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Awk manual. The GAWK Manual |
Thursday, June 29, 2023
Gawk: Effective AWK Programming - GNU Project - Free Software Foundation
awk(1): pattern scanning/processing - Linux man page.
A print statement would do the same thing. Printf reveals it's real power when formatting commands are used. The first argument to the printf function is the format.
This is a string, or variable whose value is a string. This string, like all strings, can contain special escape sequences to print control characters.
Hopefully I'll provide enough examples to demonstrate the differences. See the man page ascii 7 for more information. Printf does nothing unless you specify the action. Table 3 illustrates the differences. When a string with numbers and letters are coverted into an integer, AWK will return a zero, while NAWK will convert as much as possible.
The second example, marked with "NAWK? Using format specifiers, there is another way to print a double quote with NAWK.
This demonstrates Octal, Decimal and Hexadecimal conversion. As you can see, it isn't symmetrical. Decimal conversions are done differently. This is the width field. Spaces are added so the number of printed characters equal this number.
Note that this is the minimum field size. If the field becomes to large, it will grow, so information will not be lost. Spaces are added to the left. This format allows you to line up columns perfectly. If the string is too long, then the two fields will run together, making it hard to read. You may want to consider placing a single space between the fields, to make sure you will always have one space between the fields. This is very important if you want to pipe the output to another program.
Adding informational headers makes the output more readable. Be aware that changing the format of the data may make it difficult to get the columns aligned perfectly. Consider the following script:! More complicated formats would require a lot of trial and error. You have to adjust the first printf to agree with the second printf statement. I suggest! This may not seem like it's very useful, but when you have multiple formats and multiple columns, it's very useful to have a set of templates like the above.
If you have to add an extra space to make things line up, it's much easier to find and correct the problem with a set of format strings that are together, and the exact same width. CHainging the first columne from 10 characters to 11 is easy. Left Justification The last example places spaces before each field to make sure the minimum field width is met. What do you do if you want the spaces on the right?
The Field Precision Value The precision field, which is the number between the decimal and the format character, is more complex. With the octal, decimal or hexadecimal format, it specifies the minimum number of characters. Zeros are added to met this requirement. The precision specifies the number of digits displayed, before and after the decimal point. Explicit File output Instead of sending output to standard output, you can send output to a named file. Appending to the file does not delete the old contents.
However, there is a subtle difference between AWK and the shell. Consider the shell program:! Now consider the equivalent AWK program:! If you find this to be a problem, look into PERL. I hope this gives you the skill to make your AWK output picture perfect. Flow Control with next and exit You can exit from an awk script using the exit command. Let's say to expect all lines of a file to be 60 characters, and you want to use an awk program as a filter to exit if the number of characters is not Some sample code could be!
The "next" command will also change the flow of the program. It causes the current processing of the pattern space to stop. The program reads in the next line, and starts executing the commands again with the new line. When you add a few functions, AWK becomes even more, mmm, functional. There are three types of functions: numeric, string and whatever's left.
I bet millions, if not dozens, of my readers have been waiting for me to discuss trigonometry. Personally, I don't use trigonometry much at work, except when I go off on a tangent.
Sorry about that. I don't know what came over me. I don't usually resort to puns. I'll write a note to myself, and after I sine the note, I'll have my boss cosine it. Now stop that! I hate arguing with myself. I always lose. Thinking about math I learned in the year 2 B. Before Computers seems to cause flashbacks of high school, pimples, and shudder times best left forgotten. The stress of remembering those days must have made me forget the standards I normally set for myself.
Besides, no-one appreciates obtuse humor anyway, even if I find acute way to say it. I better change the subject fast. Combining humor and computers is a very serious matter. Here is a NAWK script that calculates the trigonometric functions for all degrees between 0 and It also shows why there is no tangent, secant or cosecant function.
They aren't necessary. All this in a thin veneer of demonstrating why we learned trigonometry in the first place. What more can you ask for? Oh, in case you are wondering, I wrote this in the month of December.
