Contribute to marshalljj/book development by creating an account on GitHub. Regular expressions are accepted input for grep, sed, awk, perl, vim and other unix commands. • It's all about syntax. (and because it's UNIX. Linux Systems Administration: Sed, Awk, and Perl. Lesson 1: Int roduct ion t o Script ing. Before We Start Coding. Logging Into the Server. What is Scripting?.
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Nutshell Handbook, the Nutshell Handbook logo, and the O'Reilly logo are registered trademarks of O'Reilly Media, Inc. sed & awk, Second Edition, the image of. All editing commands in a sed script are applied in order to each input line. • If a command changes the input, subsequent command address will be applied to. grep, awk and sed. – three VERY useful command-line utilities. Matt Probert, Uni of York grep = global regular expression print. In the simplest terms, grep.
Someone who attempts to bridge the two has a ton of work to do, both in the research and in the implementation department. This could be used to re-typeset the dictionary and to keep an updated version of the jlatex file as reference for all other components described in this section. Disregard applying the program to multiple files. An analysis like this is perfectly suited for the computer, however. The source was available in two formats: That's really the only reason I left those out. Awk sometimes uses it, sometimes not.
Next, type aizu in your shell to see what the program does. The first line! Thus, you do not need to switch to sh in order to execute a command file for sh. The second line is comment. Comment for sh, sed and awk starts by definition with a as first character in a line. Note, that comment usually does not work within, e. The echo command does what its name says: Actually, an on-line notebook as aizu at the workplace is quite convenient.
Compare the discussion of cat given below. Strings can be concatenated: To see that this works, try: The following example shows how to connect the output of aizu with another command grep in the pipe mechanism of UNIX: The important point in the above sh program is the single vertical slash.
The combination of aizu with grep fax - in this way is called a pipe which is seen by sh as a single UNIX command. One can connect any number of commands through in a pipe. The hyphen - used above stands for the virtual input file of grep in the pipe. In UNIX terminology, it is called stdin standard input.
In many cases, it can be omitted. If you are in doubt, then just include it. Usually, sh interprets reasonable expressions in a reasonable way. For better readability of programs, one may wish to spread pipes over several lines.
The following example does exactly the same as aizufax: The pattern x is sufficient to identify the second line of output of aizu, and the hyphen denoting stdin is omitted.
Usually, a UNIX command does something with a file. In that case, the name of the file is one of the arguments that is passed on. Arguments to a UNIX command are strings separated by white space consisting of blanks or tabs. The following example shows how this is achieved. Type the following four command lines in a shell without saving them to a file: For sh, the function of the literal newline character is that of the blank if it is not embedded in a string.
Moreover, cat file1 file The result can, e. Consult man cat for more details. A sed program operates on a file line-by-line. If nothing is done with a line, then it is simply copied. These are delimiters for sh and are not communicated to sed. The second substitution command shows the important technique of how to place newline characters at specific places. Then, do the following in a shell: With a one-line sed substitution as above, all occurrences of something that should be maintained only at one place e.
With a sed program similar to the above, one can reformat text which uses a non-standard phonetic, phonemic, or typographic transcription. In such a program, one is not limited to global substitution commands: Such a reformatted text can then be compared to other sources in regard to use of words, phrases and morpho-phonemic structure.
At least in part, this can be also done by machine using sed and awk. The latter sort of analysis will be outlined below in more detail. With a sed program similar to the above, it is also possible to convert phonetic transcription alphabets, e. If a sed command is terminated by a semicolon, then the next sed command can follow on the same line.
There is one exception to this: After a w command and some separating white space, everything that follows on the same line is understood as the filename to which the command is supposed to write. One can also store the sed commands listed above without the two single quotes but with the backslash in a file say sedCommands and use sed -f sedCommands instead of or in the above sh program. Observe that if a sed program is used with a separate file of commands in a UNIX pipe, then this makes reading an additional file necessary.
This may slow down the overall process. The replacement in a substitution can be empty. This can, e. Roughly speaking, it can be seen as the current, possibly already altered line. More precisely, every line of input is put into the pattern space and is worked on therein by every command line of the entire sed program from top to bottom.
This is called a cycle. After the cycle is over, the resulting pattern space is printed. Lines that were never worked on are consequently copied by sed. Each sed command that is applied to the content of the pattern space may alter it. In that case, the previous version of the content of the pattern space is lost. Subsequent sed commands are always applied to the current content of the pattern space and not the original input line. There is a separate print command p for printing the pattern space.
The second buffer used by sed is the hold space. The pattern space can be stored in the hold space for, e. The hold space is not erased if a new cycle is begun.
The content of the pattern space can be overwritten by the content of the hold space. In addition, appending one of the two buffers to the other is possible. If pce is applied to a file, then, first, all strings george in a line are replaced by strings bill.
These two actions comprise the cycle per line. AddressCommand Address can be omitted. In that case, Command is then applied to every pattern space. If an Address is given, then Command is applied to the pattern space only in the case the latter matches Address. Patterns are matched by sed as the longest, non-overlapping strings possible.
As already illustrated above, some of the sed commands allow the placement of newline characters in the pattern space. All special characters: They must not be repeated in the replacement in a substitution command. The following five rules must be observed: The backslash only represents itself.
