Bash

Add a new line whenever the first column changes 3

 in Bash / GNU/Linux  tagged awk / bash / new line by

Recently we were processing some results from an SQL query on the command line, we were grouping the results based on the first column of the query results and we wanted to add an empty line between each group of data.

Instead of messing with SQL specifics, we decided to use awk to finish the task which resulted in the following code:

awk -F '|' -v column=1 'NR>1 && $column != previous { print "" } { previous = $column } { print $0 }'

Explanation:

  • -F fs or --field-separator fs: This option allowed us to choose the input field separator, in other words the character that should be used to split our text into columns. We used the value | because that character is the default column delimiter for sqlite.
  • -v var=val or --assign var=val: We used the -v option to define a variable to be used later on in the script. The value 1 was assigned to the variable column before execution of the program began and it was available event to the BEGIN rule of the AWK program. We did this to make the code a bit more modular, we could have just hardcoded the number in.
  • NR>1 && $column != previous { print "" } : Here we defined an if statement that checks two options: First we make sure that we are not on the first line of the input by using the NR>1 (and thus avoid creating an empty line which will be the first line of the output).
    Second, we check that the last value we had for the column of interest did change since last time. (We still did not define the value of previous, it is on the next step). When both statements are true (we are not on the first row and the value of the column in the current row is different than the value of the column in the previous row) it will print out an empty line.
  • { previous = $column }: This part is executed on ALL lines (even the first one) no matter what the values are. What this line does is to translate the value of the column variable from being a number (the index of the column that we are interested in) into the actual value that the column has at that specific line. That value is then copied to the previous variable to allow us to perform the check in the previous point once we move to the next line.
  • { print $0 }: Finally, this part is also executed on all lines and it instructs awk to print the input row whole and as is. This whole part could be replaced by a true value like the value 1. In awk as you see in this example, you define a series of operations. Each operation is constructed by a pattern to be matched and an action. Each pattern is evaluated for each input line, and in the cases where the pattern matches, the action is executed. The user can choose to omit either the pattern or the action for any operation. When a pattern is omitted, the action is executed on every line. When the action is omitted, then awk will execute { print $0 }. So, by adding a true value on its own it will be translated as on each line execute { print $0 } which prints the whole row as is.

Example

1|1|0.0564904019731175
1|2|0.103176086258974
1|3|0.12910406904073
1|4|0.188592489201024
1|5|0.169676224898487
1|6|0.164690820027741
1|7|0.128458728519047
1|8|0.18549773544014
1|9|0.155677575617836
1|10|0.153941343314285
2|1|0.217221158956016
2|2|0.23390973064067
2|3|0.180231657220626
2|4|0.257673927303071
2|5|0.261393785194329
2|6|0.273441488895552
2|7|0.242815632929545
2|8|0.262269697286057
2|9|0.256054399760891
2|10|0.262613705138411
3|1|0.378589461360716
3|2|0.33008177312116
3|3|0.380973166776554
3|4|0.340431190160728
3|5|0.38189416214207
3|6|0.364842933594872
3|7|0.372958396398964
3|8|0.350010176652464
3|9|0.355815612501188
3|10|0.380553180349294

Will become

1|1|0.0564904019731175
1|2|0.103176086258974
1|3|0.12910406904073
1|4|0.188592489201024
1|5|0.169676224898487
1|6|0.164690820027741
1|7|0.128458728519047
1|8|0.18549773544014
1|9|0.155677575617836
1|10|0.153941343314285

2|1|0.217221158956016
2|2|0.23390973064067
2|3|0.180231657220626
2|4|0.257673927303071
2|5|0.261393785194329
2|6|0.273441488895552
2|7|0.242815632929545
2|8|0.262269697286057
2|9|0.256054399760891
2|10|0.262613705138411

3|1|0.378589461360716
3|2|0.33008177312116
3|3|0.380973166776554
3|4|0.340431190160728
3|5|0.38189416214207
3|6|0.364842933594872
3|7|0.372958396398964
3|8|0.350010176652464
3|9|0.355815612501188
3|10|0.380553180349294

Bash: Problem with reading files with spaces in the name using a for loop

 in Bash  tagged bash / find / for / for loop by

Recently we were working on a bash script that was supposed to find and process some files that matched certain criteria. The script would process the files one by one and the criteria would be matched using the find command. To implement our solution, we returned the results of the find back to the for loop in an attempt to keep it simple and human readable.