This AWK script plots the values from 0 to for the basic trigonometry functions but first - a review: Note to the editor - the following diagram assumes a fixed width font, like Courier. If you know the length of 2 sides, and the angles, you can find the length of the third side. Also - if you know the length of the sides, you can calculate the angles.
As you come to, you realize your feet are touching the trunk of the tree, and your eyes are 6 feet from the bottom of your frostbitten toes. While counting the stars that spin around your head, you also realize the top of the tree is located at a 65 degree angle, relative to your eyes. You suddenly realize the tree is Not many people memorize the tangent table, or can estimate angles that accurately.
I was telling the truth about the stars spinning around the head, however. The atan2 function returns a value from negative pi to positive pi. Exponents, logs and square roots The following script uses three other arithmetic functions: log , exp , and sqrt. I wanted to show how these can be used together, so I divided the log of a number by two, which is another way to find a square root.
I then compared the value of the exponent of that new log to the built-in square root function. I then calculated the difference between the two, and converted the difference into a positive number. This example isn't too exciting, except to those who enjoy nitpicking.
Expect the program to reach 3 million before you see any errors. I'll give you a more exciting sample soon. This truncates a number, making it an integer. It can be used to round numbers by adding 0. The function rand returns a random number between 0 and 1.
Here is an example that calculates a million random numbers between 0 and , and counts how often each number was used:! Experienced programmers know random number generators aren't really random, unless they use special hardware. These numbers are pseudo-random, and calculated using some algorithm. Since the algorithm is fixed, the numbers are repeatable unless the numbers are seeded with a unique value.
This is done using the srand function above, which is commented out. Typically the random number generator is not given a special seed until the bugs have been worked out of the program. There's nothing more frustrating than a bug that occurs randomly. The srand function may be given an argument. If not, it uses the current time and day to generate a seed for the random number generator. The Lotto script I promised a more useful script. This may be what you are waiting for. It reads two numbers, and generates a list of random numbers.
I call the script "lotto. First, initialize the seed srand ; How many numbers are needed? String Functions Besides numeric functions, there are two other types of function: strings and the whatchamacallits.
Associative arrays and trigonometric functions are somewhat esoteric features, that new users embrace with the eagerness of a chain smoker in a fireworks factory.
I suspect most users add some simple string functions to their repertoire once they want to add a little more sophistication to their AWK scripts. I hope this column gives you enough information to inspire your next effort. There are four string functions in the original AWK: index , length , split , and substr.
These functions are quite versatile. The Length function What can I say? The length function calculates the length of a string. I often use it to make sure my input is correct. The following command centers all lines shorter than 80 characters:! The number specified the location of the substring.
If the substring consists of 2 or more characters, all of these characters must be found, in the same order, for a non-zero return value. Like the length function, this is useful for checking for proper input conditions. The Substr function The substr function can extract a portion of a string.
There are two ways to use it: substr string , position substr string , position , length where string is the string to search, position is the number of characters to start looking, and length is the number of characters to extract default is 1. A simple example is to use it to search or extract a specific string at a fixed location. If you wanted to print columns 40 through 50, use!
This can easily be done using! If you wanted to process some mail addresses, where you search for a " " character, and split the text into username and hostname, the following code fragment might do the job:! The first is the string, the second is the position. The optional third argument is the length of the string to extract. If the third argument is missing, the rest of the string is used. The substr function can be used in many non-obvious ways. As an example, it can be used to convert upper case letters to lower case.
It takes three arguments: the string, an array, and the separator. The function returns the number of pieces found. Here is an example:! If a longer string is used, only the first letter is used as a separator. These functions take strings, so you can reduce the above script to a single line:! Sub performs a string substitution, like sed.
The function returns 1 if a substitution occurs, and 0 if not. If no slashes are given in the first argument, the first argument is assumed to be a variable containing a regular expression.
