What does two at (@) signs surrounding a string mean in a shell script?

Question

For example,

# Execute the pre-hook.
export SHELL=@shell@
param1=@param1@
param2=@param2@
param3=@param3@
param4=@param4@
param5=@param5@
if test -n "@preHook@"; then
    . @preHook@
fi

For context, this is from a shell script in a commit from 2004 in the Nixpkgs repo; tried to see if this maybe a reference feature but string "shell" only occurs once (in a case-sensitive search) in the entire file.

Answer 1

The answer by Chris Dodd is correct, insofar as there's no intrinsic meaning to the shell -- and @foo@ is thus commonly used as a sigil. Insofar as you encountered this in nixpkgs, it provides some stdenv tools specifically for implementing this pattern.

As documented at https://nixos.org/manual/nixpkgs/stable/#ssec-stdenv-functions, nixpkgs stdenv provides shell functions including substitute, substituteAll, substituteInPlace &c. which will replace @foo@ values with the content of corresponding variables.

---

In the context of the linked commit, subsitutions of that form can be seen being performed in pkgs/build-wrapper/gcc-wrapper/builder.sh:

    sed \
        -e "s^@gcc@^$src^g" \
        -e "s^@out@^$out^g" \
        -e "s^@bash@^$SHELL^g" \
        -e "s^@shell@^$shell^g" \
        < $gccWrapper > $dst

...is replacing @out@ with the value of $out, @bash@ with the value of $SHELL, etc.

Answer 2

The @ symbol has no meaning to the shell -- it is a punctuation character that will pretty much never occur in any actual shell script.

This makes it a good choice to use for patterns in script templates -- the basic idea being that a simple search-and-replace process will be used (perhaps with a sed script as in the link you show) to rewrite the template into an actual shell script. Every string of the form @name@ in the template will be replaced by some other string related to the environment in which the script is being installed.