the command.