The sub only changes the first occurrence. The gsub function is similar to the g option in sed : all occurrence are converted, and not just the first. That is, if the patter occurs more than once per line or string , the substitution will be performed once for each found pattern. The following script:! The Match function As the above demonstrates, the sub and gsub returns a positive value if a match is found.
However, it has a side-effect of changing the string tested. If you don't wish this, you can copy the string to another variable, and test the spare variable. NAWK also provides the match function. If match finds the regular expression, it sets two special variables that indicate where the regular expression begins and ends.
Here is an example that does this:! I could use the word "miscellaneous", but it's too hard to spell. Darn it, I had to look it up anyway. It returns the exit status of the program. Note that the output isn't sent to the NAWK program. You could send it to a file, and open that file for reading. There is another solution, however. For each line, it prints the line number on a 6 character-wide field and then the line contents.
For example, when executed on this input:. As a special case, when the first part of a range pattern is constantly true, e. Similarly, if the second part is constantly false, e. For example,. Word frequency using associative arrays :. Note that separators can be regular expressions. After that, we get to a bare action, which performs the action on every input line. In this case, for every field on the line, we add one to the number of times that word, first converted to lowercase, appears.
Finally, in the END block, we print the words with their frequencies. The line. This is different from most languages, where such a loop goes through each value in the array.
The loop thus prints out each word followed by its frequency count. This program can be represented in several ways. The first one uses the Bourne shell to make a shell script that does everything. It is the shortest of these methods:. There are alternate ways of writing this. This shell script accesses the environment directly from within awk:.
The shell script makes an environment variable pattern containing the first argument, then drops that argument and has awk look for the pattern in each file. Note that a regular expression is just a string and can be stored in variables.
The next way uses command-line variable assignment, in which an argument to awk can be seen as an assignment to a variable:. Finally, this is written in pure awk, without help from a shell or without the need to know too much about the implementation of the awk script as the variable assignment on command line one does , but is a bit lengthy:.
Note the if block. This means that. If you explicitly set ARGC to 1 so that there are no arguments, awk will simply quit because it feels there are no more input files.
Therefore, you need to explicitly say to read from standard input with the special filename -. On Unix-like operating systems self-contained AWK scripts can be constructed using the shebang syntax.
For example, a script that prints the content of a given file may be built by creating a file named print. The -f tells AWK that the argument that follows is the file to read the AWK program from, which is the same flag that is used in sed. Since they are often used for one-liners, both these programs default to executing a program given as a command-line argument, rather than a separate file. AWK was originally written in and distributed with Version 7 Unix.
In its authors started expanding the language, most significantly by adding user-defined functions. To avoid confusion with the incompatible older version, this version was sometimes called "new awk" or nawk.
This implementation was released under a free software license in and is still maintained by Brian Kernighan see external links below. Jump to content Navigation. Help Learn to edit Community portal Recent changes Upload file.
Download as PDF Printable version. In other projects. For short to medium-length awk programs, it is most convenient to enter the program on the awk command line. This is best done by enclosing the entire program in single quotes. This is true whether you are entering the program interactively at the shell prompt, or writing it as part of a larger shell script:.
Once you are working with the shell, it is helpful to have a basic knowledge of shell quoting rules. Before diving into the rules, we introduce a concept that appears throughout this Web page, which is that of the null , or empty, string. The null string is character data that has no value. In other words, it is empty. It is written in awk programs like this: "". In the shell, it can be written using single or double quotes: "" or ''. Although the null string has no characters in it, it does exist.
For example, consider this command:. Here, the echo utility receives a single argument, even though that argument has no characters in it.
In the rest of this Web page, we use the terms null string and empty string interchangeably. Now, on to the quoting rules:. Because certain characters within double-quoted text are processed by the shell, they must be escaped within the text.
