home | O'Reilly's CD bookshelfs | FreeBSD | Linux | Cisco | Cisco Exam  

sed & awk

sed & awkSearch this book
Previous: 11.4 Epilogue Chapter 12 Next: 12.2 Generating a Formatted Index

12. Full-Featured Applications

This chapter presents two complex applications that integrate most features of the awk programming language. The first program, spellcheck , provides an interactive interface to the UNIX spell program. The second application, masterindex , is a batch program for generating an index for a book or a set of books. Even if you are not interested in the particular application, you should study these larger programs to get a feel for the scope of the problems that an awk program can solve.

12.1 An Interactive Spelling Checker

The UNIX spell program does an adequate job of catching spelling errors in a document. For most people, however, it only does half the job. It doesn't help you correct the misspelled words. First-time users of spell find themselves jotting down the misspelled words and then using the text editor to change the document. More skilled users build a sed script to make the changes automatically.

The spellcheck program offers another way - it shows you each word that spell has found and asks if you want to correct the word. You can change each occurrence of the word after seeing the line on which it occurs, or you can correct the spelling error globally. You can also choose to add any word that spell turns up to a local dictionary file.

Before describing the program, let's have a demonstration of how it works. The user enters spellcheck , a shell script that invokes awk, and the name of the document file.


spellcheck ch00

Use local dict file? (y/n)


If a dictionary file is not specified on the command line, and a file named dict exists in the current directory, then the user is asked if the local dictionary should be used. spellcheck then runs spell using the local dictionary.

Running spell checker ...

Using the list of "misspelled" words turned up by spell , spellcheck prompts the user to correct them. Before the first word is displayed, a list of responses is shown that describes what actions are possible.

	Change each occurrence, 
	Global change, 
	Add to Dict, 
	CR to ignore: 
1 - Found SparcStation (C/G/A/H/Q/):

The first word found by spell is "SparcStation." A response of "a" (followed by a carriage return) adds this word to a list that will be used to update the dictionary. The second word is clearly a misspelling and a response of "g" is entered to make the change globally:

2 - Found languauge (C/G/A/H/Q/):


Globally change to:


Globally change languauge to language? (y/n):


> and a full description of its scripting language. 
1 lines changed. Save changes? (y/n)


After prompting the user to enter the correct spelling and confirming the entry, the change is made and each line affected is displayed, preceded by a ">". The user is then asked to approve these changes before they are saved. The third word is also added to the dictionary:

3 - Found nawk (C/G/A/H/Q/):


The fourth word is a misspelling of "utilities."

4 - Found utlitities (C/G/A/H/Q/):


These utlitities have many things in common, including
Change to:


Change utlitities to utilities? (y/n):


Two other utlitities that are found on the UNIX system
Change utlitities to utilities? (y/n):


>These utilities have many things in common, including
>Two other utilities that are found on the UNIX system
2 lines changed. Save changes? (y/n)


The user enters "c" to change each occurrence. This response allows the user to see the line containing the misspelling and then make the change. After the user has made each change, the changed lines are displayed and the user is asked to confirm saving the changes.

It is unclear whether the fifth word is a misspelling or not, so the user enters "c" to view the line.

5 - Found xvf (C/G/A/H/Q/):


tar xvf filename
Change to:


After determining that it is not a misspelling, the user enters a carriage return to ignore the word. Generally, spell turns up a lot of words that are not misspellings so a carriage return means to ignore the word.

After all the words in the list have been processed, or if the user quits before then, the user is prompted to save the changes made to the document and the dictionary.

Save corrections in ch00 (y/n)? 


Make changes to dictionary (y/n)? 


If the user answers "n," the original file and the dictionary are left unchanged.

Now let's look at the spellcheck.awk script, which can be divided into four sections:

  • The BEGIN procedure, that processes the command-line arguments and executes the spell command to create a word list.

  • The main procedure, that reads one word at a time from the list and prompts the user to make a correction.

