If you are doing a substitution with sed but need to exclude a specific line or pattern, that can be accomplished by prefixing an exclusion and using “!”. For example, to substitute “hello” for “goodbye” in the following content, but to skip any line containing “world”, use syntax like the following: $ sed “/world/! s/hello/goodbye/g” … Bash: sed substitution with an exclusion pattern
As with any programming language, Python has a multitude of ways to accomplish the same task. In this article, I will explore the idea of taking a string and checking if it ‘startswith’ any of the strings from a predetermined list. As context for the example code, RFC1918 sets out several IPv4 ranges that can … Python: exploring the use of startswith against a list: tuple, regex, list comprehension, lambda
One of the features of the ‘lineinfile‘ regexp parameter is the ability to use regular expression capture groups in the line output. That allows you to extract values on a found line when constructing the output line. Specifically, that can mean pulling information such as hostname/port, file path, or preserving the yaml indentation of an … Ansible: regex capture groups with lineinfile to preserve yaml indentation
If your Ansible automation includes modifying an existing configuration file (versus managing your own fully templatized version of the config), then you will need to account for variations in that existing file when using ‘lineinfile‘ for key/value pairs. A naive lineinfile replacement will not find commented-out keys and might not even find valid keys that … Ansible: lineinfile with regex to robustly populate key/value pairs in config file
If you need to output only lines before (or after) a unique line identified by a regular expression, then here are examples of using sed. All examples in this article can be found in github. Consider the following lines of content in the variable “$lines” $ echo “$lines” ape swings frog jumps logs dog barks … Bash: output all lines before/after line identified by regex
sed is a powerful utility for transforming text. One of the nice tricks with sed is the ability to reuse capture groups from the source string in the replacement value you are constructing. For example, if you have have the following kernel parameters in “/etc/default/grub” $ grep GRUB_CMDLINE_LINUX_DEFAULT /etc/default/grub GRUB_CMDLINE_LINUX_DEFAULT=”quiet splash” And wanted to append … Bash: Appending to existing values using sed capture group
The =~ binary operator provides the ability to compare a string to a POSIX extended regular expression in the shell. Take note, the right-hand side regex cannot be surrounded by quotes or it will be treated as a regular string, it cannot contain spaces, and must conform to POSIX regex rules and use character classes … Bash: Using BASH_REMATCH to pull capture groups from a regex
Logstash has a rich set of filters, and you can even write your own, but often this is not necessary since there is a out-of-the-box filter that allows you to embed Ruby code directly in the configuration file. Using logstash-filter-ruby, you can use all the power of Ruby string manipulation to parse an exotic regular expression, … ELK: Using Ruby in Logstash filters