The leading backslash is stripped first. Thus, the example seen previously in Running awk Without Input Files :. In the second case, awk attempts to use the text of the program as the value of FS , and the first file name as the text of the program!
This results in syntax errors at best, and confusing behavior at worst. Mixing single and double quotes is difficult. You have to resort to shell quoting tricks, like this:. This program consists of three concatenated quoted strings. The first and the third are single-quoted, and the second is double-quoted.
Another option is to use double quotes, escaping the embedded, awk -level double quotes:. This option is also painful, because double quotes, backslashes, and dollar signs are very common in more advanced awk programs. A third option is to use the octal escape sequence equivalents see Escape Sequences for the single- and double-quote characters, like so:.
Here, the two string constants and the value of sq are concatenated into a single string that is printed by print. The following example, courtesy of Jeroen Brink, shows how to escape the double quotes from this one liner script that prints all lines in a file surrounded by double quotes:. In MS-Windows escaping double-quotes is a little tricky because you use backslashes to escape double-quotes, but backslashes themselves are not escaped in the usual way; indeed they are either duplicated or not, depending upon whether there is a subsequent double-quote.
The MS-Windows rule for double-quoting a string is the following:. Many of the examples in this Web page take their input from two sample data files. The second data file, called inventory-shipped , contains information about monthly shipments. In both files, each line is considered to be one record. The columns are aligned using spaces. The data file inventory-shipped represents information about shipments during the year. Each record contains the month, the number of green crates shipped, the number of red boxes shipped, the number of orange bags shipped, and the number of blue packages shipped, respectively.
There are 16 entries, covering the 12 months of last year and the first four months of the current year. An empty line separates the data for the two years:. This type of pattern is called a regular expression , which is covered in more detail later see Regular Expressions.
The pattern is allowed to match parts of words. In an awk rule, either the pattern or the action can be omitted, but not both. If the pattern is omitted, then the action is performed for every input line. If the action is omitted, the default action is to print all lines that match the pattern. By comparison, omitting the print statement but retaining the braces makes an empty action that does nothing i.
Many practical awk programs are just a line or two long. Following is a collection of useful, short programs to get you started.
Most of the examples use a data file named data. This is just a placeholder; if you use these programs yourself, substitute your own file names for data. Its output looks like this:. The sixth, seventh, and eighth fields contain the month, day, and time, respectively, that the file was last modified. Finally, the ninth field contains the file name. For future reference, note that there is often more than one way to do things in awk.
At some point, you may want to look back at these examples and see if you can come up with different ways to do the same things shown here:. The sole rule has a relational expression as its pattern and has no action—so it uses the default action, printing the record.
This example differs slightly from the previous one: the input is processed by the expand utility to change TABs into spaces, so the widths compared are actually the right-margin columns, as opposed to the number of input characters on each line. This is an easy way to delete blank lines from a file or rather, to create a new file similar to the old file but from which the blank lines have been removed.
The awk utility reads the input files one line at a time. For each line, awk tries the patterns of each rule. If several patterns match, then several actions execute in the order in which they appear in the awk program. If no patterns match, then no actions run. After processing all the rules that match the line and perhaps there are none , awk reads the next line. However, see The next Statement and also see The nextfile Statement.
This continues until the program reaches the end of the file. For example, the following awk program contains two rules:. If a line contains both strings, it is printed twice, once by each rule. This is what happens if we run this program on our two sample data files, mail-list and inventory-shipped :.
This example shows how awk can be used to summarize, select, and rearrange the output of another utility. This command prints the total number of bytes in all the files in the current directory that were last modified in November of any year.
As a result, when awk has finished reading all the input lines, sum is the total of the sizes of the files whose lines matched the pattern. This works because awk variables are automatically initialized to zero. After the last line of output from ls has been processed, the END rule executes and prints the value of sum. In this example, the value of sum is These more advanced awk techniques are covered in later sections see Actions.
