Tag Archives: awk

How to print unique errors(with count) from Apache error logs

After I moved to WordPress Multisite recently, I had to closely watch the Apache error log file for a couple of days to make sure that there weren’t any configuration mismatch.

Initially I used run the tail command to view the errors, but then I quickly realized that if there is a frequent error, then it becomes extremely difficult to find out other non frequent errors.


After some fiddling with the awk and sort commands, I came up with the following one liner that prints the unique errors with their count.


awk script is executed for each line in the error log file.

Field Separator

The -F directive is used to specify field separator. If you look at a single line in the apache error log file, you will notice that each field is enclosed by [ and ].

awk allows us to specify multiple field separators by enclosing them between []. That’s what we are specifying in -F[\[\]]

Filtering out lines that contain error

After splitting the fields, we need to filter out the lines that contain the term error in the fourth field. That’s what happens in the next part of the command. Basically the part $4 == "error"{print $7} prints the seventh field, which has the real error message if the fourth field is equal to the string “error”.

Sorting the error messages

The next step is to sort all the error messages. This is done by piping the output to the sort command.

Finding unique error messages and counting them

The next step is to find the unique error messages and then count them. This is done by piping the output to the uniq command. The -c flag does the counting.

Sorting the messages by frequency

The last step is to sort the messages again by frequency and print them. This is done by the last sort command. The -n flag sorts by numbers and the flag -r does the sorting by descending order.

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Sum up all values of a column in a text file

Recently, I had to sum up all integers of a column in a text file (similar to how you do in excel). After some digging up, I came up with a awk one liner to do it.

Following my tradition of documenting one liners, I am going to document this one as well 🙂

Input and output data

Here is the super simplified version of input data that I was using.

I wanted to find the sum of all values present in the 3rd column. So in the case, the output that I was expecting was 145


Here is the awk one liner, which does this.


awk script is executed for each line and the first part of the command creates a variable s that stores the sum of all values in the 3rd column.

When the end of file is reached, the second part of the command is executed, which just prints the value of the variable.

Field separator

If the columns are separated by a comma or by any other non whitespace character, then you have to just specify it by adding FS=',' to the above command.

The more I dig deeper into awk, the more I like it and it is really fascinating to see how much you can do with this tool.

I learned a lot about awk and hopefully this teaches something to you as well 😉

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Remove duplicate lines based on a field

Recently while working on formatting some data files for further processing, I had to remove duplicate lines from the file based on a particular field. After trying out cut and grep commands, I was finally able to solve it with a very concise awk command/script.

The command was so concise but still was packed with so much information and it helped me to learn more about the awk scripting language. I thought of writing about it here so that it is useful for others and also I know where to search for it, when I needed it 🙂

Feel free to use it in whatever way you want, if it solves your problem as well.

Input and output data

Let me first explain the input data I had and the output that I was expecting.

Consider a file which has the following lines. Each line has four fields.

Now assume that we want to remove duplicate lines by comparing only the second field. We want the output to look like this.


Get ready for the surprise. The actual command is just this.


awk script execution and printing

awk script is executed for each line and if the result is true then the line is printed. If the result is false then the line is not printed.

Associative arrays

The awk language supports associate arrays, similar to the ones found in PHP. The script x[$2]++ fills up an associate array. The key used here is $2 which refers to the second field and x is the variable name. You can use any name for it.

The array is populated for every line. This is how the array would look like after each line.

Conditional evaluation

The ! operator results in a boolean evaluation which determines whether a particular line should be passed on to the output (printed) or not.

When the field is not present in the array, then it results in a zero value which is false. The ! (not) operator evaluates it to non-zero, which results in a true value and the line is passed on to the output (printed). When a duplicate is found, the array returns a non-zero count, which is true, but the ! converts it to false and that line is not passed on to the output.

The expanded version of the above command would be

But what is the fun in using the expanded version 😉

Field separator

In the input file that I had, the fields were separated by whitespace, so I didn’t have to specify the field separators. But if you are using a non-whitespace field separators, then you can specify it by adding FS="," to the above command.

This one-liner actually thought me that awk supports a full programming language that can be used to create scripts and also increased my understanding of the way awk command works. Hopefully this teaches something for you as well 🙂

I know that this is already a concise version, but if you think that this can be improved, then do let me know.

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Parse Apache error log and list down all missing images

Recently I had to parse Apache error log and find out all images that are missing.

After referring to a couple of man pages, I came up with this one liner. I am sure I will need it again, so thought of noting it here so that I know where to look when I need it again 😉

Feel free to use it in whatever way you want, if it solves your problem as well.


This assumes that each line in your apache error log looks like this.


Filter all 404 lines from the log file

The first step is to filter all lines that that contain the “File does not exist” text. This is done by using sed.

  • By default, sed prints out all lines. This is prevented by the -n.
  • The second option is the regular expression followed by the p flag. This option prints out all lines which match the text.
  • The third option is the name of the error log file.

Extract the last column of matching lines

The next step is to retrieve the file name from the matching lines. This is done by using awk.

  • By default awk uses space as the delimiter and splits the lines into different columns. If you look at each line, we want the last column.
  • NF is a special variable which points to the last column.
  • print $NF prints the last column

Filter only images

The next step is to filter out only the images. This is done again by using sed.

  • I use -n again to prevent sed from printing all lines.
  • The -r is added, so that we can use extended regular expression
  • The regular expression (jpg|jpeg|png|gif)$ filters out all images and p at the end prints out only lines that match

Sort and find uniques

The sort and uniq commands sort the list and find the unique lines.

Write to a file

The final output is written to a file by using the redirection > operator. If you want to append to a file then we may have to use >> operator.

More to come

It is really amazing like how you can combine these tools to do amazing things. I am planning to document other one liners which I end up creating to solve my problems. So stay tuned 🙂

Also if you think this can be improved, then do let me know as well.

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