The Stream Editor sed

What is sed?
sed is the stream editor. sed copies the standard input to standard output, edited according to a script of commands. It is extremely useful for manipulating strings or variables in shell scripts. The command script is applied to each line of the input.
Quick example of sed.
If we wanted just the list of users that were logged on we could enter:
```
$ who | sed 's/ .*$//'
operator
operator
kochis
wang
king
nobles
gary
www
$
```
sed editing commands.
The general format of an editing command is:
[address1,address2][function][arguments]
One or both of the addresses may be omitted. Any number of blanks or tabs may separate the addresses from the function. The function must be present. arguments may or may not be required depeneding on the function. sed commands are terminated by the end of line.
Options for the sed command.
Two useful options when using sed are:

-n
tells sed not to copy all lines to output. Only those lines specified with p functions or p flags are copied.
-f
tells sed to take its commands from a file. One command per line.

Test file
All of the sed examples willl use this data file. The fields are separated by a tab.

able	old	100
able	new	200
able	blue	200
baker	old	300
baker	new	300
baker	blue	200
charlie	old	100
charlie	new	400
charlie	blue	400
dog 	old	230
dog 	new	410
dog	blue	700

Addresses.
Addresses may be either line addresses or context addresses.
1. Line number addresses
  A line address is a decimal integer. As each line is read from the input, a line number counter is icremented. A line address selects a line when the line number is equal to line number counter. The counter runs cumulatively through multiple input files; it is not reset when a new input file is opened. As a special case, the character $ matches the last input line.
2. Context Addresses
  A context address is a regular expression enclosed in slashes('/'). Each line that matches the regular expression is operated on.
3. Number of addresses.
  A command may have 0,1,or 2 addresses. If a command has no addresses, it is applied to every line. One address commands are applied to all lines that match that address. Two address commands are applied to the first line that matches which matches the first address, and to all subsequent lines until (and including) the next line to match the second address. Two addresses are separated by a comma. Two addresses form a range.
4. Examples
  1. No address, remove the first word from every line.
```
$ sed 's/^[a-z][a-z]*.//' sedfile
old     100
new     200
blue    200
old     300
new     300
blue    200
old     100
new     400
blue    400
old     230
new     400
blue    700
```
  2. One line number. Print the 5th line.
```
$ sed -n '5p' sedfile
baker   new     300
```
  3. One context address, print all the blue lines.
```
$ sed -n '/blue/p' sedfile
able    blue    200
baker   blue    200
charlie blue    400
dog     blue    700
```
  4. Two line numbers, Remove lines 5,8 from the output.
```
$ sed '5,8d' sedfile
able    old     100
able    new     200
able    blue    200
baker   old     300
charlie blue    400
dog     old     230
dog     new     410
dog     blue    700
```
  5. Two context addresses. Print the lines between 400 and 230.
```
$ sed -n '/400/,/230/p' sedfile
charlie new     400
charlie blue    400
dog     old     230
```

Line Functions

d delete
lines from the file. All matched lines are not written to the output.

$ sed '1,4d' sedfile
baker   new     300
baker   blue    200
charlie old     100
charlie new     400
charlie blue    400
dog     old     230
dog     new     410
dog     blue    700

$ sed '/blue/d' sedfile        
able    old 	100
able    new 	200
baker   old 	300
baker   new     300
charlie old     100
charlie new     400
dog     old     230
dog     new     410

a append
Append to file.

$ cat data
$a\
edward  new 410\
edward  old 345

$ sed -f data sedfile
able    old     100
able    new     200
able    blue    200
baker   old     300
baker   new     300
baker   blue    200
charlie old     100
charlie new     400
charlie blue    400
dog     old     230
dog     new     410
dog     blue    700
edward  new     410
edward  old     345

i insert
Insert works the same as append except the data is written before the matched line.

c change
Change deletes lines and replaces them with text.

$ cat data
1,3c\
edward  new 410\
edward  old 345

$ sed -f data sedfile
edward  new 	410
edward  old 	345
baker   old 	300
baker   new 	300
baker   blue    200
charlie old 	100
charlie new 	400
charlie blue    400
dog     old 	230
dog     new 	410
dog     blue    700

s substitute.
The substitute function changes part of selected lines.

$sed 's/blue/red/' sedfile
able    old 	100
able    new 	200
able    red 	200
baker   old 	300
baker   new 	300
baker   red 	200
charlie old 	100
charlie new 	400
charlie red 	400
dog     old 	230
dog     new 	410
dog     red 	700

The substitute command uses the g, p and w flags.

g

The g is global. Replace all occurrences of the pattern in the line.

$ sed 's/0//g' sedfile
able    old     1
able    new     2
able    blue    2
baker   old 	3
baker   new 	3
baker   blue    2
charlie old 	1
charlie new 	4
charlie blue    4
dog     old 	23
dog     new 	41
dog     blue    7

p

The p flag prints the line only if a replace is made.

$ sed -n 's/old/y2k/p' sedfile
able    y2k 100
baker   y2k 300
charlie y2k 100
dog     y2k 230

w

The w flag write the line to a line if the line is altered.

$ sed  's/old/y2k/w new' sedfile  
able    y2k 	100
able    new 	200
able    blue    200
baker   y2k 	300
baker   new 	300
baker   blue    200
charlie y2k 	100
charlie new 	400
charlie blue    400
dog     y2k 	230
dog     new 	410
dog     blue    700

$ cat new
able    y2k 	100
baker   y2k 	300
charlie y2k 	100
dog     y2k 	230

Input and output functions.

p

Print the matched line is usually used in conjunction with the -n option.

w

Write specified line to the file.

$ sed '1,5w new' sedfile
able    old 	100
able    new 	200
able    blue    200
baker   old 	300
baker   new 	300
baker   blue    200
charlie old 	100
charlie new 	400
charlie blue    400
dog     old 	230
dog     new 	410
dog     blue    700

$ cat new
able    old 	100
able    new 	200
able    blue    200
baker   old 	300
baker   new 	300

r

Reads a file and appends the text.

$ cat new  
mable   old 	100
mable   new 	200
mable   blue    200

$ sed '3r new' sedfile
able    old 	100
able    new 	200
able    blue    200
mable   old 	100
mable   new 	200
mable   blue    200
baker   old 	300
baker   new 	300
baker   blue    200
charlie old 	100
charlie new 	400
charlie blue    400
dog     old 	230
dog     new 	410
dog     blue    700