Before you can move on to more advanced awk programming, you have to know how awk interprets your input and displays your output. By manipulating fields and using print statements, you can produce some very useful and impressive-looking reports. Most often, each line in an awk program is a separate statement or separate rule, like this:.
A newline at any other point is considered the end of the statement. The backslash must be the final character on the line in order to be recognized as a continuation character. A backslash followed by a newline is allowed anywhere in the statement, even in the middle of a string or regular expression. We have generally not used backslash continuation in our sample programs. For this same reason, as well as for clarity, we have kept most statements short in the programs presented throughout the Web page.
Backslash continuation is most useful when your awk program is in a separate source file instead of entered from the command line. You should also note that many awk implementations are more particular about where you may use backslash continuation. For example, they may not allow you to split a string constant using backslash continuation. Thus, for maximum portability of your awk programs, it is best not to split your lines in the middle of a regular expression or a string. But the C shell behaves differently!
There you must use two backslashes in a row, followed by a newline. Note also that when using the C shell, every newline in your awk program must be escaped with a backslash. To illustrate:. To have the pattern and action on separate lines, you must use backslash continuation; there is no other option. Another thing to keep in mind is that backslash continuation and comments do not mix. In this case, it looks like the backslash would continue the comment onto the next line.
When awk statements within one rule are short, you might want to put more than one of them on a line. This also applies to the rules themselves. Thus, the program shown at the start of this section could also be written this way:.
NOTE: The requirement that states that rules on the same line must be separated with a semicolon was not in the original awk language; it was added for consistency with the treatment of statements within an action. The awk language provides a number of predefined, or built-in , variables that your programs can use to get information from awk. There are other variables your program can set as well to control how awk processes your data.
In addition, awk provides a number of built-in functions for doing common computational and string-related operations. As we develop our presentation of the awk language, we will introduce most of the variables and many of the functions. They are described systematically in Predefined Variables and in Built-in Functions. By using utility programs, advanced patterns, field separators, arithmetic statements, and other selection criteria, you can produce much more complex output.
The awk language is very useful for producing reports from large amounts of raw data, such as summarizing information from the output of other utility programs like ls.
See A More Complex Example. Programs written with awk are usually much smaller than they would be in other languages. This makes awk programs easy to compose and use. Often, awk programs can be quickly composed at your keyboard, used once, and thrown away.
Because awk programs are interpreted, you can avoid the usually lengthy compilation part of the typical edit-compile-test-debug cycle of software development.
Complex programs have been written in awk , including a complete retargetable assembler for eight-bit microprocessors see Glossary , for more information , and a microcode assembler for a special-purpose Prolog computer.
If you find yourself writing awk scripts of more than, say, a few hundred lines, you might consider using a different programming language. The shell is good at string and pattern matching; in addition, it allows powerful use of the system utilities.
Python offers a nice balance between high-level ease of programming and access to system facilities. This chapter covers how to run awk , both POSIX-standard and gawk -specific command-line options, and what awk and gawk do with nonoption arguments. There are two ways to run awk —with an explicit program or with one or more program files. Here are templates for both of them; items enclosed in […] in these templates are optional:. Doing so makes little sense, though; awk exits silently when given an empty program.
If --lint has been specified on the command line, gawk issues a warning that the program is empty. Options begin with a dash and consist of a single character. GNU-style long options consist of two dashes and a keyword. The keyword can be abbreviated, as long as the abbreviation allows the option to be uniquely identified. If a particular option with a value is given more than once, it is usually the last value that counts. The long and short options are interchangeable in all contexts.
Read the awk program source from source-file instead of in the first nonoption argument. This option may be given multiple times; the awk program consists of the concatenation of the contents of each specified source-file. See Changing The Namespace , for more information on this advanced feature. Set the variable var to the value val before execution of the program begins. Provide an implementation-specific option. These options also have corresponding GNU-style long options.