  • The END procedure, that saves the working copy of the file, overwriting the original. It also appends words from the exception list to the current dictionary.

  • Supporting functions, that are called to make changes in the file.

We will look at each of these sections of the program.

12.1.1 BEGIN Procedure

The BEGIN procedure for spellcheck.awk is large. It is also somewhat unusual.

# spellcheck.awk -- interactive spell checker
# AUTHOR: Dale Dougherty
# Usage: nawk -f spellcheck.awk [+dict] file 
# (Use spellcheck as name of shell program) 
# SPELLDICT = "dict" 
# SPELLFILE = "file"

# BEGIN actions perform the following tasks: 
#	1) process command-line arguments
#	2) create temporary filenames
#	3) execute spell program to create wordlist file
#	4) display list of user responses

# Process command-line arguments
# Must be at least two args -- nawk and filename
	if (ARGC > 1) {
	# if more than two args, second arg is dict 
		if (ARGC > 2) {
		# test to see if dict is specified with "+"  
		# and assign ARGV[1] to SPELLDICT
			if (ARGV[1] ~ /^\+.*/) 
				SPELLDICT = "+" ARGV[1]
		# assign file ARGV[2] to SPELLFILE 
		# delete args so awk does not open them as files
			delete ARGV[1]
			delete ARGV[2]
	# not more than two args
		else {
		# assign file ARGV[1] to SPELLFILE 
		# test to see if local dict file exists
			if (! system ("test -r dict")) {
			# if it does, ask if we should use it
				printf ("Use local dict file? (y/n)")	
				getline reply < "-"
			# if reply is yes, use "dict" 
				if (reply ~ /[yY](es)?/){
					SPELLDICT = "+dict"
	} # end of processing args > 1 
	# if args not > 1, then print shell-command usage 
	else {
		print "Usage: spellcheck [+dict] file"
		exit 1
# end of processing command line arguments
# create temporary file names, each begin with sp_
	wordlist = "sp_wordlist"
	spellsource = "sp_input"
	spellout = "sp_out"

# copy SPELLFILE to temporary input file
	system("cp " SPELLFILE " " spellsource)

# now run spell program; output sent to wordlist
	print "Running spell checker ..."
		SPELLCMD = "spell " SPELLDICT " "
		SPELLCMD = "spell "
	system(SPELLCMD spellsource " > " wordlist )

# test wordlist to see if misspelled words turned up
	if ( system("test -s " wordlist ) ) {
	# if wordlist is empty (or spell command failed), exit
		print "No misspelled words found."
		system("rm " spellsource " " wordlist)

# assign wordlist file to ARGV[1] so that awk will read it.	
	ARGV[1] = wordlist

# display list of user responses 
	responseList = "Responses: \n\tChange each occurrence," 
	responseList = responseList "\n\tGlobal change," 
	responseList = responseList "\n\tAdd to Dict,"  
	responseList = responseList "\n\tHelp," 
	responseList = responseList "\n\tQuit" 
	responseList = responseList "\n\tCR to ignore: "
	printf("%s", responseList)

} # end of BEGIN procedure

The first part of the BEGIN procedure processes the command-line arguments. It checks that ARGC is greater than one for the program to continue. That is, in addition to "nawk," a filename must be specified. This file specifies the document that spell will analyze. An optional dictionary filename can be specified as the second argument. The spellcheck script follows the command-line interface of spell , although none of the obscure spell options can be invoked from the spellcheck command line. If a dictionary is not specified, then the script executes a test command to see if the file dict exists. If it does, the prompt asks the user to approve using it as the dictionary file.

Once we've processed the arguments, we delete them from the ARGV array. This is to prevent their being interpreted as filename arguments.

The second part of the BEGIN procedure sets up some temporary files, because we do not want to work directly with the original file. At the end of the program, the user will have the option of saving or discarding the work done in the temporary files. The temporary files all begin with "sp_" and are removed before exiting the program.

