The Beauty of Unix Pipelines

  • 时间: 2020-06-05 06:33:38

The Unix philosophy lays emphasis on building software that is simple and extensible. Each piece of software must do one thing and do it well. And that software should be able to work with other programs through a common interface – a text stream. This is one of the core philosophies of Unix which makes it so powerful and intuitive to use.

This is an excerpt from The Unix Programming Envirnonment

Even though the UNIX system introduces a number of innovative programs and techniques, no single program or idea makes it work well. Instead, what makes it effective is the approach to programming, a philosophy of using the computer. Although that philosophy can’t be written down in a single sentence, at its heart is the idea that the power of a system comes more from the relationships among programs than from the programs themselves. Many UNIX programs do quite trivial things in isolation, but, combined with other programs, become general and useful tools.

I think that explains it pretty well. Also, watch Brian Kernighan being a complete chad and explaining fundamentals of the UNIX OS where he also goes through an example of using pipes.

In this post though, I would like to show some examples of this philosophy in action – of how one can use different unix tools together to accomplish something powerful.


  • Printing a leaderboard of authors based on number of commits to a git repo
  • Browse memes from /r/memes and set your wallpaper from /r/earthporn
  • Get a random movie from an IMDb list

Example 1 - Printing a leaderboard of authors based on number of commits in a git repo

Let’s start with a simple one – display a list of authors/contributors of a git repo sorted based on the number of commits and sort the list in descending order (most commits contributed at the top). This is a simple task when you think of it in terms of piplines. git log is used to display commit logs. We can pass the --format=<format> option to it and mention what format we want the commits to be displayed in. --format='%an' just prints the author’s name for each commit.

$ git log --format='%an'AliceBobDeniseDeniseCandiceDeniseAliceAliceAlice

Now we can use the sort utility to sort them alphabetically.

$ git log --format='%an' | sortAliceAliceAliceAliceBobCandiceDeniseDeniseDenise

Next we use uniq

$ git log --format='%an' | sort | uniq -c    4 Alice    1 Bob    1 Candice    3 Denise

According to uniq 's man page:

uniq- report or omit repeated lines

Filter adjacent matching lines from INPUT (or standard input), writing to OUTPUT (or standard output).

So uniq prints out repeated lines, but only those that appear adjacent to eachother . That is why we had to pass the output first to sort . The -c flag prefixes each line by the number of occurrences.

You can see the output is still sorted alphabetically. So now all that is remaining is sort it numerically. There’s a flag for that in sort , the -n flag. It considers the numbers based on their numerical value.

$ git log --format='%an' | sort | uniq -c | sort -nr    4 Alice    3 Denise    1 Candice    1 Bob

The -r flag was also included to print the list in reverse order. By default it sorts it in the ascending order. And their you have it – A list of authors sorted according to number of commits.

Example 2 - Browse memes from /r/memes and set your wallpaper from /r/earthporn

Did you know that you can just append “ .json ” to a reddit url to get a json response instead of the usual html? This allows for a world of possibilities! One such is browsing memes right from the command line (well not entirely, because the actual image will be displayed on a GUI program). We can simply curl or wget the url –

$ wget -O - -q '''{"kind": "Listing", "data": {"modhash": "xyloiccqgm649f320569f4efb427cdcbd89e68aeceeda8fe1a", "dist": 27, "children":[{"kind": "t3", "data": {"approved_at_utc": null, "subreddit": "memes","selftext": "More info available at....'......More lines......

I use wget here because it seems like the Curl User-Agent gets treated differently. Obviously, you can get around this by simply changing the ‘User-Agent’ header, but I just went with wget . Wget has a -O to provide the output filename. Most programs that take such an option also allow a value of - which represents the standard output or input depending on the context. The -q option just tells wget to be quite and not print things like progress status. Now we get a big JSON structure to work with. Now, to parse and use this JSON data meaningfully on the command line, we can use jq . jq can be thought of as sed / awk for JSON. It has a simple intuitive language of it’s own you can refer from it’s man page.

