Sed

sed (Stream EDitor) refers to a Unix utility which (a) parses text files and (b) implements a programming language which can apply textual transformations to such files. It reads input files line by line (sequentially), applying the operation which has been specified via the command line (or a "sed script"), and then outputs the line. It was developed from 1973 to 1974 as a Unix utility by Lee E. McMahon of Bell Labscite web |url=http://sed.sourceforge.net/sedfaq2.html#s2.1 |title=Frequently-Asked Questions about sed, the stream editor |format= |work= |accessdate=2008-05-13] , and is available today for most operating systems.cite web |url=http://sed.sourceforge.net/sedfaq2.html#s2.2 |title=Frequently-Asked Questions about sed, the stream editor |format= |work= |accessdate=2008-05-13]

Usage

The following example shows a typical use of sed, where the "-e" option indicates that the sed expression follows: sed -e 's/oldstuff/newstuff/g' inputFileName > outputFileNameIn many versions, the "-e" is not required to precede the expression. The "s" stands for substitute. The "g" stands for global, which means that all matching occurrences in the line would be replaced. The regular expression (i.e. pattern) to be searched is placed after the first delimiting symbol (slash here) and the replacement follows the second symbol. Slash is the conventional symbol. Any other could be used to make syntax more readable if it does not occur in the pattern or replacement (see below).

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, and then 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 uses the "d" command to delete lines that are either blank or 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, allowing it to serve as a simple, but highly specialised, programming language. Flow of control, for example, can be managed by the use of a label (a colon followed by a string) and the branch instruction "b". An instruction "b" followed by a valid label name will move processing to the block following that label. If the label does not exist then the branch will end the script.

History

sed is one of the very early Unix commands built for command line processing of data files. It evolved as the natural successor to the popular grep command.cite web
title = On the Early History and Impact of Unix
url = http://www.columbia.edu/~rh120/ch001j.c11] Cousin to the later AWK, sed allowed powerful and interesting data processing to be done by shell scripts.

sed and AWK are often cited as the progenitors and inspiration for Perl. The "s///" syntax shown above is part of Perl's syntax and originated with ed, the precursor to sed.

sed's language does not have variables and has only primitive GOTO and branching functionality; nevertheless, the language is Turing-complete.cite web
title = Implementation of a Turing Machine as Sed Script
url = http://sed.sourceforge.net/grabbag/scripts/turing.txt] cite web
title = turing.sed
url = http://sed.sourceforge.net/grabbag/scripts/turing.sed]

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

"Super-sed" is an extended version of sed that includes regular expressions compatible with Perl.

Another variant 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.Fact|date=May 2008 It is used on embedded systems and is the version of sed provided with Minix.Fact|date=May 2008

Samples

In this example, sed, which usually only works on one line, removes 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/ / /;P;D;'

* (N) add the next line to the work buffer
* (s) substitute
* (/ /) match: (newline character in Unix) 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

The context address

Conditioned commands are possible using the "context address": /addr-pattern/s/search-pattern/replacement/flagsor more readable variants: /addr-pattern/s~search-pattern~replacement~flags

/addr-pattern/s,search-pattern,replacement,flags

Here, 'addr-pattern' is the address, which defines lines, where substitution of 'search-pattern' with 'replacement' will be done.

Likewise: /addr-pattern/!s,search-pattern,replacement,flagssubstitute will be executed if no 'addr-pattern' was matched.

For example, replace "world" with "mom", but only on lines which contain the word "you": sh$ sed -e '/you/s,world,mom,g' <<" EOF" Hello world. Hello world. I love you. EOF Hello world. Hello mom. I love you. sh$

Negation will be: sh$ sed -e '/you/!s,world,mom,g' <<" EOF" Hello world. Hello world. I love you. EOF Hello mom. Hello world. I love you. sh$

Exotic examples

Despite the inherent limitations, sed scripts exist for games as sokoban, arkanoidcite web
url = http://sed.sourceforge.net/#gamez
title = sed.sf.net - The SED $HOME] , and a recent implementation of tetris.cite web
url = http://uuner.livejournal.com/55238.html
title = author's livejournal page]

Further reading

*cite book|title=sed & awk|edition=2nd Edition|month=March|year=1997|author=Dale Dougherty & Arnold Robbins|publisher=O'Reilly|id=ISBN 1-56592-225-5
*cite book|title=sed and awk Pocket Reference|edition=2nd Edition|month=June|year=2002|author=Arnold Robbins|publisher=O'Reilly and Associates|id=ISBN 0-596-00352-8
*cite book|title=UNIX AWK and SED Programmer's Interactive Workbook (UNIX Interactive Workbook)|author=Peter Patsis|publisher=Prentice Hall|year=1998-12-30|id=ISBN 0-13-082675-8
* [http://sed.sourceforge.net/sedfaq.html The sed FAQ]
* [http://www.gnu.org/software/sed/manual/sed.html GNU sed manual]

References

ee also

*List of Unix programs

External links

* [http://sed.sourceforge.net Major sources for sed scripts, files, usage]
* [http://sed.sourceforge.net/sed1line.txt Handy one-line sed scripts]
* [http://sed.sourceforge.net/grabbag/scripts/ Sed script archive]
* [http://www.pement.org/sed/ A home page for sed, some focus on Windows/DOS]
* [http://www.npcguild.org/~ksb/hack/math.sed A calculator written in sed]
* [http://www.gnu.org/directory/text/editors/super-sed.html Super-sed]
* [http://www.gnulamp.com/sed.html sed Tutorial]
* [http://www.grymoire.com/Unix/Sed.html The sed tutorial from Grymoire]
* [http://unxutils.sourceforge.net GNU utilities for Win32]
* [http://users.cybercity.dk/~bse26236/batutil/help/SED.HTM#11.20 More on the address command and sub-matched replacements]
* [http://www.exactcode.de/oss/minised/ Minised homepage]
* [http://www.pcre.org/ PCRE - Perl Compatible Regular Expressions]
* [http://www.computerworld.com.au/index.php/id;1726534212;fp;4;fpid;611908207 The A-Z of Programming Languages: AWK]
* [http://www.ibm.com/developerworks/linux/library/l-sed1.html Common threads: Sed by example, Part 1] by Daniel Robbins
* [http://www.ibm.com/developerworks/linux/library/l-sed2.html Common threads: Sed by example, Part 2] by Daniel Robbins
* [http://www.ibm.com/developerworks/linux/library/l-sed3.html Common threads: Sed by example, Part 3] by Daniel Robbins