CS 403: Programming Languages Fall 2004 Department of Computer Science University of Alabama Joel Jones

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CS 403: Programming Languages

Fall 2004

Department of Computer Science

University of Alabama

Joel Jones


Other Filters (cont.) tr inputChars outputChar(s)

tr a-z A-Z maps lower case to upper case Flags: -s squeezes multiple occurences of a

character in the input to a single character in the output; -c takes the complement of the first argument, e.g. tr -c ab matches every character except a and b. tr also understands character ranges.

uniq removes duplicate adjacent lines Flags: -c adds count of duplicate lines at beginning

‘\012’ is a new linePair Up: Write a pipeline that prints the 10 most frequent words in its input.


Printing 10 most common wordscat $* | # tr doesn’t take filename argumentstr -sc A-Za-z ‘\012’ | # all non alpha become newlinesort |uniq -c | # get the countsort -n | # sort by counttail # prints 10 by default

Use the man command to look at for help man sort


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


sed (cont.)

sed ‘s/^/ /’ file

Pair Up: Write a sed command line that indents a line by adding four spaces to the beginning of the line

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


sed (cont.) Commands (cont.)

/re/d deletes lines matching re /re/p prints lines matching re

Options -n turns off automatic printing -f filename takes sed commands from filename

Pair Up: What does the sed command sed ‘/the/p’ < file do?

Pair Up: Write a sed command line that does the same thing as:

grep re file

sed -n ‘/re/p’ file


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


awk (cont.)

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 linePair Up: Write an awk command line that does the same thing as:

grep re file

awk ‘/re/’ filePair Up: Write an awk command line that does the same thing as:

cat filenames …

awk ‘{ print }’ filenames …


awk (cont.)

Variables $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.


awk (cont.)

awk -F: ‘{ print $1 }’ /etc/passwd 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


awk (cont.)

More on patterns Print user name of people who have no password$2 == “” 2nd field is empty$2 ~ /^$/ 2nd field matches empty string$2 !~ /./ 2nd field doesn’t match any characterlength($2) == 0 Length of 2nd field is zeroPair Up: Write an awk command line that prints lines of input that have an

odd number of fields.

awk ‘NF % 2 != 0’ filesPair Up: Write an awk command line that is the shortest equivalent of cat.

awk '/^/' files

Documents

CS 403: Programming Languages Fall 2004 Department of Computer Science University of Alabama Joel Jones