Sed

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For other uses, see Sed (disambiguation).
The correct title of this article is sed. The initial letter is shown capitalized due to technical restrictions.

sed (which stands for Stream EDitor) is a simple and powerful computer program used to apply various pre-specified textual transformations to a sequential stream of text data. It reads input files line by line, edits each line according to rules specified in its simple language (the sed script), and then outputs the line. While originally created as a Unix utility by Lee E. McMahon of Bell Labs from 1973 to 1974, sed is now available for virtually every operating system that supports a command line.

Contents

[edit] Functions

sed is often thought of as a non-interactive text editor. It differs from conventional text editors in that the processing of the two inputs is inverted. Instead of iterating once through a list of edit commands applying each one to the whole text file in memory, sed iterates once through the text file applying the whole list of edit commands to each line. Because only one line at a time is in memory, sed can process text files with an arbitrarily-large number of lines. Some implementations of sed can only process lines of limited lengths.

sed's command set is modeled after the ed editor, and most commands work similarly in this inverted paradigm. For example, the command 25d means if this is line 25, then delete (don't output) it, rather than go to line 25 and delete it as it does in ed. The notable exceptions are the copy and move commands, which span a range of lines and thus don't have straightforward equivalents in sed. Instead, sed introduces an extra buffer called the hold space, and additional commands to manipulate it. The ed command to copy line 25 to line 76 (25t76) for example would be coded as two separate commands in sed (25h; 76g), to store the line in the hold space until the point at which it should be retrieved.

[edit] Usage

The following example shows a typical usage of sed, where the -e option indicates that the sed expression follows:

sed -e 's/oldstuff/newstuff/g' inputFileName > outputFileName

The s stands for substitute; the g stands for global, which means that all matching occurrences in the line would be replaced. After the first slash is the regular expression to search for and after the second slash is the expression to replace it with. The substitute command (s///) is by far the most powerful and most commonly used sed command.

Under Unix, sed is often used as a filter in a pipeline:

generate_data | sed -e 's/x/y/g'

That is, generate the data, but make the small change of replacing x with y.

Several substitutions or other commands can be put together in a file called, for example, subst.sed and then be applied using the -f option to read the commands from the file:

sed -f subst.sed inputFileName > outputFileName

Besides substitution, other forms of simple processing are possible. For example, the following deletes empty lines or lines that only contain spaces:

sed -e '/^ *$/d' inputFileName

This example used some of the following regular expression metacharacters:

  • The caret (^) matches the beginning of the line.
  • The dollar sign ($) matches the end of the line.
  • A period (.) matches any single character.
  • The asterisk (*) matches zero or more occurrences of the previous character.
  • A bracketed expression delimited by [ and ] matches any of the characters inside the brackets.

Complex sed constructs are possible, to the extent that it can be conceived of as a highly specialised, albeit simple, programming language. Flow of control, for example, can be managed by use of a label (a colon followed by a string which is to be the label name) and the branch instruction b; an instruction b followed by a valid label name will move processing to the block following the label; if the label does not exist then the branch will end the script.

[edit] History

sed is one of the very early Unix commands that permitted command line processing of data files. It evolved as the natural successor to the popular grep command. Cousin to the later AWK, sed allowed powerful and interesting data processing to be done by shell scripts. sed was probably the earliest Unix tool that really encouraged regular expressions to be used ubiquitously. In terms of speed of operation, sed is generally faster than Perl in execution and markedly faster than AWK.

sed and AWK are often cited as the progenitors and inspiration for Perl; in particular the s/// syntax from the example above is part of Perl's syntax.

sed's language does not have variables and has only primitive GOTO and branching functionality; nevertheless, the language is Turing-complete. [1]

GNU sed includes several new features such as in-place editing of files (i.e., replace the original file with the result of applying the sed program). In-place editing is often used instead of ed scripts: for example,

sed -i 's/abc/def/' file

can be used instead of

ed file
1,$ s/abc/def/
w
q

There is an extended version of sed called Super-sed (ssed) that includes regular expressions compatible with Perl.

Another version of sed is minised, originally reverse-engineered from the 4.1BSD sed by Eric S. Raymond and currently maintained by René Rebe. minised was used by the GNU project until the GNU project wrote a new version of sed based on the new GNU regular expression library. The current minised contains some extensions to BSD sed but is not as feature-rich as GNU sed. Its advantage is that it is very fast and uses little memory. It is used on embedded systems and is the version of sed provided with Minix.

[edit] Samples

This example will enable sed, which usually only works on one line, to remove newlines from sentences where the second sentence starts with one space.

Consider the following text:

This is my cat
 my cat's name is betty
This is my dog
 my dog's name is frank

The sed script below will turn it into:

This is my cat my cat's name is betty
This is my dog my dog's name is frank

Here's the script:

sed 'N;s/\n / /;P;D;'
  • (N) add the next line to the work buffer
  • (s) substitute
  • (/\n /) match: \n and one space
  • (/ /) replace with: one space
  • (P) print the top line of the work buffer
  • (D) delete the top line from the work buffer and run the script again

[edit] The Address Command (submatches)

More complex substitutions are possible using the "Address" command:

/pattern1/s/pattern2/replacement/flags
will replace pattern2 with replacement where pattern1 is matched.

Likewise:

/pattern1/!s/pattern2/replacement/flags
will replace pattern2 where pattern1 is *not* matched.

For example, if you have a file (text.txt) containing the following lines:

Hello world.
Hello world. I love sed.

And you want to replace "world" with "mom", but only on those lines that contain the word "sed", you can use:

sed -e '/sed/s/world/mom/g' text.txt

will result in:

Hello world.
Hello mom.  I love sed.

You can negate this behavior with:

sed -e '/sed/!s/world/mom/g' text.txt

which will result in the opposite:

Hello mom.
Hello world.  I love sed.

[edit] Further reading

[edit] See also

[edit] External links

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