Solution: Crafting Linux User Error Strings
Answer: BORN→HUNT FIRM→HAFT LOCK→SALT ROLE→PEST

Written by Amos Eshel

Solve the cryptic clues on the left and compare the 4-letter words to CASE as the Mastermind keyword to get a series of blacks & whites (or bulls & cows, etc.). Then solve the cryptics & function calls on the right.

Secret code: CASEMastermind resultSecret code: output
Leave / it after equinox ends
E[quino]X+IT
 W
FLUE
Lead vermin keel over
STAR<
 WW
Essentially, demonstrative pronoun!
[essen]T[ially] + HIS (&lit)
Rule / every other alliance
aLlIaNcE
 B
HELM
Secure / time machine failed to start
[c]LOCK
 W
ANEW
Took off / flower
ROSE (ddef.)
 BB
Breathe heavily / during pop anthem
[po]P ANT[hem]
Touched, / left sadly
FELT*
 W
input
BORN | FIRM | LOCK | ROLE
Understand / outspoken rejection
“NO” → KNOW
 miss
Console has leading Kickstarter / site
WI(K[ickstarter])I

We then combine the words on the right with the mastermind results, and try to find a word that fits those constraints.

This means we are looking for a word that shares 1 letter not in the correct position with FLUE, 2 letters not in the correct position with THIS, ..., 1 letter not in the correct position with [BORN | FIRM | LOCK | ROLE], ...

The blank for the input is on the right, which is why we get four different results.

PuzzleInputOutput
LOGICBORNHUNT
LISTFIRMHAFT
Meta ILOCKSALT
Meta IIROLEPEST

Author’s Notes

The exact mechanics of this puzzle were figured out pretty late in the process (using Mastermind was the driving force from the start). This is unfortunate as I think if CASE had been replaced by input, a lot more freedom would have existed for the input/output pairs. As it is, the original idea of this puzzle acting on 7 letter words (and providing CONTROL to the final meta phrase) had to be abandoned for something constructible at 4 letters, and this was by far the most constrained construction of the 3 content-filled puzzles.