The closing bracket ] must be the first character in what in order to be recognized as itself. Ranges of the type a-z, A-Z, in what are permitted. The hyphen - must be at the beginning or the end of what in order to be recognized as itself. The rules R1—R4 set under 5 also apply here. As indicated in the last two examples, patterns are matched by sed as the longest, non- overlapping strings possible. If one wants to process overlapping pattern, then one can use the t command described below.
In the next sections, we shall explore the possibilities in using patterns in substitution commands. This is in our experience the most frequent use of patterns. Patterns as addresses and other types of addresses will be discussed afterwards. Alternatively, the code given below may be included in larger sed programs when needed. However, dividing processes into small entities as given in the examples below is a very useful technique to isolate reusable components and to avoid programming mistakes resulting from over-complexity of single programs.
In what follows, we shall refer to this program as addBlanks. All ranges in the sed program contain a blank and a tab. Then, a single blank is placed at the beginning and the end of the pattern space.
Finally, any resulting white pattern space is cleared from blanks and tabs in the last substitution command. To identify the first liberal, sed needs the blank in the string which is then not available to identify the second.
Recall that sed matches non-overlapping patterns. Instead of repeating the first pattern, one could loop over it once. Looping with sed will be explained below. If one preprocesses the source file with addBlanks, only the first pattern is needed once.
Thus, a sed based search program for Liberal and liberal is shortened and faster. The following program is a variation of addBlanks. It can be used to isolate words in text in a somewhat crude fashion. In fact, abbreviations and words that contain a hyphen or an apostrophe are not properly identified. The white ranges in the sed program contain a blank and a tab each. Then, a single blank is placed at the beginning and the end of a line.
In what follows, we shall refer to this program as adjustBlankTabs. Every range contains a blank and a tab. All white strings are replaced by a single blank in the last substitution command. This is useful if one wants to analyze sentences and, e. The following program folds all lines in a text inserts newline characters after the first string of blank or tabs following every string of at least 10 characters.
All ranges contain a blank and a tab. A newline character is inserted in the pattern space after every sequence of characters specified in the combined pattern. Some editors allow sending files via e-mail from within the editor. This leads to particularly long lines in e-mail messages which, e. If one intends to process such e-mail messages automatically, then a customized version of the above program that folds after characters can be used to counter this effect.
It can be used for extending, deviding and rearranging patterns and their parts. More detail about the usage of tagged regular expressions is given in the following examples: The following program shows a first application of the techniques introduced so far. We shall refer to it as markDeterminers. The first substitution command replaces the triple period as, e. After the tagging is completed, the triple period is restored. For example, the string "A liberal? A note on addBlanks: Instead of using addBlanks one may be tempted to work, e.
However, this substitution command causes the string "Another? A collection of tagging programs such as markDeterminers can be used for ele- mentary grammatical analysis. The following program shows how one can properly identify words in text. We shall refer to it as leaveOnlyWords in the sequel. This is the longest program listing in this paper.
Next lines , strings of the type letters. For example, v. Next lines comes a collection of substitution commands that replaces the period in standard abbreviations with an underscore character. Then line 7 , all period characters are replaced by blanks and subsequently all underscore characters by periods. Next line 8 , every apostrophe which is embedded in between two letters is replaced by an underscore character.
All other apostrophes are then replaced by blanks, and subsequently all underscore characters are replaced by apostrophes. Finally line 9 , the hyphen is treated in a similar way as the apostrophe. We shall refer to this program as doubleLetterWords. In the second substitution command, all unmarked words are deleted. To illustrate this by an example consider the following: Finally, the underscore char- acters are deleted.
Use only one tagged regular expression for the latter. In that case, retain also the word that follows the word containing the string ing. In what follows, we shall refer to this program as hideUnderscore. In what follows, we shall refer to this inverse program as restoreUnderscore. Observe that sed scannes the pattern space from left to right.
This technique has already been demonstrated above in leaveOnlyWords and doubleLetterWords. Framed by underscore characters, these keywords are easily distinguishable from regular words in the text. Another application is to recognize the ending of sentences in the case of the period character. The period appears also in numbers and in abbreviations. By first replacing the period in the two latter cases by an underscore character and then interpreting the period as marker for the ending of sentences is, with minor additions, one way to generate a file which contains whole sentences per line.
In that case, only the format of numbers changes occasionally. Usually, the format of numbers in text sources is not checked for the purpose of language analysis. As outlined at the end of the last section, one has to implement the following steps: This can be achieved by a program such as hideUnderscore.
This has to be done in such a way that the pattern matching done under 3 does not apply to the special cases marked here. Thus, rearrangement of tagged regular expressions is possible in the replacement in a substitution command. The following program acts on short sentences on single lines. Line 42 is the last line that is put into the pattern space and is processed copied by quitting. Consult man more and man less for alternatives to the above program.
Line numbers are cumulative over several files to which a sed program is applied. For example, the following two lines are the same: The following program deletes the first and the last two lines in a file: Consult man tail and man less for alternatives to the above program.
The addresses 1 resp. Address1 specifies where on which line resp. Address2 specifies where actions end. The following program indents the code by two blanks. In fact, non-empty code lines are indented only. All white ranges contain a blank and a tab. The period in the line addressed by the range matches only the first character in a non- empty pattern space since there is no g trailing the substitution command. The source code for this document contains several test programs for the claims made about sed commands in the next section.
