Sep 26

sed - stream editor

Read in files from input stream one line at a time and perform some operations on them

For this lecture, consider the following file called faculty.details:

Name: Lenwood S. Heath Office: 2160J Torgersen Hall Course: CS 4124
Name: Chris North Office: 634 McBryde Hall Course: CS 5764
Name: Godmar Back Office: 2160A Torgersen Hall Course: CS 5204
Name: Manuel Perez Office: 638 McBryde Hall Course: CS 1706
Name: Naren Ramakrishnan Office: 2160L Torgersen Hall Course: CS 2204
Name: Dennis Kafura Office: 660 McBryde Hall Course: CS 5204
Name: Liqing Zhang Office: 2160K Torgersen Hall Course: CS 5045

'd' for delete

Delete lines from the output stream, not original file

sed 'd' faculty.details reads in one line at a time into a buffer (work space), deletes it, and prints the contents of the buffer (in this case, empty).
sed '1d' faculty.details reads in one line at a time into the buffer, deletes it if it is line 1, and prints the buffer contents onto output. In this case, all lines except 1 would be output.
sed '$d' faculty.details does the same, but for the 'last' line.
sed '2,4d' faculty.details deletes lines from 2, upto and including line 4.
sed '/Back/,/Kafura/d' faculty.details deletes lines starting from one that matches "Back", upto and including one that matches "Kafura".
sed '/Back/,/Kafura/!d' faculty.details "negates" the meaning, i.e., do not delete these lines, and delete others.

'p' for print

Print lines from the buffer

sed 'p' faculty.details reads in one line at a time into the buffer, prints it. Notice that by default sed prints what is in the buffer. So, you will get two copies of each line!
In sed -n 'p' faculty.details, the "-n" gets rid of the default action of sed. So, this is the equivalent of doing a cat.
We can use the same addressing commands as before, e.g., sed -n 4,6 'p' faculty.details prints lines 4 through 6.

More sed jargon

Beginnings of a complete command language

= prints (just) the line number.
a appends text at the end of the buffer. Use it as a\, followed by what you want to append.
s substitutes one text for another. Use it as s/find string/replace string/. Can use "\1", "\2" to trap matches and refer to them in the replace string. Need to use parentheses "(" and ")" to group the named matches. Must also escape them.
b branches out of pattern matching, i.e., stop attempting to make more matches.

You can also put in the sed commands in a file and invoke it with the -f option.

Worked out problems

Write sed commands/scripts to:

Delete the blank lines in the file.
```
sed '/^$/d' faculty.details
```
Print the lines pertaining to faculty who have offices in Torgersen Hall.
```
sed -n '/Torgersen Hall/p' faculty.details
```
Find the line numbers describing faculty who teach undergraduate courses.
```
sed -n '/CS [1234][0-9][0-9][0-9]/=' faculty.details
```
You are given that Heath, Kafura are full professors. Ramakrishnan is an associate professor. All others are assistant professors. Print each professor's designation on a separate line, after the given line, in the form "Designation: ".
```
/Heath/ {
    a\
    Designation: Full Professor
    p
    b
}
/Kafura/ {
    a\
    Designation: Full Professor
    p
    b
}
/Rama/ {
    a\
    Designation: Associate Professor
    p
    b
}
{
    a\
    Designation: Assistant Professor
    p
}
```
(put the above in a file, say designation.f and invoke it as: sed -f designation.f faculty.details) Note that the "b" commands are important, otherwise since the last command is supposed to work for all lines (note the lack of addresses), everybody will also be listed as an assistant professor. Also note that you can append multiple lines, each must be followed by a "\" except the last line (observe the "\" after the a command). The braces "{" and "}" must be where they are, i.e., the "{" must end the first line and the "}" must be on a line by itself.
Print the lines in the format 'name:office:course' i.e., strip the headers "Name:" and "Office:" and "Course:".
```
sed 's/Name: $.*$ Office: $.*$ Course: $.*$/\1:\2:\3/' faculty.details
```

Print the lines in the format 'course:office:name'.

sed 's/Name: \(.*\) Office: \(.*\) Course: \(.*\)/\3:\2:\1/' faculty.details

Break down every entry onto three lines.

sed 's/Name: \(.*\) Office: \(.*\) Course: \(.*\)/\1\n\2\n\3/' faculty.details