UNIX Filters

Some Simple UNIX Filters(Commands That Use Both Standard Input &

Standard Output)

$ pr$ cmp, comm, diff$ sort$ uniq

$ head,tail$ cut, paste$ tr$ grep

File Level Content Level

There are a lot of others!!

Formatting Output: pr Command

pr prepares files for printing by adding formatting, headers, footers, …

Some options: -k prints in k columns

-n numbers the lines of output

-d double spaces output

-l n sets length to n lines

-w m sets width to m chars

Example: a.out | pr –n –d –l 64 Prints output with line

numbers, double spacing,

and 64 lines per page

Comparing Files: cmp Command

cmp compares 2 files and stops when it finds a difference

The comparison is character by character (byte by byte).

Option: -l lists all byte differences in the files

Examples: cmp file1 file2

file1 file2 differ: char 12, line 3

cmp –l file1 file2 | wc –l

Displays the number of differences in the 2 files.

Comparing Files: comm Commandcomm compares 2 files and lists 3 columns of information:1. lines unique to 1st file 2.lines unique to 2nd file 3. lines common to both files

The files must be sorted.

Options: -1, -2, -3 indicates the columns to drop in the output

Examples: comm -3 file1 file2 Lists all the unique lines in

both files.

comm –l2 file1 file2 Lists all the lines common to

both files.

Comparing Files: diff Command

diff compares 2 files and lists the instructions needed

to make the files the same. Here’s an example:

$ diff file1 file2 3c3 Change line 3

< This is line 3 of file 1. from this

--- > This is line 3 of file 2. to this.

7a8 > This is line 8 from file 2. Add this line after

line 7 of file1.

Extracting Vertical Data: cut Commandcut extracts vertical slices of data from a file. Either columns (-c

option) or fields (-f option) of data may be extracted. A delimiter (-d

option) may be defined to separate fields. Default delimiter is tab.

Examples: cut –c1-4,8,15- file1 Extracts chars 1 thru 4, the 8th

character, and characters 15 thru end of each line from file1. No

whitespace in the column list!!

cut –d: -f1-3 file2 Extracts fields 1 thru 3 from file2. The

fields are separated by the : character

who | cut –d” “ –f1 Lists the names of all users logged in.

Joining Vertical Data: paste Command

paste vertically joins 2 files together. A delimiter (-d option)

may be defined to separate fields. Default delimiter is tab. The

-s option joins lines of a single file together.

Examples: paste file1 file2 Displays file1 and file2 side by side.

paste –s –d”::\n” addressbook

rick

rick@att.com >>>> rick:rick@att.com:1234567890

1234567890

Displaying Files: head and tail Commands

head displays the top of a file. (1st 10 lines, by default).

tail displays the end of the file (last 10 lines, by default).

Options: -n x or –x displays 1st (last) x lines of the file.

-f continuously displays the end of a file as it grows. This

option is for the tail command only. You must use the

interrupt key to stop monitoring the file growth.

Examples: ls –t | head –n 1 Displays the file most recently edited.

tail –f install.log Continuously displays the log file as

it grows. Use interrupt key to stop.

Ordering Files: sort Command

sort reorders the lines of a file in ascending (descending) order.

The default order is ASCII: whitespace, numbers, uppercase, and

finally, lowercase letters.

Options: -k n sort on the nth field of the line

-tchar use char as the field delimiter

-n sort numerically

-r reverse order sort

-u remove repeated lines

-m list merge sorted files in list

sort Examples

Examples: sort –t: -k 2 list Sort on the 2nd field of file list. Fields are

separated by :

sort –t: -k 5.7 –r list Sort file list in reverse order on the

7th character of 5th field. Fields separated by :

sort –n list Numerically sort file list, assumed to contain numbers.

sort –m file1 file2 Sorted files file1 and file2 are merged.

cut –d: -f3 list | sort –u Extract the 3rd field from list &

sort that field, removing the repeated lines.

Removing Duplicates: uniq Commanduniq displays a presorted file, removing all the duplicate lines from it.

If 2 files are specified, uniq reads from the first and writes to the second.

Options: -u lists only the lines that are unique

-d lists only the lines that are duplicates

-c counts the frequency of occurrences

Examples: sort list | uniq – xlist Sorts file list; uniq reads from

stdin and writes the output to xlist.

uniq –c list Displays count of each unique line in the file list.

Character Manipulation: tr Command

tr translates characters from one format to another. Input always comes

from standard input. Arguments don’t include filenames. General form is:

tr options expression1 expression2 standard input

expression1 is the set of characters to change; expression2 is what

they change to. (The expressions should be equal length.)

Examples: tr ‘+-’ ‘*/’ < math In the file math, replace all +’s with *’s and

replace all -’s with /’s.

head –n 3 list | tr ‘[a-z]’ ‘[A-Z]’ The 1st 3 lines

of the file list are translated to uppercase.

tr Command Options

Options: -d delete characters from the input stream

-s compress multiple consecutive characters (squeeze)

-c complementing value of expression

Examples: tr –d ‘/’ < dates Remove all /’s from the file dates.

tr –s ‘ ’ < names Replace all strings of blanks with a single

blank in the file names.

tr –cd ‘:’ < file1 In the file file1, delete everything that isn’t

a colon (:). All that’s left is a file full of :’s

Finding Patterns in Files with grepgrep searches a file and displays the lines containing a pattern. Form is: grep options pattern filesIf more than 1 file is listed, the filename is also displayed in the output.

Some options: -i ignore case when matching

-n display line numbers as well as lines

-c displays a count of the number of occurrences

Examples: grep “professor” college.lst Displays all lines in file, college.lst, that contain the string professor.

grep –i “Rick” college.lst Displays all lines in the file, college.lst, that contain the string Rick. (Also finds rick, RICK, rIcK, …)

Regular Expressions in grepRegular expressions are metacharacter patterns used in ways differentfrom how the shell uses them.

Regular Expression Meaning

* 0 or more of the previous character

. a single character

[pqr] a single p or q or r

[c1-c2] a single char in the ASCII range of c1 thru c2

[^pqr] a single character not p nor q nor r

^abc abc at the beginning of line

abc$ abc at the end of the line

Example Regular Expressions with grep

grep “g*” file1 Displays all lines in file1 that contain nothing or g, gg, ggg, …

grep “.*” file1 Displays all lines in file1 that contain nothing or any # of chars

grep “[1-3]” file1 Displays all lines in file1 that contain a digit between 1 & 3.

grep “[^a-zA-Z]” file1 Displays all lines in file1 that contain a non-alphabetic character

grep “^Rick$” file1 Displays all lines in file1 that contain only Rick

grep “^$” file1 Displays all lines in file1 that contain nothing

grep “R[aeiou]ck” file1 Displays all lines in file1 that contain Rack, Reck, Rick, Rock or Ruck

Putting It All Together

An author wants to count the frequency of words used in a book chapter.

1. Put each word on a separate line:

tr “ \011” “\012\012” < chapter

2. Strip out everything that isn’t an alphabetic character or newline:

tr -cd “[a-zA-Z\012]”

3. Sort the list: sort

4. Count the word frequency: uniq –c

5. Put it all together:

tr “ \011” “\012\012” < chapter |

tr –cd “[a-zA-Z\012]” | sort | uniq -c