If you take a look at the response JSON, it looks something like this:

{    "kind": "Listing",    "data": {        "modhash": "awe40m26lde06517c260e2071117e208f8c9b5b29e1da12bf7",        "dist": 27,        "children": [],        "after": "t3_gi892x",        "before": null    }}

So here we have some response of the type “Listing” and we can see we have an array of “children”. Each element of that array is a post.

This is what one of the elements of the ‘children’ array looks like:

{    "kind": "t3",    "data": {        "subreddit": "memes",        "selftext": "",        "created": 1589309289,        "author_fullname": "t2_4amm4a5w",        "gilded": 0,        "title": "Its hard to argue with his assessment",        "subreddit_name_prefixed": "r/memes",        "downs": 0,        "hide_score": false,        "name": "t3_gi8wkj",        "quarantine": false,        "permalink": "/r/memes/comments/gi8wkj/its_hard_to_argue_with_his_assessment/",        "url": "",        "upvote_ratio": 0.93,        "subreddit_type": "public",        "ups": 11367,        "total_awards_received": 0,        "score": 11367,        "author_premium": false,        "thumbnail": "",        "gildings": {},        "post_hint": "image",        ".................."        "more lines skipped"        ".................."    }}

I have reduced the number of key value pairs in data . In total there were 105 items. As you can see there are many interesting data attributes you can fetch about a post. The one of our interest is url of the post. This isn’t the url of the actual reddit post but rather it’s the url of the content of the post. If the post url is what you want then that’s permalink . So in this case, the url field is the url to the meme’s image.

We can simply get the list of of all the urls of of every post using:

$ wget -O - -q | jq '.data.children[] |.data.url'""""""""""""......

Ignore the first two links, those are basically sticky posts that the mods put, whose ‘url’ is same as the ‘permalink’.

jq reads from the standard input and it’s fed the JSON we saw earlier. .data.children is referring to the array of posts I mentioned earlier. And – .data.children[] | .data.url means, “iterate through every element in the array and print the ‘url’ field which is in the ‘data’ field of every element”.

So we get a list of all the urls of the “hot” posts of /r/memes . If you wanted to get the “top” posts of the this week then you can hit . For top posts of all time? t=all , year? t=year and so on.

Once we have a list of all the URLs, we can now just pipe it into xargs . Xargs is a really useful utility to build command lines from standard input. This is what xarg’s man page says:

xargs reads items from the standard input, delimited by blanks (which can be protected with double or single quotes or a backslash) or newlines, and executes the command (default is /bin/echo) one or more times with any initial-arguments followed by items read from standard input. Blank lines on the standard input are ignored

So running something like:

$ echo "" | xargs wget -O meme.jpg -q

would be equavalent to running:

$ wget -O meme.jpg -q ""

Now, we can just pass the list of URLs to an image viewer, like feh or eog that accept a URL as a valid argument.

$ wget -O - -q | jq '.data.children[] |.data.url' | xargs feh

Now, feh pops up with the memes and I can just browse through them using the arrow keys like they were on my local disk.

Feh screen

Or I could simply just download all of the images using wget, by replacing feh with wget above.

And the possibilities are endless. Another good use of this reddit JSON data is setting the wallpaper of your desktop to the top upvoted image of /r/earthporn from the “hot” section.

$ wget -O - -q | jq '.data.children[] |.data.url' | head -1 | xargs feh --bg-fill

You can then, if you want, set this up as a cron-job that runs every hour or so. I use the head command here to just print the first line, which would be the top upvoted post. By it’s own, head seems to do something very trivial and unuseful, but in this case, working with other programs, it becomes an important part.

You see the power of Unix pipelines? That one single line does everything from fetching JSON data, parsing and getting the relevant data out of it and then again fetching the image from the URL and finally setting it as the wallpaper.