The third part of the procedure executes spell and creates a word list. We test to see that this file exists and that there is something in it before proceeding. If for some reason the spell program fails, or there are no misspelled words found, the wordlist file will be empty. If this file does exist, then we assign the filename as the second element in the ARGV array. This is an unusual but valid way of supplying the name of the input file that awk will process. Note that this file did not exist when awk was invoked! The name of the document file, which was specified on the command line, is no longer in the ARGV array. We will not read the document file using awk's main input loop. Instead, a while loop reads the file to find and correct misspelled words.

The last task in the BEGIN procedure is to define and display a list of responses that the user can enter when a misspelled word is displayed. This list is displayed once at the beginning of the program as well as when the user enters "Help" at the main prompt. Putting this list in a variable allows us to access it from different points in the program, if necessary, without maintaining duplicates. The assignment of responseList could be done more simply, but the long string would not be printable in this book. (You can't break a string over two lines.)

12.1.2 Main Procedure

The main procedure is rather small, merely displaying a misspelled word and prompting the user to enter an appropriate response. This procedure is executed for each misspelled word.

One reason this procedure is short is because the central action - correcting a misspelled word - is handled by two larger user-defined functions, which we'll see in the last section.

# main procedure, executed for each line in wordlist.
#	Purpose is to show misspelled word and prompt user
#	for appropriate action.

# assign word to misspelling
	misspelling = $1 
	response = 1
# print misspelling and prompt for response
	while (response !~ /(^[cCgGaAhHqQ])|^$/ ) {
		printf("\n%d - Found %s (C/G/A/H/Q/):", word, misspelling)
		getline response < "-"
# now process the user's response
# CR - carriage return ignores current word 
# Help
	if (response ~ /[Hh](elp)?/) {
	# Display list of responses and prompt again.
		printf("%s", responseList)
		printf("\n%d - Found %s (C/G/A/Q/):", word, misspelling)
		getline response < "-"
# Quit
	if (response ~ /[Qq](uit)?/) exit
# Add to dictionary
	if ( response ~ /[Aa](dd)?/) { 
		dict[++dictEntry] = misspelling
# Change each occurrence
	if ( response ~ /[cC](hange)?/) {
	# read each line of the file we are correcting
		newspelling = ""; changes = ""
		while( (getline < spellsource) > 0){
		# call function to show line with misspelled word
		# and prompt user to make each correction 
		# all lines go to temp output file
			print > spellout
	# all lines have been read 
	# close temp input and temp output file
	# if change was made
		if (changes){ 
		# show changed lines
			for (j = 1; j <= changes; ++j)
				print changedLines[j]
			printf ("%d lines changed. ", changes) 
		# function to confirm before saving changes
# Globally change
	if ( response ~ /[gG](lobal)?/) {
	# call function to prompt for correction
	# and display each line that is changed.
	# Ask user to approve all changes before saving.
} # end of Main procedure

The first field of each input line from wordlist contains the misspelled word and it is assigned to misspelling . We construct a while loop inside which we display the misspelled word to the user and prompt for a response. Look closely at the regular expression that tests the value of response :

while (response !~ /(^[cCgGaAhHqQ])|^$/)

The user can only get out of this loop by entering any of the specified letters or by entering a carriage return - an empty line. The use of regular expressions for testing user input helps tremendously in writing a simple but flexible program. The user can enter a single letter "c" in lower- or uppercase or a word beginning with "c" such as "Change."

The rest of the main procedure consists of conditional statements that test for a specific response and perform a corresponding action. The first response is "help," which displays the list of responses again and then redisplays the prompt.

The next response is "quit." The action associated with quit is exit , which drops out of the main procedure and goes to the END procedure.

If the user enters "add," the misspelled word is put in the array dict and will be added as an exception in a local dictionary.

