The blog post laments the absence of a simple, built-in command-line tool in common Unix systems for sorting IPv6 addresses correctly. Standard sorting tools like sort treat IPv6 addresses as strings, leading to incorrect ordering. The author explores several workarounds, including converting addresses to a sortable format using expansion and zero-padding, leveraging specialized tools like ip6calc, or scripting solutions. Ultimately, the post highlights the surprising complexity of this seemingly straightforward task and calls for a more elegant, standardized solution within core Unix utilities.
The blog post "The lack of a good command line way to sort IPv6 addresses" by Chris Siebenmann explores the challenges of sorting IPv6 addresses using standard Unix command-line tools. Siebenmann begins by highlighting the inherent difficulty stemming from the mixed hexadecimal and colon-separated representation of IPv6 addresses. A naive lexicographical sort, as performed by sort, produces incorrect results because it treats the hexadecimal characters as strings, disregarding their numerical values and the structure of the IPv6 address.
He then delves into several potential solutions, analyzing their effectiveness and limitations. One approach involves converting the addresses to a sortable format, such as a 128-bit integer. This can be achieved using tools like Perl, Python, or dedicated network utilities, which are capable of parsing the IPv6 address and representing it in its canonical numerical form. However, this method requires external tools and introduces complexity to the otherwise simple task of sorting.
The post further examines the use of sort -n, which sorts numerically. While this handles the hexadecimal digits correctly, it fails to address the colons, which still influence the sorting order in an undesirable way. Siebenmann emphasizes that even if the colons were removed, sort -n would still be inadequate for sorting compressed IPv6 addresses (those with ::) because the compression represents a variable number of omitted zeros. Correctly interpreting these compressed addresses requires specialized knowledge of the IPv6 address format.
Siebenmann also considers the possibility of using dedicated IP address manipulation tools. While some utilities offer sorting capabilities, they might not be readily available in all environments or provide the desired flexibility.
The post concludes by lamenting the absence of a simple, readily available command-line utility or shell built-in function specifically designed for sorting IPv6 addresses. The author emphasizes the need for a solution that correctly handles both the hexadecimal representation and the complexities of IPv6 address compression, ideally within the standard Unix toolset, offering the convenience and portability expected from command-line operations. The current lack of such a tool necessitates resorting to more complex workarounds, involving external scripts or programs, ultimately hindering efficient manipulation of IPv6 addresses on the command line.
Summary of Comments ( 41 )
https://news.ycombinator.com/item?id=44026414
HN commenters generally agree that sorting IPv6 addresses from the command line is tricky. Several suggest using
sort -k, potentially with some preprocessing viaawkorsedto isolate the relevant parts of the address for numerical sorting. Some note the complications introduced by mixed representations (e.g., compressed vs. expanded addresses) and the need to handle various formats like CIDR notation. One commenter highlights the difficulty of sorting IPv6 addresses lexicographically as opposed to numerically. Another commenter suggests a Python solution using theipaddressmodule. Several commenters point out that thesort -V(version sort) option likely won't work correctly for IPv6 addresses, reinforcing the original poster's frustration.The Hacker News post "The lack of a good command line way to sort IPv6 addresses" has several comments discussing the challenges and potential solutions for sorting IPv6 addresses on the command line.
Several commenters point out that the standard
sortcommand doesn't handle IPv6 addresses correctly because it sorts lexicographically, which doesn't reflect the numerical value of the addresses. This leads to incorrect ordering, especially when addresses have varying numbers of leading zeroes or use mixed representations (e.g., compressed vs. full expansion).One commenter suggests converting the IPv6 addresses to 128-bit integers before sorting and then converting them back. This approach ensures correct numerical sorting. They even provide a small Python script to perform this conversion and sorting. This solution is generally well-received, with other commenters appreciating its simplicity and effectiveness.
Another commenter suggests using the
-n(numeric sort) option withsort, but others quickly point out that this only works for decimal numbers and not for the hexadecimal representation often used with IPv6 addresses.There's a discussion about the potential use of specialized tools like
gawkor Perl for more complex sorting scenarios, especially when dealing with mixed representations of IPv6 addresses. However, these solutions are considered less convenient for simple sorting tasks.The original poster highlights the lack of a built-in, straightforward command-line tool specifically designed for sorting IPv6 addresses, similar to how
sort -nhandles numerical values. This sentiment is echoed by other commenters who express the need for a more user-friendly solution.Some commenters discuss the complexities of IPv6 address representation, including the use of zero compression and mixed notation, which contribute to the difficulty of sorting.
One commenter mentions the
sipcalctool, suggesting it might have capabilities for sorting IPv6 addresses, although this isn't confirmed within the thread.Overall, the comments highlight the existing gap in command-line tools for easy and accurate sorting of IPv6 addresses, while also offering some workarounds using scripting or existing tools with some manipulation. The most compelling comments are the ones offering practical Python and Perl snippets to convert the addresses for accurate sorting and the ones highlighting the need for a more dedicated tool for this specific purpose.