https://yusuf.fyi/posts/linux-text-manipulation

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A white young man wearing a coat, outside a home, early morning,
misty weather Yusuf Bouzekri
 Thursday, Mar 28, 2024

Linux text manipulation

Today, I wanted to make a module for my AwesomeWM status bar that
shows the currently playing Spotify song, it'd be nice if it could
show something human-readable like this: Wild World by Yusuf/Cat
Stevens.

Through a quick search, I found a tiny bash script, sp, that does
what I need. except the output must be cleaned up.

joe@popos> sp current
Album        Tea For The Tillerman (Remastered 2020)
AlbumArtist  Yusuf / Cat Stevens
Artist       Yusuf / Cat Stevens
Title        Wild World

I could ask chatgpt to format this or write a Lua script, but that
wouldn't be much fun, would it? in this article, we'll learn how to
use awk, sed and other commands to get the desired output.
Intended Audience
this is intended for people who have used Linux but are otherwise new
to the terminal, if you've at least created a file from the command
line, you're good to go, I'll cover everything else throughout the
article

If you want to follow along, download the bash script, and experiment
with the commands we'll learn, but that's not required per se.

Mini Problem #1

To get started sp current returns the album name, album artist, song
artist, and song title, whereas we only need the artist and title. we
can use sed to pick the specific lines we need.

sed works by looping through lines of the input and doing a given
action, by default it prints the resulting lines to stdout. we can
give sed a file to work with but more commonly you'll pipe a command
into sed, as we'll do later on.

    What do you mean by pipe? Generally, in Linux, a command's input
    is read from the keyboard and its output (along with any errors)
    is printed on your screen. we can, however, change this behavior.

    piping essentially lets you pass a command's output onto another
    command's input. programs can choose whether or not to support
    piping, but most basic Linux programs (and all the ones we'll
    discuss today) support it.

let's look into sed in more detail

sp current | sed '' # no action given, sed will print the lines as is
sp current | sed 's/Cat/Dog/g' # replace Cat with Dog and print every line

# the n flag suppresses the automatic printing
# this will replace Cat with Dog but print nothing!
sp current | sed -n 's/Cat/Dog/g' 

# p prints a line
# so this will print each line twice (default + p action)
sp current | sed 'p'
  
# if we combine -n with the p action we can do this
# this is equivalent to plain sed ''
sp current | sed -n 'p'
  
# we can print a specific line too (we'll need this later!)
# this will print every line AND print the second line an extra time
sp current | sed '2p'

# finally, we can chain actions.
# this substitutes Cat with Dog and prints ONLY the 2nd line, because of the n flag.
sp current | sed -n 's/Cat/Dog/g;2p'

and now--as you may have already figured out--we can print the artist
and song name only like so:

sp current | sed -n '3p;4p'

and we end up with

Artist       Yusuf/Cat Stevens
Title        Wild World

Mini Problem #2

Next, we need to get rid of the column labels, we can use cut for
this. As with sed, cut accepts a file as its input, if no file is
given it'll read from stdin.

Here is how we usually work with cut, you give it a delimiter and a
set of columns to print, and it will split each line by that
delimiter and return the selected columns.

in our case, columns are separated with a space, so we can do the
following

# we're splitting the input by spaces and picking the 2nd column
sp current | cut -d ' ' -f2

our columns are seperated by multiple spaces though, so cut will
treat the empty-ness between each space as its own column, to avoid
this we can inject the command tr to trim the extra spaces:

# the s flag specifies what to trim
sp current | tr -s ' ' | cut -d ' ' -f2

But this is still not totally right. if a song's title contains more
than one word we'll get this

Artist       Yusuf/Cat Stevens
Title        Wild World
 
Yusuf/Cat
Wild

you can pass cut a column range to print, as in cut -d ' ' -f2-4 to
select the 2nd, 3rd, and 4th columns, if the range is open-ended, cut
will print every column until the end of the line, so let's
incorporate that into our script.

sp current | tr -s ' ' | cut -d ' ' -f2- | sed -n '3p;4p'

output:

Yusuf/Cat Stevens
Wild World

note: as I've shown you tr needs to be applied before cut on the
contrary, cut and sed can be swapped because they serve entirely
different purposes.

Mini Problem #3

Now, what's left is to simply join the lines and format them nicely.
we can use awk to do this.

awk is based on a condition-script model, It's similar to sed it
checks if the current line matches the condition and executes the
appropriate script, although I find awk to have a more expressive
syntax.

awk scripts are written in quotes like this:

sp current | awk '{print $1}'

let's look at a few examples. any of these can be passed to awk in
quotes.

{print} # no condition given, prints every line.
  
# NR==1 matches the 1st line only
# thus, this will print the 1st line
NR==1{print} 
  
# any subsequent script isn't part of the condition. 
# despite them being on the same line.
# this will print the print every line + the first line again
NR==1{print}{print}

# awk has pre-defined globals corresponding to columns in your input
# this behaves like sed and will print the first column.
{print $1}
 
# $0 matches the entire line
# this is equivalent to {print}
{print $0}

Lastly, you can assign globals to variables,

this assigns a to the 1st column of the 1st line and then prints x
for every line in the input. note that the first script only runs
once, thus a will stay the same for every line

NR==1{x=$1}{print x}

now we can combine everything we know to get this script:

sp current | ... | awk 'NR==1{a=$0; next}{print $0 " by " a}'

this does the following:

 1. we assign a to the artist name (entire first line)
 2. skip tells awk to skip the execution of the second script and to
    move to the next line
 3. we're on the second line now so NR==1 isn't matched and the first
    script is skipped.
 4. print the song name ($0) then the string literal by then the
    artist name (content of variable a)

Yusuf/Cat Stevens
Wild World
 
Wild World by Yusuf/Cat Stevens

Final Thoughts

That concludes our script, here is what we ended up with:

sp current | tr -s ' ' | cut -d ' ' -f2- | sed -n '3p;4p' | awk 'NR==1{a=$0; next}{print $0 " by " a}'

Now this script could be optimized in different ways, but instead of
showing you how to do it here, I leave that as an exercise to the
reader. here are a couple of things you can do, ranging from easiest
to hardest:

  * replace the cut command with an additional sed action
  * re-write the entire thing from memory
  * make the script as short as possible
  * write the entire script in awk

Finally, I hope you learned something new today and if you attempt
any of these, please share what you did either in the mastodon
replies or through an article on your blog. I promise to take a look
and I'll probably learn something myself!

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