1 Day 16 Sed and Awk

2 Looking through output We already know what “grep” does. –It looks for something in a file. –Returns any line from the file that contained the word we searched for. Very often what it returns isn’t exactly what we wanted. –Sometimes we only want on column of output –Sometimes we need to change part of its output.

3 sed Used to search for and replace things. sed s/search/replace/g Example: –sed s/a/b/g –Search for all occurrences of “a” and replace them with “b” –sed s/blah//g –Search for all occurrences of “blah” and remove it…[replace it with nothing].

4 Searching and replacing Lets imagine you run some command such as ls Lots of your files end in.txt, but you don’t want to show that on the screen. ls | sed s/.txt//g

5 Other Example Look through a file and replace all occurrences of the word “edna” with “enda” cat file.txt | sed s/edna/enda/g > newfile.txt Note that sed doesn’t actually change the file, it just outputs the new file, which you can then redirect.

6 Exercise Write a script which takes 3 arguments: replace myfile.txt a b This should run through the file myfile.txt and replace all a’s with b’s. It should output the result to the screen.

7 Regular Expressions Sed actually supports an entire language called regular expressions. They allow you to do many complex things, however they are beyond the scope of this class. For more information –man regex

8 Information in Columns Sometimes the text you want is only one column of the output. For example –w Gives you way too much information. What if you only care about who is logged in. –You can use awk to find out

9 awk awk is an entire programming language. It was specifically designed to help you parse lots of text and do stuff with it. For this class we are just going to use it to show us one column of text at a time.

10 Using awk awk ‘{print $1}’ –The $1 here has nothing to do with shell parameters. –This would mean print out only the first column of the input. awk ‘{print $2}’ –This would print out the 2nd column. etc.

11 Who’s on If the only thing we care about is WHO is logged in, we can do: w | awk ‘{print $1}’ | sort | uniq | more This would give us a sorted list of every person logged in.

12 One Column Imagine you have a grade file, and you do: grep -i hw grades HW1: 80 HW2: 90 HW3: 20 However you just want the actual grades. grep -i hw grades | awk ‘{print $2}’ Will print out only the second column.

13 More than one column awk can print more than one column at a time Perhaps you want file name, and size: ls -l | awk ‘{print $9 “ - “ $5 }’

14 Exercise Write a script which: –Takes one argument, (a users name) –Uses the last command to find out when that user last logged in. –However it should one line, like this: endalast logged in Mar 18 at 23:55

15 last on last enda | awk '{print $1 " last logged in " $5 $6 " at " $7}' | head -1 –This does a last on enda. –Looks for the first, fifth, sixth, and seventh column. –Only shows us the most recent line