The "Change" and "Global" responses cause the program's real work to begin. It's important to understand how they differ. When the user enters "c" or "change," the first occurrence of the misspelled word in the document is displayed. Then the user is prompted to make the change. This happens for each occurrence in the document. When the user enters "g" or "global," the user is prompted to make the change right away, and all the changes are made at once without prompting the user to confirm each one. This work is largely handled by two functions, make_change() and make_global_change() , which we'll look at in the last section. These are all the valid responses, except one. A carriage return means to ignore the misspelled word and get the next word in the list. This is the default action of the main input loop, so no conditional need be set up for it.

12.1.3 END Procedure

The END procedure, of course, is reached in one of the following circumstances:

  • The spell command failed or did not turn up any misspellings.

  • The list of misspelled words is exhausted.

  • The user has entered "quit" at a prompt.

The purpose of the END procedure is to allow the user to confirm any permanent change to the document or the dictionary.

# END procedure makes changes permanent.
# It overwrites the original file, and adds words
# to the dictionary.
# It also removes the temporary files.

# if we got here after reading only one record, 
# no changes were made, so exit.
	if (NR <= 1) exit
# user must confirm saving corrections to file
	while (saveAnswer !~ /([yY](es)?)|([nN]o?)/ ) {
		printf "Save corrections in %s (y/n)? ", SPELLFILE
		getline saveAnswer < "-"
# if answer is yes then mv temporary input file to SPELLFILE
# save old SPELLFILE, just in case
	if (saveAnswer ~ /^[yY]/) {
		system("cp " SPELLFILE " " SPELLFILE ".orig")
		system("mv " spellsource " " SPELLFILE)
# if answer is no then rm temporary input file
	if (saveAnswer ~ /^[nN]/)
		system("rm " spellsource) 

# if words have been added to dictionary array, then prompt
# to confirm saving in current dictionary. 
	if (dictEntry) {
		printf "Make changes to dictionary (y/n)? "
		getline response < "-"
		if (response ~ /^[yY]/){
		# if no dictionary defined, then use "dict"
			if (! SPELLDICT) SPELLDICT = "dict"
		# loop through array and append words to dictionary
			sub(/^\+/, "", SPELLDICT)
			for ( item in dict )
				print dict[item] >> SPELLDICT
		# sort dictionary file 
			system("sort " SPELLDICT "> tmp_dict")
			system("mv " "tmp_dict " SPELLDICT)
# remove word list
	system("rm sp_wordlist")
} # end of END procedure

The END procedure begins with a conditional statement that tests that the number of records is less than or equal to 1. This occurs when the spell program does not generate a word list or when the user enters "quit" after seeing just the first record. If so, the END procedure is exited as there is no work to save.

Next, we create a while loop to ask the user about saving the changes made to the document. It requires the user to respond "y" or "n" to the prompt. If the answer is "y," the temporary input file replaces the original document file. If the answer is "n," the temporary file is removed. No other responses are accepted.

Next, we test to see if the dict array has something in it. Its elements are the words to be added to the dictionary. If the user approves adding them to the dictionary, these words are appended to the current dictionary, as defined above, or if not, to a local dict file. Because the dictionary must be sorted to be read by spell , a sort command is executed with the output sent to a temporary file that is afterwards copied over the original file.

12.1.4 Supporting Functions

There are three supporting functions, two of which are large and do the bulk of the work of making changes in the document. The third function supports that work by confirming that the user wants to save the changes that were made.

When the user wants to "Change each occurrence" in the document, the main procedure has a while loop that reads the document one line at a time. (This line becomes $0.) It calls the make_change() function to see if the line contains the misspelled word. If it does, the line is displayed and the user is prompted to enter the correct spelling of the word.

