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fish-shell/share/functions/eval.fish

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function eval -S -d "Evaluate parameters as a command"
if not set -q argv[2]
# like most builtins, we only check for -h/--help
# if we only have a single argument
switch "$argv[1]"
case -h --help
__fish_print_help eval
return 0
end
end
# If we are in an interactive shell, eval should enable full
# job control since it should behave like the real code was
# executed. If we don't do this, commands that expect to be
# used interactively, like less, wont work using eval.
set -l mode
if status --is-interactive-job-control
set mode interactive
else
if status --is-full-job-control
set mode full
else
set mode none
end
end
if status --is-interactive
status --job-control full
end
# rfish: To eval 'foo', we construct a block "begin ; foo; end <&3 3<&-"
# Note the redirections are also within the quotes.
#
# We then pipe this to 'source 3<&0.
#
# You might expect that the dup2(3, stdin) should overwrite stdin,
# and therefore prevent 'source' from reading the piped-in block. This doesn't happen
# because when you pipe to a builtin, we don't overwrite stdin with the read end
# of the block; instead we set a separate fd in a variable 'builtin_stdin', which is
# what it reads from. So builtins are magic in that, in pipes, their stdin
# is not fd 0.
#
# source does not apply the redirections to itself. Instead it saves them and passes
# them as block-level redirections to parser.eval(). Ultimately the evald code sees
# the following redirections (in the following order):
# dup2 0 -> 3
# dup2 pipe -> 0
# dup2 3 -> 0
# where the pipe is the pipe we get from piping echo to source. Thus the redirection
# effectively makes stdin fd0, instead of the thing that was piped to source
echo "begin; $argv "\n" ;end <&3 3<&-" | source 3<&0
set -l res $status
status --job-control $mode
return $res
end