Our original code was the following:
(do not use it, see explanation below)

for file in `find $search_path -type f -name '*.kml'`; do
  # Formatting KML file to be human friendly.
  xmllint --format "$file" > "$output_path/$file";
done

Soon we realized that we had a very nasty bug, the way we formatted the command it would break filenames that had spaces in them into multiple for loop entries and thus we would get incorrect filenames back to process.

To solve this issue we needed a way to force our loop to read the results of find one line at a time instead of one word at a time. The solution we used in the end was fairly different than the original code as it had the following significant changes:

  • the results of the find command were piped into the loop
  • the loop was not longer a for loop and a while loop was used instead
  • it used the read command that reads one line at a time to fill in the filename variable
    (the -r parameter does not allow backslashes to escape any characters)

Solution

find $search_path -type f -name '*.kml' | 
while read -r file; do
  # Formatting KML file to be human friendly.
  xmllint --format "$file" > "$output_path/$file";
done

Create an encrypted 7zip archive with encrypted header as well (no filenames are visible)

 in Bash  tagged 7z / 7zip / encryption / filenames / headers / password / protection by

In case you come to a scenario where you need to encrypt, password protect the contents of a 7zip archive and make sure that not even the filenames of the contents are visible, 7zip has your back! As you can see in the following example you can implement the above requirements very easily.

7z a -p"pbVfdPs27Dc" -mhe hello.7z file1.bin file2.doc files.*

The structure of the above 7z command is the following:

#Based on: 7z <command> [<switches>...] <archive_name> [<file_names>...]
7z a -p"Some Password!.32@" -mhe <archive_name> [<file_names>...]

To break it down, it goes like this:

  • We used the <command> a, which instructs the tool to add the listed files to the listed archive (if the archive does not exist, it will create it).
  • The <switch> -p, allows you to set the password for the archive.
  • The second <switch> -mhe (or -mhe=on) it enables data and header archive encryption.
    In case you cannot find this switch at the manual, check the examples in the man page (This command works on GNU/Linux, it was tested on Fedora).

NetCat (nc) as a webserver

 in Bash / GNU/Linux  tagged nc / NetCat / webserver by

Recently, we needed to perform some tests in a network. Specifically, we wanted to check the configuration of a firewall and see what IP are blocked and/or which ports are allowed to go through. To do so, we used NetCat to setup a small web-server to perform our tests.

Netcat (often abbreviated to nc) is a computer networking utility for reading from and writing to network connections using TCP or UDP. Netcat is designed to be a dependable back-end that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and investigation tool, since it can produce almost any kind of connection its user could need and has a number of built-in capabilities.

From: https://en.wikipedia.org/wiki/Netcat


while true;
do
  echo -e "HTTP/1.1 200 OK\r\n\r\n<h1>$(hostname) is live</h1>$(date)" | nc -vl -p 5555;
done

or in one line


while true; do echo -e "HTTP/1.1 200 OK\r\n\r\n<h1>$(hostname) is live</h1>$(date)" | nc -vl -p 5555; done

Explanation of code:

  • The above code creates an infinite loop that calls nc in listening mode, we had to do this as nc will terminate as soon as it serves one client.
  • Using echo we create an html 200 response along with a small “webpage”.
  • While generating the webpage, echo -e will execute the commands hostname and date to get the current system values adding them to the resulted text.
  • The resulted text is then piped to nc to be served as a response to any incoming clients.
  • The date and time that nc will show to the client is not the current date and time when visiting the webpage but the one that was when echo was executed.

nc parameters:

  • -v, --verbose Sets the verbosity level and it can be used several times to increase it even further
  • -l, --listen Instructs nc to bind and listen for incoming connections (just like a web-server)
  • -p, --source-portwith port parameter specifies the source port to be used by nc