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  \emph{environment files} which contain lists of environment variables
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  \emph{environment files} which contain lists of environment variables
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  to be set, and \emph{profile files} which contain \R code.
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  to be set, and \emph{profile files} which contain \R code.
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  Lines in a site or user environment file should be either comment
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  Lines in a site or user environment file should be either comment
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  lines starting with \samp{#}, or lines of the form
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  lines starting with \samp{#}, or lines of the form
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  \samp{\var{name}=\var{value}}. The latter sets the environmental
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  \code{\var{name}=\var{value}}. The latter sets the environmental
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  variable \samp{\var{name}} to \samp{\var{value}}, overriding an
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  variable \code{\var{name}} to \code{\var{value}}, overriding an
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  existing value.  If \samp{\var{value}} contains an expression of the
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  existing value.  If \code{\var{value}} contains an expression of the
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  form \samp{${foo-bar}}, the value is that of the environmental
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  form \samp{${foo-bar}}, the value is that of the environmental
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  variable \samp{foo} if that is set, otherwise \samp{bar}.  For
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  variable \samp{foo} if that is set, otherwise \samp{bar}.  For
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  \samp{${foo:-bar}}, the value is that of \samp{foo} if that is set to
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  \samp{${foo:-bar}}, the value is that of \samp{foo} if that is set to
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  a non-empty value, otherwise \samp{bar}.  (If it is of the form
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  a non-empty value, otherwise \samp{bar}.  (If it is of the form
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  \samp{${foo}}, the default is \code{""}.)  This construction can be
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  \samp{${foo}}, the default is \code{""}.)  This construction can be
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  nested, so \code{bar} can be of the same form (as in
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  nested, so \code{bar} can be of the same form (as in
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  \samp{${foo-${bar-blah}}}).  Note that the braces are essential: for
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  \samp{${foo-${bar-blah}}}).  Note that the braces are essential: for
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  example \samp{$HOME} will not be interpreted.
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  example \samp{$HOME} will not be interpreted.
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  Leading and trailing white space in \samp{\var{value}} are stripped.
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  Leading and trailing white space in \code{\var{value}} are stripped.
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  \samp{\var{value}} is then processed in a similar way to a Unix shell:
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  \code{\var{value}} is then processed in a similar way to a Unix shell:
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  in particular (single or double) quotes not preceded by backslash
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  in particular (single or double) quotes not preceded by backslash
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  are removed and backslashes are removed except inside such quotes.
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  are removed and backslashes are removed except inside such quotes.
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  For readability and future compatibility it is recommended to only use
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  For readability and future compatibility it is recommended to only use
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  constructs that have the same behavior as in a Unix shell.  Hence,
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  constructs that have the same behavior as in a Unix shell.  Hence,