sed (Stream EDitor) refers to a Unix utility for parsing text files and the
programming language it uses to apply textual transformations to a sequential
stream of data. It reads input files line by line, 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 Labs, and is available today on most operating systems.
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, 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 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 (
$) matches the end of the line.
- A period (
.) matches any single character.
- The asterisk (
*) matches zero or more occurrences of the previous
character.
- A bracketed expression delimited by
[ and ] matches any of the
characters inside the brackets.
Complex sed constructs are possible, to the extent that it can be conceived of as a highly specialised, albeit simple,
programming language. Flow of control, for example, can be managed by use of a
label (a colon followed by a string which is to be the label name) and the branch instruction b; an instruction b
followed by a valid label name will move processing to the block following the label; if the label does not exist then the branch
will end the script.
History
sed is one of the very early Unix commands that permitted command line processing of data files. It evolved as the natural
successor to the popular grep command. Cousin to the later AWK, sed allowed powerful and interesting data processing to be done by shell scripts. sed
was probably the earliest Unix tool that really encouraged regular expressions to be used ubiquitously. In terms of speed of
operation, sed is generally faster than Perl in execution and markedly faster than AWK.
sed and AWK are often cited as the progenitors and inspiration for Perl; in particular the s///
syntax from the example above is part of Perl's syntax.
sed's language does not have variables and has only primitive GOTO and branching functionality;
nevertheless, the language is Turing-complete. [1]
GNU sed includes several new features such as in-place editing of files (i.e., replace the
original file with the result of applying the sed program). In-place editing is often used instead of ed scripts: for example,
sed -i 's/abc/def/' file
can be used instead of
ed file
1,$ 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.
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
The Address Command (submatches)
More complex substitutions are possible using the "Address" command:
/pattern1/s/pattern2/replacement/flags
Here, 'pattern1' is the address. The command following the address will only execute if the current line being processed
matches the pattern (address) specified. This 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 lamp.
And you want to replace "world" with "mom", but only on those lines that contain the word "lamp", you can use:
sed -e '/lamp/s/world/mom/g' text.txt
will result in:
Hello world.
Hello mom. I love lamp.
You can negate this behavior with:
sed -e '/lamp/!s/world/mom/g' text.txt
which will result in the opposite:
Hello mom.
Hello world. I love lamp.
Exotic examples
Despite the limited possibilities of sed, there exist sed scripts for games as sokoban or
arkanoid. Even some debuggers in sed have been made. [2]
Further reading
- Dale Dougherty & Arnold Robbins (March 1997). sed & awk, 2nd
Edition, O'Reilly. ISBN 1-56592-225-5.
- Arnold Robbins (June 2002). sed and awk Pocket Reference, 2nd Edition,
O'Reilly and Associates. ISBN 0-596-00352-8.
- Peter Patsis (1998-12-30). UNIX AWK and SED
Programmer's Interactive Workbook (UNIX Interactive Workbook). Prentice Hall. ISBN
0-13-082675-8.
- The sed FAQ
- GNU sed manual
See also
External links
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