Note that the long options may be abbreviated, as long as the abbreviations remain unique. The full list of gawk -specific options is provided next. Signal the end of the command-line options. It is also useful for passing options on to the awk program; see Processing Command-Line Options. Cause gawk to treat all input data as single-byte characters. In addition, all output written with print or printf is treated as single-byte characters.
This can often involve converting multibyte characters into wide characters internally , and can lead to problems or confusion if the input data does not contain valid multibyte characters. Specify compatibility mode , in which the GNU extensions to the awk language are disabled, so that gawk behaves just like BWK awk.
Also see Downward Compatibility and Debugging. Print a sorted list of global variables, their types, and final values to file. If no file is provided, print this list to a file named awkvars. No space is allowed between the -d and file , if file is supplied. Having a list of all global variables is a good way to look for typographical errors in your programs.
This is a particularly easy mistake to make with simple variable names like i , j , etc. Enable debugging of awk programs see Introduction to the gawk Debugger.
By default, the debugger reads commands interactively from the keyboard standard input. The optional file argument allows you to specify a file with a list of commands for the debugger to execute noninteractively.
No space is allowed between the -D and file , if file is supplied. Provide program source code in the program-text. This option allows you to mix source code in files with source code that you enter on the command line. This makes building the total program easier. However, this is no longer true. If you have any scripts that rely upon this feature, you should revise them.
See Changing The Namespace , for more information. Similar to -f , read awk program text from file. There are two differences from -f :.
This option is particularly necessary for World Wide Web CGI applications that pass arguments through the URL; using this option prevents a malicious or other user from passing in options, assignments, or awk source code via -e to the CGI application. Analyze the source program and generate a GNU gettext portable object template file on standard output for all string constants that have been marked for translation.
See Internationalization with gawk , for information about this option. Read an awk source library from source-file. This option is completely equivalent to using the include directive inside your program. It is very similar to the -f option, but there are two important differences. First, when -i is used, the program source is not loaded if it has been previously loaded, whereas with -f , gawk always loads the file.
Second, because this option is intended to be used with code libraries, gawk does not recognize such files as constituting main program input.
Thus, after processing an -i argument, gawk still expects to find the main source code via the -f option or on the command line. Print the internal byte code names as they are executed when running the program.
The trace is printed to standard error. Load a dynamic extension named ext. Extensions are stored as system shared libraries. The correct library suffix for your platform will be supplied by default, so it need not be specified in the extension name. An alternative is to use the load keyword inside the program to load a shared library. This advanced feature is described in detail in Writing Extensions for gawk.
Warn about constructs that are dubious or nonportable to other awk implementations. No space is allowed between the -L and value , if value is supplied. Some warnings are issued when gawk first reads your program. Others are issued at runtime, as your program executes.
The optional argument may be one of the following:. Cause lint warnings become fatal errors. This may be drastic, but its use will certainly encourage the development of cleaner awk programs.
Only issue warnings about things that are actually invalid are issued. This is not fully implemented yet. Some warnings are only printed once, even if the dubious constructs they warn about occur multiple times in your awk program. Thus, when eliminating problems pointed out by --lint , you should take care to search for all occurrences of each inappropriate construct. As awk programs are usually short, doing so is not burdensome.
Select arbitrary-precision arithmetic on numbers. As of version 5. As long as this situation remains stable, MPFR will be supported. If it changes, the MPFR support will be removed from gawk. Enable automatic interpretation of octal and hexadecimal values in input data see Allowing Nondecimal Input Data. Use with care. Also note that this option may disappear in a future version of gawk.
Enable pretty-printing of awk programs. Implies --no-optimize. By default, the output program is created in a file named awkprof. The optional file argument allows you to specify a different file name for the output. No space is allowed between the -o and file , if file is supplied. NOTE: In the past, this option would also execute your program. This is no longer the case.
At the moment, this includes just simple constant folding. Optimization is enabled by default. This option remains primarily for backwards compatibility.
