Awk what is 0

Associative because the indexes could be text, not only numeric. The reference to the array at some key for example a[one] if the line is one instantiates creates the array space in memory to store it. If, in the future, there is another line that also has the value of one the same key , no new memory is used, as the array position has already been created. For each unique line in the input file, that line is stored as an index value in the memory.

If there are many unique lines, a lot of memory might get used. This step has raised a lot of controversy in previous posts. Why first multiply? Because it is higher in the precedence table, and then add as it is at the lowest last level.

There is no way around that IMO. It actually generates two results. One is the value of the variable technically an lvalue that is incremented. That ends stored at a memory location. The second one is the value that is given as the result of the expresion as evaluated up to that point. That is given to the next element on the expression:.

It doesn't matter. Or, said the other way around but not necessarily the sequence of what happens is: the value is incremented after it is used in the expression. The reason on why the exact sequence might not be exactly defined is because, in most implementations, the order of the actions is:. Some languages wait until all the parts of the expression have been evaluated in the hopes that the value in the register B is used again somewhere in the expression and doesn't need to be retrieved again from memory and then return the value in register B to memory.

Then, on the non-incremented value for a post-increment we apply the! With a result of 1 , the line gets printed. On successive lines it might be incremented to bigger numbers unless it overflow but it won't return to 0. Sign up to join this community. The best answers are voted up and rise to the top. For example suppose you have a list of numbers in file2 single column as follows Print rows based on separator. For below lines in a file. Print various rows in one row. I have this in a file Dear All, I have a data file input.

Only five column shown here for example. Awk: Print out overlapping chunks of file - rows ,, etc. First time poster, but the forum has saved my bacon more times than Anyway, I have a text file, and wanted to use Awk or any other sensible program to print out overlapping sections, or arbitrary length. To describe by example, for file 1 2 3 4 5 etc I want the out put Print individual rows. Code :. Last edited by glev; at AM..

Originally Posted by glev Originally Posted by birei. Using next instruction: Code :. Who -r interpret? Need to interpret code. Shell Programming and Scripting. Can someone interpret this -- not sure. Was wondering if someone could interpret this for me -- I'm not sure what everything means.

I think, if I understand How to interpret TOP. But I don't know how to interpret the output from TOP. In this case, the single argument is "-f". Another difference between the command line option and the internal variable is the ability to set the input field separator to be more than one character.

You cannot do this on the command line. There is a third advantage the internal variable has over the command line option: you can change the field separator character as many times as you want while reading a file. Well, at most once for each line. You can even change it depending on the line you read.

Suppose you had the following file which contains the numbers 1 through 7 in three different formats. Lines 4 through 6 have colon separated fields, while the others separated by spaces. You don't have to reset it for each line. Sounds simple, right? However, I have a trick question for you. What happens if you change the field separator while reading a line? That is, suppose you had the following line One Two:Three:4 Five and you executed the following script:!

However, if you deleted the first print statement, it would print out "Three" once! I thought this was very strange at first, but after pulling out some hair, kicking the deck, and yelling at muself and everyone who had anything to do with the development of UNIX, it is intuitively obvious. You just have to be thinking like a professional programmer to realize it is intuitive.

I shall explain, and prevent you from causing yourself physical harm. If you change the field separator before you read the line, the change affects what you read. If you change it after you read the line, it will not redefine the variables. You wouldn't want a variable to change on you as a side-effect of another action. A programming language with hidden side effects is broken, and should not be trusted.

AWK allows you to redefine the field separator either before or after you read the line, and does the right thing each time. Once you read the variable, the variable will not change unless you change it. To illustrate this further, here is another version of the previous code that changes the field separator dynamically. When the line contains a colon, the field separator is a colon, otherwise, it is a space. Here is a version that worked with older versions of awk:!

In the first case, the two positional parameters are concatenated together and output without a space. In the second case, AWK prints two fields, and places the output field separator between them. Normally this is a space, but you can change this by modifying the variable "OFS". If you wanted to copy the password file, but delete the encrypted password, you could use AWK:! You can make the output field separator any number of characters.

You are not limited to a single character. You may want to have your script change its operation based on the number of fields. As an example, the command "ls -l" may generate eight or nine fields, depending on which version you are executing.

If you wanted to print the owner and filename then the following AWK script would work with either version of "ls:"! This allows you to print the last field of any column! There is a limit of 99 fields in a single line. PERL does not have any such limitations. This tells you the number of records, or the line number. You can use AWK to only examine certain lines.

This example prints lines after the first lines, and puts a line number before each line after ! If you set it to an empty string, then AWK will read the entire file into memory. You can combine this with changing the "FS" variable. This example treats each line as a field, and prints out the second and third line:! Also this will only work if the input file is less than lines, therefore this technique is limited.

You can use it to break words up, one word per line, using this:! If there is a tab or punctuation inside, it would not. This can be set to be a newline and carriage return, if you need to generate a text file for a non-UNIX system. Normally you use standard input to provide AWK with information. You can also specify the filenames on the command line. If the above script was called "testfilter", and if you executed it with testfilter file1 file2 file3 It would print out the filename before each change.

I have used this when I want to put some information before and after a filter operation. The prefix and postfix files special data before and after the real data.

