Return to Answer

die above could be:

A standard equivalent of that GNU grep command could be:

LC_ALL=C sed '/^[Cc][Oo][Nn][Tt][Ee][Nn][Tt]-[Ll][Ee][Nn][Gg][Tt][Hh]:[[:space:]]*\([0-9]\{1,\}\).*/!d;s//\1/;q'

With the caveat that if the header is not found, it wouldn't return a false exit status like grep does so you'd need an additional check for [ -n "$remote_size" ].

die above could be:

• if there are several occurrences of the header, it will be return the first one only
• it will complain if the value doesn't start with a digit optionally preceded with a +, but still needs to be sanitised as whatever characters there are after that are passed along.
• it does support folded headers, but rejects a content-length whose first line has an empty value.

• if there are several occurrences of the header, it will return the value for the first one only
• it will complain if the value doesn't start with a digit optionally preceded with a +, but still needs to be sanitised as whatever characters there are after that are passed along.
• it does support folded headers, but rejects a content-length whose first line has an empty value.

With curl 7.84.0 or newer, you can also get curl to give you the value of that header directly with:

remote_size=$(curl -w '%header{content-length}' -sIo /dev/null -- "$url") || die

Through testing, I find that

• if there are several occurrences of the header, it will be return the first one only
• it will complain if the value doesn't start with a digit optionally preceded with a +, but still needs to be sanitised as whatever characters there are after that are passed along.
• it does support folded headers, but rejects a content-length whose first line has an empty value.