In bash, what does the leading assignment in the command `LC_ALL=C sort names.txt` do, and how is it different from putting `LC_ALL=C` on its own line before the sort?

## The temporary environment Bash lets any simple command be preceded by one or more `NAME=value` assignments: ```bash LC_ALL=C sort names.txt TZ=UTC date API_TOKEN=$tok curl -sf https://example.invalid/api ``` Those assignments are added to the environment handed to that command and to nothing else. The shell's own variable table is not modified, so nothing leaks into the lines that follow. It is the sharpest scoping tool bash offers for environment: narrower than `export`, and it cannot be forgotten about later. ## Why the separate line silently fails ```bash LC_ALL=C sort names.txt # sorts in whatever locale was already in the environment ``` The first line creates a shell variable. Shell variables are not part of the environment unless they are exported, and `sort` is a separate program that reads only its environment. So the setting has no effect at all — and, worse, it has no effect *silently*: the sort succeeds, it just produces a different order. Locale-sensitive sorting, `date` output and numeric formatting are exactly the places where a wrong-but-successful result does the most damage. The fix on a separate line is `export LC_ALL=C`, but note what you have bought: the value is now in the environment of every command the script runs from that point, including things you did not intend to affect. ## The expansion-order trap The prefix affects the command's *environment*, not the shell's expansions on that same line — and expansions happen first: ```bash FOO=outer FOO=inner echo "$FOO" # prints outer FOO=inner printenv FOO # prints inner ``` Bash expands `"$FOO"` while building the command line, before the temporary assignment is applied, so `echo` receives the old value as an argument. `printenv` reads the environment at runtime and sees the new one. This is a favourite interview trick and a real source of confusion in scripts that try to use the variable they just set on the same line. ## Scope details worth knowing - **Multiple assignments are allowed** and are applied left to right: `TZ=UTC LC_ALL=C date`. - **The value is expanded by the current shell**, so `PATH=/opt/bin:$PATH mytool` builds the new value from the shell's current `PATH` and hands the result to `mytool` — without changing the shell's `PATH`. - **It works for external commands and, most usefully, is the safe form for them.** When the command word is a shell function or a special builtin, the persistence rules differ and depend on POSIX mode and the bash version, so do not build logic on assignments prefixed to those — use an explicit `export` inside a subshell if you need it. - **`env` does the same thing from outside bash**: `env LC_ALL=C sort names.txt` is equivalent and is what you reach for when the command word would otherwise be ambiguous, or when you are writing for a shell that lacks the prefix form. ## Scrubbing instead of adding The complement of adding one variable is removing all of them. `env -i cmd` runs `cmd` with an *empty* environment, and `env -u NAME cmd` removes a single one. `env -i` is the practical way to reproduce the stripped-down environment that a script gets when something other than your interactive session starts it — you set only the variables you are willing to depend on and see whether the script still works: ```bash env -i PATH=/usr/bin:/bin HOME="$HOME" ./myscript.sh ``` If the script only passes with your full environment, it has an undeclared dependency on something you happen to have set. ## Choosing between the three forms - **Prefix** (`VAR=x cmd`) — one command needs it. Default choice; no cleanup, no leakage. - **`export VAR=x`** — many commands in this script need it, and you accept that every descendant gets it too. - **`env -i` / `env -u`** — you want to *reduce* what a command inherits, usually to reproduce a minimal environment or to keep a secret out of a child. A script that reads clearly tends to use the prefix for the exceptional case and export sparingly, near the top, for the handful of values genuinely shared across the whole run.