Explain Bash parameter expansion with examples.

[bob]

Bash grabs variables and changes them on the fly during your commands. You put the name inside braces to unlock extra options. I twist these expansions to fix missing data without extra checks. You add a colon and dash to pick a fallback value if nothing exists. Then your command keeps going instead of choking. But watch how the braces wrap everything tight. I tried this on a log script once and it cut my debug time in half. Your output stays clean even with empty inputs. Perhaps you test it by clearing a variable first. The trick feels odd at first yet it sticks quick once you play around.
You strip parts from the front or back of a string value too. I remove prefixes by using a hash sign after the braces. Then the rest of the text comes out ready for use. But you can also chop suffixes with a percent sign in the same spot. Your paths shorten fast without calling other tools. I mix this with length checks to see how big the result got. You count characters by slapping an exclamation inside the braces before the name. Then a number pops up telling you the size. Perhaps the string holds a filename and you trim the extension away. It saves steps when you build new names from old ones.
Case flips happen the same way with carets or commas. I switch text to upper or lower by adding those marks after the braces. Then your variables match whatever format the next command expects. But you combine removals and flips in one go for complex tweaks. Your scripts handle dirty data from users without breaking. I once built a backup name generator this way and it ran for months without edits. You avoid loops entirely and keep things simple. Perhaps a variable arrives with mixed case and you force it lower before logging. The whole thing flows better in long chains of commands.
Or you extract slices by adding offsets and lengths inside the braces. I pull middle sections from bigger strings when needed. Then only the key part lands in the output. But offsets start from zero so you count careful at the start. Your examples might pull dates from timestamps this way. I combine it with defaults to handle short strings too. You end up with fewer errors overall. Perhaps the expansion grabs a port number from a longer address line. It works across many shells without changes.
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