These programs are eliminated from the document through preprocessing with a one-line sed program. This is done in a similar fashion as above using a begin and an end address and the delete command d addressed by the range begin,end. In it, the number at the end of each section is the number of addresses possible.
Usually, the command labeled by an address range is executed for every line in the range. We shall mention those commands that behave differently. The others may be skipped on first reading. LastLine append: This prints Line1 through LastLine at the end of the current cycle, i. What is appended is not put into the pattern space and not subject to the following sed commands.
The appended text is printed even if the pattern space is deleted afterwards in the cycle or the quit command is executed. Branch to the: If there is no whereTo, then branch to the end of the script.
In that case, another b command has to be used to leave the loop. Or, an address in front of the b com- mand must deactivate the loop eventually. LastLine change: This prints Line1 through LastLine. The current content of the pattern space is deleted, and a new cycle is started. Consequently, what is printed is not subject to the following sed commands. If an address range is given, then printing is done at the end of the address range.
However, the current content of the pattern space is deleted for the full address range. Thus, with an address range one can exchange, e. The initial segment of the pattern space through including the first newline character is deleted, and a new cycle is started with the remaining pattern space. In those cases, a newline character separating old and new is appended first.
The following program results in an endless loop: Replace the contents of the pattern space by the contents of the hold space. If the hold space is empty, then this results in an empty pattern space. This is useful for repeated analysis of the original input line which can be stored in the hold space with the command h.
Append the contents of the hold space to the pattern space. This in- cludes appending a newline character first separating old and new. Replace the contents of the hold space by the contents of the pattern space. Storing the pattern space in the hold space makes it possible to reinvestigate the original line or an intermediate state of the pattern space.
Append the contents of the pattern space to the hold space. This includes appending a newline character first separating old and new.
Both, G and H add newline characters while appending. Thus, an H-G sequence may create many empty lines due to double newline characters. This prints text before the current content of the pattern space is processed and possibly printed with the sed program. What is inserted is not subject to the following sed commands. In connection with the first line address 1, the i command can be used to prepend something to a document.
This lists the pattern space on the output in an unambiguous form. This can be used to identify Japanese characters [Lunde ] in bilingual text.
This prints the pattern space. In addition, the next line of input is put into the pattern space. The current line number changes. However, a new cycle is not started from the top. Instead, the sed program is continued at the current program line for the pattern space with the new content.
If there is no interference by other commands, then the switch by the n command in the pattern space is done for every second line of input. In the case of an address range, the addresses will only work, if the pattern space is matched before the n command is executed. Compare the example given next. If sed is invoked as sed -n, then printing is suppressed, and only the next line of input is put into the pattern space. The n command behaves as follows: The lines 2, 4 and 6 were only subject to the first substitution command.
The lines 3, 5 and 7 were only subject to the final substitution command. Note that 7ay was obtained after 6bxE. This shows that the n command may have consequences one line beyond an address range associated with it. The 8by in the output shows that executing n stopped at 6bxE since both substitution commands were applied to the line containing 8ax. In contrast to that, 1ax 2axS 3ax 4ax 5axE 6ax 7ax 8ax yields 1by 2bxS 3ay 4bx 5ayE 6bx 7ay 8bx.
This appends the next line of input to the pattern space with an embedded newline character separating old and new. As above, N has an effect one line beyond a range and can miss an address, if the line matching the address is appended. If there is an attempt to append something beyond the the end of the file, then sed quits and misses processing and printing the last pattern space.
Thus, in the usual sed mode one gets an additional line of output if the pattern space is not deleted afterwards. However, the default printing by sed can be switched off by invoking it as sed -n.
This prints the initial segment of the pattern space through the first newline character. Print the current pattern space and terminate the sed program. Copy the file filename to the output at the end of the current cycle. What is copied is not put into the pattern space. Copying is done even if the current pattern space is deleted or the q command is executed afterwards in the cycle. If no n or N commands are used, then the copying is done before processing the next input line. Substitute pattern with replacement.
Substitute only for the nth occurrence of the pattern. Substitute globally for all non-overlapping occurrences of the pattern rather than just the first one.
Print the pattern space, if a substitution was made. Append the pattern space to the file filename, if a substitution was made. One can print to at most 10 different files. In case one has to use more files, one can split the sed program and use a pipe in which every piece uses up to 10 files.
Larger text files that are processed may contain exceptional cases to patterns that are manipulated. Branch to the label: If there is no whereTo, then start a new cycle. Creating loops with the t command can be used to re substitute in overlapping patterns. It can also be used for reprocessing if the pattern in a particular preceeding substitution is possibly generated by a subsequent substitution.
Append the pattern space to the file filename. The w command can be used to sort pieces of a file into several files. After a w command, everything that follows after some white space on the same line is understood as the filename to which the command is supposed to write.
Thus, after a w command no other command can follow on the same line. Exchange the pattern and the hold space. The lengths of string1 and string2 must be equal.
Ranges are not allowed. A substitution for it can be achieved by an additional s command. The y command can, e. Address for the b whereTo and t whereTo commands. Print the current line number of the input file on a separate line.
