scripting

What is sed ? “sed” is used in “Loklak Server” one of the most popular projects of FOSSASIA. “sed” is acronym for “Stream Editor” used for filtering and transforming text, as mentioned in the manual page of sed. Stream can be a file or input from a pipeline or even standard input. Regular expressions are used to filter the text and transformation are carried out using sed commands, either inline or from a file. So, most of the time writing a single line does the work of text substitution, removal or to obtaining a value from a text file. Basic Syntax of “sed” $sed [options]... {inline commands or file having sed commands} [input_file]... Loklak Server uses a config.properties file - contains key-value pairs - which is the basis of the server as it contains configuration values, used by the server during runtime for various operations. Let’s go through a simple sed example that prints line containing word “https” at the beginning in the config.properties file. $sed -n '/^https/p' config.properties Here “-n” option suppresses automatically printing of pattern space (pattern space is where each line is put that is to be processed by sed). Without “-n” option sed will print the whole file. Now, the regular expression part,  “/^https” matches all the lines that has “https” at the start of line and “/p” is print command to print the output in console. Finally we provide the filename i.e. config.properties. If filename is not provided then sed waits for input from standard input. Use cases of “sed” in Loklak Server Displaying proper port number in message while starting or installing Loklak Server The default port of loklak server is port number 9000, but it can be started in any non-occupied port by using “-p” flag with bin/start.sh and bin/installation.sh like $ bin/installation.sh -p 8888 starts installation of Loklak Server in port 8888. To display the proper localhost address so that user can open it in a browser the port number in shortlink.urlstub parameter in config.properties needs to be changed. This is carried out by the function change_shortlink_urlstub in bin/utility.sh. The function is defined as Now let's try to understand what the sed command is doing. “-i” option is used for in-place editing of the specified file i.e. config.properties in conf directory. “s” is substitute command of sed. The regular expression can be divided into two parts, between “/”: \(shortlink\.urlstub=http:.*:\)\(.*\) this is used to find the match in a line. \1'"$1" is used to substitute the matched string in part 1. The regular expressions can be split into groups so that operations can be performed on each group separately. A group is enclosed between “\(“ and “\)”. In our 1st part of regular expressions, there are two groups. Dissecting the first group i.e. \(shortlink\.urlstub=http:.*:\): “shortlink\.urlstub=http:” will match the expression “shortlink.urlstub=http:”, here “\” is used as an escape sequence as “.” in regex represents any character. “.*:”, “.” represents any character and “*” represents 0 or more characters of the previous character. So, it…