1. CS 403: Programming Languages
Lecture 22
Fall 2003
Department of Computer Science
University of Alabama
Joel Jones

2. Overview Announcements Puzzle from last time sed awk
Story Hour, Houser 108, 3PM Friday
Puzzle from last time
sed
awk
Lecture 22

3. Printing 10 most common words
cat $* | # tr doesn’t take filename arguments
tr -sc A-Za-z ‘\012’ | # all non alpha become newline
sort |
uniq -c | # get the count
sort -n | # sort by count
tail # prints 10 by default
Use the man command to look for help
man sort
Lecture 22

4. sed
sed [options]‘list of commands’ filenames …
Commands
s/re1/re2/ substitute regular expression r1 with r2, first instance on every line
s/re1/re2/g substitute regular expression r1 with r2, every instance on every line
#command, does command for # times
E.g. sed 3q prints first 3 lines
/re1/q prints lines up to first one matching re1, then quits
Lecture 22

5. sed (cont.) sed ‘s/^/ /’ file More commands sed ‘/./s/^/ /’ file # or
Pair Up: Write a sed command line that indents a line by adding four spaces to the beginning of the line
sed ‘s/^/ /’ file
More commands
/re1/s/re2/re3/ substitute regular expression re2 with re3, first instance on every line matching re1
Pair Up: What does the above sed command do with empty lines? Write a sed command line that fixes this problem.
sed ‘/./s/^/ /’ file # or
sed ‘/^$/!s/ /’ file
Lecture 22

6. sed (cont.) Commands (cont.) Options -n turns off automatic printing
/re/d deletes lines matching re
/re/p prints lines matching re
Pair Up: What does the sed command sed ‘/the/p’ < file do?
Options
-n turns off automatic printing
-f filename takes sed commands from filename
Pair Up: Write a sed command line that does the same thing as:
grep re file
sed -n ‘/re/p’ file
Lecture 22

7. awk
awk [options]‘program’ filenames …
Like sed, but program is different: pattern { action } pattern { action } …
awk reads input in filenames one line at a time & when pattern matches, executes corresponding action
Patterns
Regular expressions
C-like expressions
Lecture 22

8. awk (cont.) awk ‘/re/’ file awk ‘{ print }’ filenames …
Pattern or action is optional
Pattern missing—perform action on every line
Action missing—print every line matching pattern
Simple action
print—without argument prints current line
Pair Up: Write an awk command line that does the same thing as:
grep re file
awk ‘/re/’ file
Pair Up: Write an awk command line that does the same thing as:
cat filenames …
awk ‘{ print }’ filenames …
Lecture 22

9. awk (cont.) Variables Options $0—entire line, $1-$NF—fields of line
E.g. awk ‘{ print $2 }’ textFile prints 2nd field of every line of textFile
E.g. who | awk ‘{ print $5, $1 }’ | sort prints name and login sorted by time
NF is number of fields on current line
NR is number of records (lines) read so far
Options
-Fchar—sets char as the field separator
Pair Up: Write an awk command line that prints user names out of /etc/passwd, where the user name is the first field and fields are colon separated.
Lecture 22

10. awk (cont.) awk -F: ‘{ print $1 }’ /etc/passwd Field breaking
N.B. most unix systems don’t store users in /etc/passwd anymore
Field breaking
Default is on space and tab and multiple contiguous white space counts as a single white space and leading separators are discarded
Setting separator causes leading separators to be counted
Lecture 22

11. awk (cont.) More on patterns awk ‘NF % 2 != 0’ files
Print user name of people who have no password $2 == “” nd field is empty $2 ~ /^$/ 2nd field matches empty string $2 !~ /./ 2nd field doesn’t match any character length($2) == Length of 2nd field is zero
Pair Up: Write an awk command line that prints lines of input that have an odd number of fields.
awk ‘NF % 2 != 0’ files
Pair Up: Write an awk command line that is the shortest equivalent of cat.
Lecture 22