As with the p command, printing is done immediately. Commands can be on separate lines or be separated by semicolons ;. Using a framing pair of parentheses, a non-address range command such as i can be applied to a range. The following is legal: Note the semicolon termi- nating the s command. Do not execute function if the pattern space matches the address pattern preceeding!. An address range is only allowed if function allows it. For example, a header containing an address may be inserted in a document several times.
Or, a certain piece of code such as the declaration of a standard set of variables is used in many function definitions. This should be done only if the headers resp. If a header or footer is always added to a document, then using the UNIX command cat mentioned above together with separate files that contain the additions is best. The following program is another useful variation of addBlanks. It isolates non-white strings of characters in a text and puts every such string on a separate line.
We shall call this oneItemPerLine in the sequel. For non-white lines, white characters at the beginning and the end of lines are removed. Finally, all remaining strings of white characters are replaced by newline characters.
The following program finds all four-letter-words in a text. We shall refer to it as findFourLetterWords in the sequel. This will occur only if a four-letter-word was found on a line. The second sed program merges corresponding numbers and lines: The following program sorts all characters 0 zero to the right of a line. This shows the typical use of the t command.
The second command exchanges all characters 0 with a neighboring non-zero to the right. The last command tests whether or not a substitution happened. If a substitution happened, then the cycle is continued at: Otherwise, the cycle is terminated. In the course of the investigation in [Abramson et al. This raised the following problems: Such control sequences had to be removed. This was done using substitution commands with empty re- placements. Some of the control sequences in the source are important in regard to the database which was generated.
In [Nelson ], Japanese is represented using kanji, kun pronuncia- tion and on pronunciation. The on pronunciation of kanji is typeset in italics characters. In the source file, the associated text is framed by a unique pair of control sequences.
Similarly, the kun pronunciation of kanji is represented by small caps printing. Though quite regular already, it contains a certain collection of describable irregularities. For example, the ranges of framing pairs of control sequences overlap some- times. In order to match kun pronunciation and on pronunciation in the source file of [Nelson ] properly, a collection of commutation rules for control sequences was implemented to achieve that the control sequences needed for pattern matching only frame a piece of text and no other control sequences.
These commutation rules were implemented in a similar way as the last example shows. We shall refer to this program as quadrupleWords. The third command tests whether or not a substitution happened. If no substitution happened, then the cycle is continued in the next line of the program. In the last line, all pattern spaces are deleted that do not contain a triple underscore character corresponding to a quadruple word in the original line of input.
In the last command of the program, everything after the first word is deleted. This will be outlined below in greater detail. We shall refer to it as sortByVowel in the sequel. An alternative is to use the UNIX rm-command.
Consult man rm for more details. Observe the use of the single quotes. Note that output by the w command is always appended to an existing file. Thus, the files have to be removed or empty versions have to be created in case the program has been used before on the same file. There is no direct output by this UNIX command. It is clear how to generalize this procedure to a more significant analysis, e.
Recall that everything after a w command and separating white space until the end of the line is understood as the filename the w command is supposed to write to. We shall refer to the following program as mapToLowerCase. It does what its name says. The latter pro- cedure was applied to short essays submitted by Japanese students via e-mail as homework. We were subsequently interested in selecting student-generated example sentences containing a specific problematical pattern for presentation in class.
The next two examples show such selection procedures. Also consult man grep. The grep-family of filters is designed to find lines in a file that match a certain pattern.
We shall refer to it as printPredecessorBecause in the sequel. Also consult man grep in regard to the options -n n a positive integer , -A, -B, and -C. Next, the new pattern space containing the previous line is printed by p. Then, the pattern space is overwritten by g with the current line which is also printed by p. The b command terminates the cycle. Write an awk program that does the same as the latter program.
Disregard applying the program to multiple files. Print the filename using echo. Use tagged regular expressions in order to recognize the double words. Be aware of properly processing the last line in connection with the N command. Such action can preceed the use of the generated program. Alternatively, the generation of a program and its subsequent use are part of a single UNIX command. The latter possibility is outlined next. For example, the words the, a, an, if, then, and, or, We shall refer to the following program as eliminateList in the sequel.
This is done since periods and hyphens are special characters in sed. For example, we have implemented a filter hideAbbreviations and its left inverse filter. The first recognizes abbreviations of the sort v. The second part is generated from a list of strings containing, e.
Out of those, replacement commands are generated that replace, e. This filter is used in the program that reformats essays such that lines contain whole sentences. The latter procedure will be explained below in greater detail.
Different types of words can be tagged or replaced by their grammatical type by automatically generated sed programs. For example, one could have a file verbs containing selected verbs and generate a program marking those verbs in text.
If one processes larger files, then, possibly, one should generate C programs based upon lex to perform searching and tagging. Note that a C program P1 , in which a list of words to search for is encoded, is as fast as a C program P2 , that has to read a list of words it is supposed to identify. Also, tagging can be done in regard to specialized word lists such as [Orr et al.
A collection of such search programs can be used for analysis of grammatical patterns in texts involving selected verbs, nouns and other components. By tagging a given word list, the foreign language teacher is able to do searches for grammatical trouble spots. Numerous tagging schemes are currently in use in large-scale corpora cf. Most of these are extremely detailed schemes used to explore corpora consisting of tens of millions of words.