# make_change -- prompt user to correct misspelling 
#		 for current input line.  Calls itself
# 		 to find other occurrences in string.
# 	stringToChange -- initially $0; then unmatched substring of $0
# 	len -- length from beginning of $0 to end of matched string 
# Assumes that misspelling is defined. 

function make_change (stringToChange, len,	# parameters
	line, OKmakechange, printstring, carets)	# locals
# match misspelling in stringToChange; otherwise do nothing 
  if ( match(stringToChange, misspelling) ) {
  # Display matched line 
	printstring = $0
	gsub(/\t/, " ", printstring)
	print printstring
	carets = "^"
	for (i = 1; i < RLENGTH; ++i)
		carets = carets "^"
	if (len)
		FMT = "%" len+RSTART+RLENGTH-2 "s\n"
		FMT = "%" RSTART+RLENGTH-1 "s\n"
	printf(FMT, carets)
  # Prompt user for correction, if not already defined
	if (! newspelling) {
		printf "Change to:"
		getline newspelling < "-"
  # A carriage return falls through
  # If user enters correction, confirm  
	while (newspelling && ! OKmakechange) {
		printf ("Change %s to %s? (y/n):", misspelling, newspelling)
		getline OKmakechange < "-"
		madechg = ""
	# test response
		if (OKmakechange ~ /[yY](es)?/ ) {
		# make change (first occurrence only)
			madechg = sub(misspelling, newspelling, stringToChange)
		else if ( OKmakechange ~ /[nN]o?/ ) {
			# offer chance to re-enter correction 
			printf "Change to:"
			getline newspelling < "-"
			OKmakechange = ""
	} # end of while loop

   # if len, we are working with substring of $0
	if (len) {
	# assemble it
		line = substr($0,1,len-1)
		$0 = line stringToChange
	else {
		$0 = stringToChange
		if (madechg) ++changes

   # put changed line in array for display
	if (madechg) 
		changedLines[changes] = ">" $0

   # create substring so we can try to match other occurrences
	part1 = substr($0, 1, len-1)
	part2 = substr($0, len)
   # calls itself to see if misspelling is found in remaining part 
	make_change(part2, len) 

  } # end of if

} # end of make_change()

If the misspelled word is not found in the current input line, nothing is done. If it is found, this function shows the line containing the misspelling and asks the user if it should be corrected. Underneath the display of the current line is a row of carets that indicates the misspelled word.

Two other utlitities that are found on the UNIX system

The current input line is copied to printstring because it is necessary to change the line for display purposes. If the line contains any tabs, each tab in this copy of the line is temporarily replaced by a single space. This solves a problem of aligning the carets when tabs were present. (A tab counts as a single character when determining the length of a line but actually occupies greater space when displayed, usually five to eight characters long.)

After displaying the line, the function prompts the user to enter a correction. It then follows up by displaying what the user has entered and asks for confirmation. If the correction is approved, the sub() function is called to make the change. If not approved, the user is given another chance to enter the correct word.

Remember that the sub() function only changes the first occurrence on a line. The gsub() function changes all occurrences on a line, but we want to allow the user to confirm each change. Therefore, we have to try to match the misspelled word against the remaining part of the line. And we have to be able to match the next occurrence regardless of whether or not the first occurrence was changed.

To do this, make_change() is designed as a recursive function; it calls itself to look for additional occurrences on the same line. In other words, the first time make_change() is called, it looks at all of $0 and matches the first misspelled word on that line. Then it splits the line into two parts - the first part contains the characters up to the end of the first occurrence and the second part contains the characters that immediately follow up to the end of the line. Then it calls itself to try and match the misspelled word in the second part. When called recursively, the function takes two arguments.

make_change(part2, len)

The first is the string to be changed, which is initially $0 when called from the main procedure but each time thereafter is the remaining part of $0. The second argument is len or the length of the first part, which we use to extract the substring and reassemble the two parts at the end.

The make_change() function also collects an array of lines that were changed.

# put changed line in array for display
        if (madechg)
                changedLines[changes] = ">" $0

The variable madechg will have a value if the sub() function was successful. $0 (the two parts have been rejoined) is assigned to an element of the array. When all of the lines of the document have been read, the main procedure loops through this array to display all the changed lines. Then it calls the confirm_changes() function to ask if these changes should be saved. It copies the temporary output file over the temporary input file, keeping intact the corrections made for the current misspelled word.