However, it may be used to cancel the effect of an earlier -s option see later in this list. Enable profiling of awk programs see Profiling Your awk Programs.
By default, profiles are created in a file named awkprof. The optional file argument allows you to specify a different file name for the profile file. No space is allowed between the -p and file , if file is supplied. The profile contains execution counts for each statement in the program in the left margin, and function call counts for each function. This disables all gawk extensions just like --traditional and disables all extensions not allowed by POSIX.
See Common Extensions Summary for a summary of the extensions in gawk that are disabled by this option. Also, the following additional restrictions apply:. If you supply both --traditional and --posix on the command line, --posix takes precedence. Allow interval expressions see Regular Expression Operators in regexps.
Nevertheless, this option remains for backward compatibility. Disable the system function, input redirections with getline , output redirections with print and printf , and dynamic extensions. Also, disallow adding file names to ARGV that were not there when gawk started running. Print version information for this particular copy of gawk.
This allows you to determine if your copy of gawk is up to date with respect to whatever the Free Software Foundation is currently distributing. It is also useful for bug reports see Reporting Problems and Bugs.
Mark the end of all options. Any command-line arguments following -- are placed in ARGV , even if they start with a minus sign. In compatibility mode, as long as program text has been supplied, any other options are flagged as invalid with a warning message but are otherwise ignored. The -f option may be used more than once on the command line. If it is, awk reads its program source from all of the named files, as if they had been concatenated together into one big file.
This is useful for creating libraries of awk functions. These functions can be written once and then retrieved from a standard place, instead of having to be included in each individual program. The -i option is similar in this regard. As mentioned in Function Definition Syntax , function names must be unique. After typing your program, type Ctrl-d the end-of-file character to terminate it.
Because it is clumsy using the standard awk mechanisms to mix source file and command-line awk programs, gawk provides the -e option. As with -f , the -e and -i options may also be used multiple times on the command line. If no -f option or -e option for gawk is specified, then awk uses the first nonoption command-line argument as the text of the program source code.
Arguments on the command line that follow the program text are entered into the ARGV array; awk does not continue to parse the command line looking for options. For a Bourne-compatible shell such as Bash , you would add these lines to the.
For a C shell-compatible shell, 12 you would add this line to the. Any additional arguments on the command line are normally treated as input files to be processed in the order specified.
See Assigning Variables on the Command Line. Command-line options and the program text if present are omitted from ARGV. All other arguments, including variable assignments, are included. The distinction between file name arguments and variable-assignment arguments is made when awk is about to open the next input file. At that point in execution, it checks the file name to see whether it is really a variable assignment; if so, awk sets the variable instead of reading a file.
Therefore, the variables actually receive the given values after all previously specified files have been read. The variable values given on the command line are processed for escape sequences see Escape Sequences. In some very early implementations of awk , when a variable assignment occurred before any file names, the assignment would happen before the BEGIN rule was executed.
The variable assignment feature is most useful for assigning to variables such as RS , OFS , and ORS , which control input and output formats, before scanning the data files.
It is also useful for controlling state if multiple passes are needed over a data file. Given the variable assignment feature, the -F option for setting the value of FS is not strictly necessary. It remains for historical compatibility. Consider the following:. In this case, awk reads from standard input instead of trying to open any command line files. To the unwary, this looks like awk is hanging. To demonstrate:.
Often, you may wish to read standard input together with other files. If FS is null, the input line is split into one field per character. Normally, any number of blanks separate fields.
The field separator is usually set via the -F option or from inside a BEGIN block so that it takes effect before the input is read. Pattern-action statements are separated by newlines or semicolons. Statements are terminated by semicolons, newlines or right braces. String constants are quoted "" , with the usual C escapes recognized within see printf 1 for a complete list of these. The operators! Variables may be scalars, array elements denoted x[i] or fields.
Variables are initialized to the null string. Array subscripts may be any string, not necessarily numeric; this allows for a form of associative memory.
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