Corpus linguists involved in such research require very high accuracy. However, the average foreign language teacher, working with a word list of probably words likely built up over time requires a great deal less sophistication.
A rough and very general tagging scheme like the one shown in markDeterminers is enough for most practical applications in which the human end-user can correct a small number of exceptional cases.
Such statistics are useful for a language teacher in determining which patterns students feel more secure about using i. Avoidance is a difficult aspect of language use to measure. However, using a program which analyzes sentence patterns, prints like patterns in files, and keeps statistics regarding fre- quency of use, patterns which students rarely use would be immediately apparent to the teacher.
Searching for grammatical patterns can also be used to select example sentences from a database of, e. Using the set and vector operations defined below, patterns that are used can be measured against patterns that are desirable and were introduced in class.
Usually, an input record is an input line. The chunks of the input record are called fields and, usually, are the full strings of non-white characters.
In contrast to sed, awk allows string variables and numerical variables. Consequently, one can accomplish operations on files such as accounting and keeping statistics of things. Another typical use of awk is matching and rearranging the fields in a line. Good introductions to awk are [Aho et al. In what follows, we shall sometimes include an awk version of a procedure implemented above with sed. This allows adaptation of the procedures such as inclusion into other sed or awk programs under different circumstances.
An awk program looks as follows: Or an awk program can consist of, e. As shown above, any awk command or awk statement has the following format: If a semicolon follows an awk statement, then another statement can follow on the same line.
One can also store a list of awk commands in a file say awkCommands and use awk -f awkCommands to invoke the program. However, more complicated address patterns are also possible. Compare the two listings given below. The input record is put into a pattern space as in sed. However in awk, the pattern space is divided into an array of the fields of the original input record.
Each of these fields can be manipulated separately. Since the whole input record can be stored as a string in any variable, awk does not need a hold space. After that, action2 is executed if pattern2 matches the current, possibly altered pattern space and the cycle was not terminated by action1. And so on. An action is a sequence of statements commands that are separated by semicolons ; or are on different lines.
If pattern is omitted, then the corresponding action is done all the time provided this program line is reached in the cycle. Observe that by default an awk program does not copy an input line similar to sed -n.
Thus, printing has to be triggered by an address pattern, which selects the pattern space as shown in the next example, or printing has to be triggered by a separate print statement. The following program does the same as identifyBeginI. The default action is used, i. They can also be used in the if statement of awk to define a conditional. In this section, we shall discuss those patterns in awk which are called regular expressions.
In addition to the list of patterns which we give next, there is also the possibility to define arithmetic-relational expressions, string- valued expressions and arbitrary Boolean combinations of all of the above.
The patterns different from regular expressions will be explained later. The following rules must be observed: There is no tagging in awk. The rules R1—R3 set under 5 also apply here. The backslash can sometimes by included in strings as a single backslash character. Otherwise, every character including the blank just represents itself. Strings can be concatenated by just writing them behind each other separated by blanks. This hold also for variables containing strings. Thus, in Errare humanum est.
Errare, humanum and est. One can count beyond 9 in regard to field variables. The program! In our example, that is the blank in humanum Errare. In general, a print statement has the following structure: A comma causes an output field separator string OFS default a blank to be printed. The sequence of arguments may either be empty or must end in object. After a print statement, an output record separator ORS default a newline character is printed. Note that print money is interpreted as printing the content of the variable money while print "money" really prints money.
The following program prints the first five fields in every line separated by one blank. It can be used to isolate starting phrases of sentences. The following program triple spaces the input file.
This can be useful if one wants to correct printed text by hand and needs space for inserted comments. The following program prepends the second field in a line by a newline character if there are at least two fields in the line, and then prints the pattern space possibly printing two lines. This is done only if a second field exists. The second function for printing in awk is printf.
Let us implement another version of the example which printed est. It says: Then print a blank followed by the third variable understood as string. Finally, print the string "??? In general, a printf statement has the following structure: The most important specifications are: The for loop will be explained below.
In particular, a number from the input record contained in a field variable is printed unchanged. In addition, they can be printed starting left in the field. It can be specified that zeroes rather than blanks fill a field in which a number is printed.
The following sh program shows the use of printf: Specifying formats in the way of the present example is useful to obtain nice tabular output of statistical evidence and other accounting. For the representation of special characters consult the discussion of strings in awk given above. We leave the discussion of printf at this point since it is more important in regard to numerical computation with awk rather than to text processing. In regard to the latter, print is mostly sufficient in our experience.
An output file is created if it is not in existence. One can print to at most 10 different files from within an awk program. If one needs to print to more files, then one can split the program into several pieces and use a pipe. The second field is supposed to be a non-zero integer. The white ranges contain a blank and a tab each.
The following program mimmicks the UNIX command tee. The input file is once copied line-by-line to the output by the first print statement. Again, this shows a simple technique to place arguments to sh commands into sed or awk programs. Consult man tee for more details and options on tee.
The following program is another version of sortByVowel. The strings defining the files are not stored in variables. Thus, they have to be framed by double quotes ". Otherwise, the sed and awk versions of sortByVowel are very similar.
Instead of appending output to files, one can also feed output into a pipe: But from within awk one can sort and mail different things to several recipients at the same time. Consult man mail for more details about mail. They just exist and can be set to filled with strings and numbers of different types i. Variables are initiated to the empty string automatically. The empty string is interpreted as 0 if a variable is used in a numerical computation.
