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The simple and obvious solution:
```
sed 's/^ *//;s/ *$//'
```
Many recipes you find online will erroneously add a `g` flag, but these regular expressions can only match once per line anyway.
(In some more detail, `s/from/to/g` says to replace all occurrences of `from` _on the current input line;_ but of course, if you know `from` can only match once, you don't want or need that.)
Your requirement to treat the first and last lines differently seems odd to me, but `sed` easily allows you to do that too.
```
sed '1s/^ *//;$s/ *$//'
```
This adds the address expression `1` to the first command (which matches on line number 1) and the address `$` to the last (which matches the final input line).
Many `sed` implementations will allow you to pass in the script piecemeal with multiple `-e` options.
```
sed -e '1s/^ *//' -e '$s/ *$//'
```
~~but this is somewhat less portable.~~
The regular expressions above specifically target literal spaces. If you want to target any whitespace, replace each ` ` with `[[:space:]]`, which is a POSIX character class which matches one whitespace character of any kind (space, tab, etc).
Something similar could be achieved with Awk with a clever `RS` (record separator) but I'd consider that more obscure, as well as probably slower.

The simple and obvious solution:
```
sed 's/^ *//;s/ *$//'
```
Many recipes you find online will erroneously add a `g` flag, but these regular expressions can only match once per line anyway.
(In some more detail, `s/from/to/g` says to replace all occurrences of `from` _on the current input line;_ but of course, if you know `from` can only match once, you don't want or need that.)
Your requirement to treat the first and last lines differently seems odd to me, but `sed` easily allows you to do that too.
```
sed '1s/^ *//;$s/ *$//'
```
This adds the address expression `1` to the first command (which matches on line number 1) and the address `$` to the last (which matches the final input line).
If your `sed` implementation doesn't support stringing multiple commands together with `;` as shown above, you can pass in the script piecemeal with multiple `-e` options.
```
sed -e '1s/^ *//' -e '$s/ *$//'
```
The regular expressions above specifically target literal spaces. If you want to target any whitespace, replace each ` ` with `[[:space:]]`, which is a POSIX character class which matches one whitespace character of any kind (space, tab, etc).
Something similar could be achieved with Awk with a clever `RS` (record separator) but I'd consider that more obscure, as well as probably slower.