Another silly thing I used this for was for just downloading memes off of /r/memes every two hours. This is set up as a cron job on my machine. Now I have around 19566 memes taking up 4.5G on my disk. Why did I do that? Don’t ask me…

Example 3 - Get a random movie from an IMDb list

Let’s end it with a simple one. IMDb has a feature where they allow you to make lists. You can also find lists made by other users. For example - Blow Your Mind Movies . If you append /export to the url you get the list in a .csv format.

$ curl,Const,Created,Modified,Description,Title,URL,Title Type,IMDb Rating,Runtime (mins),Year,Genres,Num Votes,Release Date,Directors1,tt0137523,2017-07-30,2017-07-30,,Fight Club,,movie,8.8,139,1999,Drama,1780706,1999-09-10,David Fincher2,tt0945513,2017-07-30,2017-07-30,,Source Code,,movie,7.5,93,2011,"Action, Drama, Mystery, Sci-Fi, Thriller",471234,2011-03-11,Duncan Jones3,tt0482571,2017-07-30,2017-07-30,,The Prestige,,movie,8.5,130,2006,"Drama, Mystery, Sci-Fi, Thriller",1133548,2006-10-17,Christopher Nolan4,tt0209144,2018-01-16,2018-01-16,,Memento,,movie,8.4,113,2000,"Mystery, Thriller",1081848,2000-09-05,Christopher Nolan5,tt0144084,2018-01-16,2018-01-16,,American Psycho,,movie,7.6,101,2000,"Comedy, Crime, Drama",462984,2000-01-21,Mary Harron6,tt0364569,2018-01-16,2018-01-16,,Oldeuboi,,movie,8.4,120,2003,"Action, Drama, Mystery, Thriller",491476,2003-11-21,Chan-wook Park7,tt1130884,2018-10-08,2018-10-08,,Shutter Island,,movie,8.1,138,2010,"Mystery, Thriller",1075524,2010-02-13,Martin Scorsese8,tt8772262,2019-12-27,2019-12-27,,Midsommar,,movie,7.1,148,2019,"Drama, Horror, Mystery, Thriller",150798,2019-06-24,Ari Aster

We can use cut to decide which fields we need to print:

$ curl | cut -d ',' -f 6TitleFight ClubSource CodeThe PrestigeMementoAmerican PsychoOldeuboiShutter IslandMidsommar

The -d option is to specify the delimiter for each field. What are the fields separated with? In this case it’s a comma ( , ). The -f option is the field number you want to print. In this case the sixth field is the Title of the movie. This also prints the csv header “Title” so to remove it we can just use sed '1 d' , which just means, d elete 1 line from the input stream.

We can then pipe the list of movies into shuf . Shuf just shuffles it’s input lines randomly and spits it out.

$ curl | cut -d ',' -f 6 | sed '1 d' | shufAmerican PsychoMidsommarSource CodeOldeuboiFight ClubMementoShutter IslandThe Prestige

Now just pipe it into head -1 or sed '1 q' which would print only the first line. Every time you run this, you should get a random selection.

$ curl | cut -d ',' -f 6 | sed '1 d' | shuf | head -1Source Code

Now let’s say you would also like the URL to be printed along with title, no problem, cut allows you to specify multiple fields to print using --field=LIST

$ curl | cut -d ',' --field=6,7 | sed '1 d' | shuf | head -1Shutter Island,

There is a problem with this though, if the Movie title has a comma in it, then you would get a totally different field value. One way to overcome this is by using a python one-liner like this:

python -c 'import csv,sys;[print (a["Title"]) for a in csv.DictReader(sys.stdin)]'
$ curl -s |\    python -c 'import csv,sys;[print (a["Title"],a["URL"]) for a in csv.DictReader(sys.stdin)]'|\    shuf | head -1Oldeuboi

These were just a few examples, there are so many things you can accomplish in a single line of shell using pipes.