The following program exchanges the first two fields in every line of a file. The following program is another version of printPredecessorBecause. Finally, every line is saved in the variable previous waiting for the next cycle. Their dimension is 1 meaning one index consisting of numbers. Their size, i.
Any element of an array that is used is initiated to the empty string which is interpreted as 0 in numerical computations. An element of an array is denoted as name[index]. Elements of arrays can hold everything that ordinary variables can, i. The empty string is allowed as index. All built-in variables can be used in the same way as other variables in computations, string manipulations and conditionals. In particular, all built-in variables can be reset.
Usually, a built-in variable is reset in the first group of statements actionB of an awk program addressed by the BEGIN pattern. This will be explained in more detail below. The default are sequences of blanks and tabs. This is slightly beyond a single character but is done for convenience. If one processes chunks of texts not words, then one may want to set the field separator to, e. In that way, one can process phrases and sentences where words are separated by blanks while the fields are separated by.
One application is to separate original and translation of phrases by. This is very important in order to loop over all fields. Usually, this is the line number if the record separator character RS is not reset or NR itself is not reassigned another value. Note that NR counts cumulatively over several files to which an awk program is applied. Consult the above section on printf and man 3 printf for details about such formats.
Primarily, a number is considered as a string as long as it was not subject to a computation. OFMT only becomes active for numbers that were involved in computations. If numbers are just copied with print, then they reappear unchanged from the input format. OFS is caused to be printed if a comma , is used in a print statement. The default is a blank character. It can only be one character long. In particular, it should not be set to the empty string.
If one uses a dummy statement, e. It seems advisable to exchange field separators and to increase spacing with sed. It is appended to the output after each print statement.
The default is a newline character. This can be used to unite lines of the input file. We shall give some examples for the use of FILENAME later in connection with set operations on files the program setIntersection and in connection with the implementation of a vocabulary trainer. The built-in variable NF is mostly used in connection with the for statement. Check the section describing the for statement for some examples using NF.
The following sh program counts the number of paragraphs in a text file. The idea in the awk program is to count and print the number of paragraphs which are put as one input records into the pattern space. As indicated above, the fields in this setting are the individual lines of text. The corrected number of records NR which equals the number of paragraphs is printed at the end. The following sh program is another version of oneItemPerLine. The range contains a blank and a tab.
Thus, all fields are on separate lines. First in every line comes a word or phrase which can contain a number. In addition to that, the final field of every line contains a number which may, e.
An example of an entry is given by limit 55 The last entry will be called the frequency of the preceeding word or phrase.
The following lists all awk operators in decreasing order of precedence, i. Note that strings other than those that have the format of numbers all have the value 0 in a numerical computations. In particular, they are used for counting in for and while loops. Two strings "aa" and "bb" can be concatenated via "aa""bb" to "aabb".
The strings in two variables x and y can be concatenated and assigned to, e. Easy to get started with, takes a long time to master. The whole point of programming is power and flexibility. Else there would be no need to move beyond logic gates. The problem is tools sold for newbies tend to take away too much power to make it easy for people to start, but then keep them, right there, all life.
When I did my comp sci undergrad degree, languages per-se was not part of what was taught in the classroom. Whatever language that class was using, be it assembler, scheme, C or some other higher level language, it was up to the student to figure out the syntax, how to run the compiler, etc.
You got some help and basic examples in the discussion sessions but it was not part of the lectures. And there was no web, no stack overflow, no google at the time. If you didn't enjoy reading, puzzling, and banging your head against the wall you would wash out after a couple of classes. And why is Python incompatible with those things? To me python seems well organized enough that it makes a great language for beginners.
Python is fine. Python is perhaps my favorite language, and I use a dozen fairly regularly. My point was about how python sometimes gets used, and how its design is especially facilitating to that use. Python is the rubber sheet analogy of programming. Aka python helps you think more like the computer does. Perl helps the computer think more like you. Note that it sounds like a snide remark but it doesn't have to be. One could interpret it as, "Do you want to simply get things done, and not bicker endlessly about microoptimizations or where the braces go?
Python is for people who simply want to complete their tasks and then go home and spend their time doing more stuff they think is fun. Note that none of this necessarily reflects my personal opinion; it is just an alternative way to read the grandparent.
The cargo cult continues. It's disgusting how much we all rely on intuition and gut feelings when evaluating large swathes of technology. The internet hates perl so people criticize it without even using it. People will use what everybody else is using without actually trying out the options. There's too much information and so we must go with what others have said, and it all becomes hearsay. Keeping up is more like wizardry than engineering.
Go with what the crowd says because I can't possibly install all of those libraries, play with the examples, and give my own evaluation. Hey look a new js framework just came out I used it quite a bit, and wrote some applications in it that are still in use a decade later. The "write-only" aspect of Perl is real -- it enables many styles and idioms, and as a result, tends to requires reading knowledge of all of them. It also has some unusual design decisions list vs.
Python is a lot more readable, and does most of the same stuff. What Perl did that was amazing was bring regular expressions "to the masses", and Perl-compatible regular expressions pcre are still the defacto standard that most subsequent libraries have used more or less. That itself is the kind of gross generalization you are criticizing.