By checking the filename, you can parse the information differently. This is also useful to report syntax errors in particular files:! AWK was the first language I found that has associative arrays. The perl language was released later, and had hash arrays, which are the same thing. But I will use the term associative arrays because that is how the AWK manual describes them. This term may be meaningless to you, but believe me, these arrays are invaluable, and simplify programming enormously.

Let me describe a problem, and show you how associative arrays can be used for reduce coding time, giving you more time to explore another stupid problem you don't want to deal with in the first place. Let's suppose you have a directory overflowing with files, and you want to find out how many files are owned by each user, and perhaps how much disk space each user owns.

You really want someone to blame; it's hard to tell who owns what file. A filter that processes the output of ls would work: ls -l filter But this doesn't tell you how much space each user is using. It also doesn't work for a large directory tree. This requires find and xargs : find. The filter has to count how many times it sees each user.

The typical program would have an array of usernames and another array that counts how many times each username has been seen. The index to both arrays are the same; you use one array to find the index, and the second to keep track of the count. I'll show you one way to do it in AWK--the wrong way:! I told you it's the wrong way to do it.

If you were a C programmer, and didn't know AWK, you would probably use a technique like the one above. Here is the same program, except this example that uses AWK's associative arrays. The important point is to notice the difference in size between these two versions:! The concept is simple. Instead of using a number to find an entry in an array, use anything you want. An associative array in an array whose index is a string. All arrays in AWK are associative.

In this case, the index into the array is the third field of the "ls" command, which is the username. We need to add some more intelligence to the AWK script, and need the right foundation to proceed. There is also a slight bug in the AWK program.

If you wanted a "quick and dirty" solution, the above would be fine. If you wanted to make it more robust, you have to handle unusual conditions. If you gave this program an empty file for input, you would get the error: awk: username is not an array Also, if you piped the output of "ls -l" to it, the line that specified the total would increment a non-existing user.

There are two techniques used to eliminate this error. The first one only counts valid input:! However, it still generates an error when an empty file is read as input.

To fix this problem, a common technique is to make sure the array always exists, and has a special marker value which specifies that the entry is invalid. Then when reporting the results, ignore the invalid entry. Apply this technique and you will make your AWK programs more robust and easier for others to use. It can. However, you don't use conventional two-dimensional arrays.

Instead you use associative arrays. Did I even mention how useful associative arrays are? Remember, you can put anything in the index of an associative array. It requires a different way to think about problems, but once you understand, you won't be able to live without it. All you have to do is to create an index that combines two other indices. It combines the three strings into the single string "1,2". Then it uses it as an index into the array. That's all there is to it.

There is one minor problem with associative arrays, especially if you use the for command to output each element: you have no control over the order of output. You can create an algorithm to generate the indices to an associative array, and control the order this way. However, this is difficult to do.

Since UNIX provides an excellent sort utility, more programmers separate the information processing from the sorting. I'll show you what I mean. This example will demonstrate these techniques, and illustrate the power and elegance of AWK.

The program is simple and common. The disk is full. Who's gonna be blamed? I just hope you use this power wisely. Remember, you may be the one who filled up the disk. Having resolved my moral dilemma, by placing the burden squarely on your shoulders, I will describe the program in detail. I will also discuss several tips you will find useful in large AWK programs. First, initialize all arrays used in a for loop.

There will be four arrays for this purpose. Selecting the names of the arrays, and the indices for each array is very important. In a complex program, it can become confusing to remember which array contains what. I suggest you clearly identify the indices and contents of each array.

Even when a quick hack comes back to haunt you three years later. I've been there. The third suggestion is to make sure your input is in the correct form. It's generally a good idea to be pessimistic, but I will add a simple but sufficient test in this example. I placed the test and error clause up front, so the rest of the code won't be cluttered. AWK doesn't have user defined functions.

The next piece of advice for complex AWK scripts is to define a name for each field used. In this case, we want the user, group and size in disk blocks. We could use the file size in bytes, but the block size corresponds to the blocks on the disk, a more accurate measurement of space.

Disk blocks can be found by using "ls -s". This adds a column, so the username becomes the fourth column, etc. Of course this is confusing. That's why it's a good idea to assign names to the fields. I've been there too. Next the AWK script will count how many times each combination of users and groups occur. That is, I am going to construct a two-part index that contains the username and groupname. Consider this: how would you calculate the total for just a user, or for just a group?

You could rewrite the script. You could do it, but it's not the AWK way to do it. If you had to examine a bazillion files, and it takes a long time to run that script, it would be a waste to repeat this task. It's also inefficient to require two scripts when one can do everything. The proper way to solve this problem is to extract as much information as possible in one pass through the files.

I don't really need 4 arrays, as I can use the format of the index to determine which array is which. But this does maake the program easier to understand for now.

The next tip is subtle, but you will see how useful it is. I mentioned the indices into the array can be anything. If possible, select a format that allows you to merge information from several arrays. I realize this makes no sense right now, but hang in there. All will become clear soon. There is a space between the two values.

What about the other three arrays? The heart of the script totals up the number and size of each file, putting the information into the right category. Also important is to sort the information in an order that is useful. You can try to force a particular output order in AWK, but why work at this, when it's a one line command for sort? The difficult part is finding the right way to sort the information. This script will sort information using the size of the category as the first sort field.

The largest total will be the one for all files, so this will be one of the first lines output. However, there may be several ties for the largest number, and care must be used. The second field will be the number of files.