If a user decides to make a "Global change," the make_global_change() function is called to do it. This function is similar to the make_change() function, but is simpler because we can make the change globally on each line.

# make_global_change --
#		prompt user to correct misspelling 
#		for all lines globally.  
#		Has no arguments
# Assumes that misspelling is defined. 

function make_global_change(    newspelling, OKmakechange, changes)
# prompt user to correct misspelled word
   printf "Globally change to:"
   getline newspelling < "-"

# carriage return falls through
# if there is an answer, confirm 
   while (newspelling && ! OKmakechange) {
		printf ("Globally change %s to %s? (y/n):", misspelling,
		getline OKmakechange < "-"
	# test response and make change
		if (OKmakechange ~ /[yY](es)?/ ) {
		# open file, read all lines 
			while( (getline < spellsource) > 0){
			# if match is found, make change using gsub
			# and print each changed line.
				if ($0 ~ misspelling) {
					madechg = gsub(misspelling, newspelling)
					print ">", $0
					changes += 1  # counter for line changes
			# write all lines to temp output file
				print > spellout
			} # end of while loop for reading file

		# close temporary files
		# report the number of changes	
			printf ("%d lines changed. ", changes) 
		# function to confirm before saving changes
		} # end of if (OKmakechange ~ y) 

	# if correction not confirmed,  prompt for new word
		else if ( OKmakechange ~ /[nN]o?/ ){
			printf "Globally change to:"
			getline newspelling < "-"
			OKmakechange = ""

  } # end of while loop for prompting user for correction

} # end of make_global_change()

This function prompts the user to enter a correction. A while loop is set up to read all the lines of the document and apply the gsub() function to make the changes. The main difference is that all the changes are made at once - the user is not prompted to confirm them. When all lines have been read, the function displays the lines that were changed and calls confirm_changes() to get the user to approve this batch of changes before saving them.

The confirm_changes() function is a routine called to get approval of the changes made when the make_change() or make_global_change() function is called.

# confirm_changes --  
#		confirm before saving changes

function confirm_changes(  savechanges) {
# prompt to confirm saving changes
	while (! savechanges ) {
		printf ("Save changes? (y/n)")
		getline savechanges < "-"
# if confirmed, mv output to input
	if (savechanges ~ /[yY](es)?/)
		system("mv " spellout " " spellsource) 

The reason for creating this function is to prevent the duplication of code. Its purpose is simply to require the user to acknowledge the changes before replacing the old version of the document file ( spellsource ) with the new version ( spellout ).

12.1.5 The spellcheck Shell Script

To make it easy to invoke this awk script, we create the spellcheck shell script (say that three times fast). It contains the following lines:

nawk -f $AWKLIB/spellcheck.awk $*

This script sets up a shell variable AWKLIB that specifies the location of the spellcheck.awk script. The symbol "$*" expands to all command-line parameters following the name of the script. These parameters are then available to awk.

One of the interesting things about this spell checker is how little is done in the shell script.[1] All of the work is done in the awk programming language, including executing 10 UNIX commands. We're using a consistent syntax and the same constructs by doing it all in awk. When you have to do some of your work in the shell and some in awk, it can get confusing. For instance, you have to remember the differences in the syntax of if conditionals and how to reference variables. Modern versions of awk provide a true alternative to the shell for executing commands and interacting with a user. The full listing for spellcheck.awk is found in Appendix C, Supplement for Chapter 12 .

[1] UNIX Text Processing (Dougherty and O'Reilly, Howard W. Sams, 1987) presents a sed-based spell checker that relies heavily upon the shell. It is interesting to compare the two versions.

Previous: 11.4 Epilogue sed & awk Next: 12.2 Generating a Formatted Index
11.4 Epilogue Book Index 12.2 Generating a Formatted Index

The UNIX CD Bookshelf Navigation The UNIX CD BookshelfUNIX Power ToolsUNIX in a NutshellLearning the vi Editorsed & awkLearning the Korn ShellLearning the UNIX Operating System