And one can criticize a language and still have respect for it. I can count the number of tool-agnostic development teams that I've met on one hand. Many more have claimed they are when they are not.
Then the mass hops on board that train too. I agree with the thrust of your comment but not the specifics. And I don't think "high-profile" people talking has much effect.
There's something like necessary complexity you can't easily abstract away. I find Rust does a fine job at cleaning up syntax. Other than that I can't think of anything that's more difficult than the underlying concept in Rust. And it's still close to C braces and functions and ML syntax type after colon and variable name, let bindings in many ways.
Oh, don't be silly. Syntactic complexity is linear with expressive power, that's why it's complex to begin with. You just "like" rust, so you view it as a good tradeoff there and not in other languages that you "dislike". My point above was that this decision is basically one of fashion and not technology. And the proof is how the general consensus about awk has evolved over 20 years as perl has declined. Are you confusing syntax with grammar? Rust has a large grammar—many reserved words, many compile-time macros, etc.
Also, given the languages that occupy the ends of said spectrum, I think it should be clear that your position on said spectrum has no correspondence with "expressive power": You said it better than I could: Awk has evolved?
GNU Awk hassomewhat. You need to compile C code to use it, and the API has been a moving target. It has a way to work with XML. It has an "in place" editing mode, and other things like bignum integer support via GMP requiring the -M option to be used. Plus various minor extensions to POSIX, like full regex support in RS record separator , a way to tokenize fields positively rather than delimit by matching their separators, a way to extract fixed-width fields, an include mechanism, a way to indirect on functions and such.
None of it adds up to very much. So Rust could be designed properly for modern use cases. Rust, in terms of GC-less languages with mostly zero-cost abstractions is the simplest language that I've seen. And having a GCless language with memory safety is not just fashion.
It's pretty much the greatest single advancement in language design since the GC itself. You might be pleasently surprised.
Somewhat relative xkcd  https: Second this. Perl replaced both awk and sed. Perl May not be as suited to writing complex programs, but for text processing tasks, it is much more elegant than awk or sed. Perl -pie is a very powerful idiom. I would rather use a subset of Perl than awk. That's the first time I'm hearing Perl being praised for its elegance of all things. Elegance is certainly in the eye of the beholder, but by default is understood in the context of programming languages as "containing only a minimal amount of syntax constructs".
In fact, I find Perl one of the ugliest languages of all time. One has to recall that there were some forces that wanted Perl to become a standard shell rather than a programming language. A shell is usually more limited in features, but it is frequently very forgiving, provides many shortcuts, and there are often multiple ways of doing things.
However, I've never believed it to be possible to have a language both as a shell language and a proper programming language for large-scale projects.
I believe the two usecases are fundamentally antithetical, but I'd be happy to be proven wrong. I'd say Powershell proves you right. Powershell has a great design, it has optional typing and access to a cornucopia of libraries via.
Even so, they had to make some compromises because of the shell parts functions return output, for example which makes is quite finicky as a "proper" programming language.
On the shell side, the very nice nomenclature which makes it very readable and discoverable makes is annoying sometimes to use as a shell. That and the somewhat unwieldy launch of non-Powershell commands. Someone who attempts to bridge the two has a ton of work to do, both in the research and in the implementation department.
I guess Oil Shell https: And it's probably still years away from release and many more years from mass adoption if that ever happens.
Why add 'tr' and 'join' to awk when they exist on their own? That's part of why people avoid perl. It's very capable, but that wide scope is counter to the unix philosophy that prefers simple, focused utilities that can be combined in pipelines.
I meant join as in perl's join - to construct a string out of array values with specified separator Some examples: I think perls niche is sort of it's downfall. Any time I ran into a perl script I had to switch my brain into Perl mode, with the understanding that what I was working on would be just as good or better in C or shell.
I had nothing against Perl as a language, but always enjoyed exorcising a perl script from the codebase. Ah, but which subset? To be clear: I like that about it too. But in the real world, we want tooling that does more stuff.
And that makes awk look clever and elegant. All I'm saying is that in perl-world which was a real place! And that to me is more surprising than awk's cleverness. I think people underestimate the importance of medium-powered tools. Perl is great, but awks limitations make it easier to write in a way that the next person can maintain.
Or just reading about perl wizardry: Can you elaborate this? What revelations did you experience? And yet in , I never think about Perl anymore, but use sed, awk, and grep daily. This is a great point. Eventually the cycle will repeat again. The moment you will have non trivial text work to do, awk will have to give way to Perl. Python hasn't actually replaced Perl in any meaningful way. Most modern distributions write their tools in Python instead of Perl. They could have done it faster if not for their system tools written in Python 2.
In the web space, Python is not huge, but it definitely supplanted the niche Perl used to have. No one I know writes web stuff in Perl anymore. I totally see your point. And in Perl-world, I would probably use Perl too — I mean, if both tools are equally ubiquitous, why not use the most powerful one? I entered the field at the tail end of the Perl era, so I've only toyed with it a long time ago.
Meanwhile, if you randomly mashed on your keyboard, it would output a perl script. I jumped to Python as soon as I found out about it in the late 90's, because it was exactly what I was looking for in self-documenting structuring. It was great for creating parsers with state-machines. I was also the user of my scripts, and I didn't want to have to relearn what I coded a year or so earlier.
