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What are the most inscrutable lines of $SHELL you've ever written? (programming.dev)

submitted 1 year ago* (last edited 1 year ago) by gamma to c/shell

31 comments fedilink hide all child comments

I'm sure some of you have absolute monstrosities of sigils (I know I do, in my .zshrc alone). Post them without context, and try and guess what other users's lines are. If you want to provide context or guess, use the markdown editor to spoiler-tag your guesses and explanations!

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[–] varsock 1 points 1 year ago (1 children)

print -rn $termcap[sc]${termcap[ch]//(%i|%p1|%d)}$termcap[ct]${(j::)s}$termcap[rc]

spoiler

The given shell script appears to be written in Zsh, a popular Unix shell. Let's break down the script step by step:

print -rn: This command is used to print the output without a newline character (-n) and without interpreting any escape sequences (-r).
$termcap[sc]: It refers to a termcap variable sc, which stands for "save cursor position." Termcap is a database and library used to control the behavior of terminals. $termcap[sc] saves the current cursor position.
${termcap[ch]//(\%i|\%p1|\%d)}: This part performs a substitution on the value of the termcap variable ch. The double slashes (//) indicate a global substitution, and the pattern (\%i|\%p1|\%d) matches any of the three percent sequences: %i, %p1, or %d. It replaces all occurrences of these sequences with an empty string.
$termcap[ct]: It refers to the termcap variable ct, which stands for "clear to end of line." It clears everything from the cursor position to the end of the current line.
${(j::)s}: This uses parameter expansion to join the array variable s using colons (:) as separators. The (j::) flag indicates that colons should be used as separators.
$termcap[rc]: It refers to the termcap variable rc, which stands for "restore cursor position." It restores the saved cursor position.

In summary, this shell script saves the cursor position, performs a substitution on the value of ch termcap variable, clears the current line from the cursor position to the end, prints the value of array variable s joined by colons, and finally restores the cursor position. The purpose and context of this script can only be determined based on the larger context or the values of the termcap variables sc, ch, ct, s, and rc.

[–] gamma 1 points 1 year ago

Corrections

First, a look at man termcap:

       ch   Move cursor horizontally only to column %1
       ct   Clear tabs

(j::) actually joins with no seperator. The character following the j is used to capture the separator. Nearly any character or paired bracket can be used, but colons are common. Other ways to write this which might have been more obvious: (j''), (j[]), (j<>). To actually join with colons, something like (j.:.) would have been used.

Full context

Zsh agressively sets tabstops to every 8 characters. This is in a function which I trap to the WINCH signal, to set the termstops to 4 using the $termcap sequences rather than the external command tput:

.on_winch(){
	# tabs
	local -a s
	repeat COLUMNS/$1 s+=(${termcap[RI]/\%p1\%d/$1}$termcap[st])

	# final
	print -rn $termcap[sc]${termcap[ch]//(\%i|\%p1|\%d)}$termcap[ct]${(j::)s}$termcap[rc]
}
.on_winch 4
trap '.on_winch 4' WINCH