Python let me pick that up, and to a lesser extent, awk.
State machine programming was really self-documenting in Python. Here's a picture of the AWK book next to the Camel book https: FractalLP 10 months ago. That is one thing I like better in Python than in Perl. In Perl, I was having difficulty with nested data structures and then realized this was something I did in Python daily without even knowing it was a thing on a conscious level.
Who hasn't made some weird dictionary with its values being lists or tuples or something like that? This is exactly backwards to my eyes. Perl's autovivification behavior assigning through a empty reference to something autocreates the object of the right type makes nested data much, much cleaner.
Who hasn't made some weird dictionary with its values being lists and forgotten to check for and write code to create it when it doesn't exist, and had to fix that later after a runtime crash?
In perl that's literally covered by the one-line assignment you used to set the field you parsed. This is why it's sad that everyone's forgotten perl.
Defaultdicts to the rescue! But I think it should be an explicit choice. If you only intend to have lists at some keys, and then accidentally mistype a key, it shouldn't in my opinion silently create a list, effectively hiding the bug. Sorry, but I didn't catch your example.
Can you explain a little more? What does Perl do better there? For example removing something from a file and then talking to a database or a web service, or do other stuff like parse a JSON or XML.
Deal with multiple files, or other advanced use cases. Unicode work, advanced regexes etc etc. In fact the whole point of Perl was Larry wall reaching the upper limits of what one could do with awk, sed and other utilities and having to use them all over the place in C.
Then realizing there was a use case for a whole new language. In fact the boilerplate you describe is mostly redundant if you use perl -p since perl -p results in your program running with this wrapper: Correct me if I'm wrong, but this will only be line based won't it? For example removing something from a file and then talking to a database or a web service, I do exactly this every day with AWK.
Solving exactly these kinds of problems features prominently in the AWK programming language book. It goes further. Awk is when you entire use case fits into the category of iterating lines in a file. Perl is that plus more things. And I would hereby like to remind you that every computing problem is fundamentally an input-output problem, and because of this intrinsic property, it is possible to reduce all problems in computing to input-processing-output.
Which is exactly the kind of problem AWK is designed to address. As a single example, have you looked at compiling openssl without perl, using awk instead? Whenever I see a perl prerequisite I question whether it is truly a requirement or whether other base utilities1 such as awk could replace it. Assuming it could be removed, how much effort is one willing to expend in order to extinguish a perl dependency? The illumos project undertook a massive effort to eradicate any and all dependencies on Perl which had also been made part of the core operating system decades prior, by themselves no less.
While they're still at it, they have managed to rip out most of it and replace it with binary executables in C or shell. Yes, writing a build engine in AWK would be perfectly doable, but the right tool for that job is Make. That's easy to do. But that's not the end of it. The whole point of Perl is avoid a salad of C and shell utils. The resulting code is often far more unreadable than anything you will ever write in Perl.
Perl is a mess whose syntax borders on hieroglyphic once poured into software. That, and awk programs are almost perfectly portable across awk implementations: This is of practical importance for portable scripts, since Debian uses mawk , RH ships gawk, and Mac OS uses nawk. AWK seems far more modern than Perl, though. Defining a function with you know, "function" and actually having parameters with names feels like a 21st century language.
Yes, I know there are packages to add named parameters to Perl, plus Perl6 has them out of the box, but it is weird that things went backwards in a lot of ways when Perl replaced AWK in the mid s. Though sadly still unfixed: You have a really good point.
AWK feels incredibly modern for being so old. Until you find that it has no local variables other than function parameters. In a function, local variables can be simulated by defining additional arguments Everything that isn't a function parameter is a global! Awk is stupidly confused whether it is a two-namespace or one-namespace language think Lisp-2 vs. For instance, this works: Actually what you show is that there is a clear rule. First, the symbol is treated as a function. If it is not in the function space, then it is used as a variable name.
To make a finer distinction awk would need some form of local variable declaration, which it clearly hasn't. That is simply not the case. Also not the case. The purely syntactic context distinguishes whether or not the identifier is being used as a variable or function.
Though having being trained in sed and awk helped with picking up Perl. Python was never intended to replace Perl. Perl was designed to extract stuff from text files. Python was designed as a scripting language for system programming. Perl is a little older than Python. Most of the ideology of Python was a reaction against Perl https: Python has always tried to be as good as Perl on everything. I use very much Perl for what it was intended: I have tried to use Python for the same tasks, in particular when some of the files were in xml xml parsing in Python is nicer than in Perl.
Regular expression usage is easier in Perl where multithreading is easier in Python. Python is very good for teaching whiteboard interview or as a scripting language around some big libraries like tensorflow. At my work, the small glue scripts are almost always in shell or in Perl. The Python applications are being rewritten in Java because of maintenance issues mainly caused by lack of proper typing.
Python does not rule here. You might want to try Ruby for some of those things you reached to Python for. It takes direct inspiration from Perl and does a lot of those things better, IMO.
Nokogiri is hands-down the best tool for dealing with XML that there is. I have much the same experience with Python and anything significant developed in Python here ends up getting rewritten in Go. Sileni 10 months ago. Not to move the post, but that's part of what makes python so great to me. It reads like pseudocode if written with that goal in mind. Its more like having a conversation with the computer.