iopus7: submit a new article from Athas - tgtimes - The Gopher Times	Err	bitreich.org	70
hgit clone git://bitreich.org/tgtimes git://enlrupgkhuxnvlhsf6lc3fziv5h2hhfrinws65d7roiv6bfj7d652fid.onion/tgtimes	URL:git://bitreich.org/tgtimes git://enlrupgkhuxnvlhsf6lc3fziv5h2hhfrinws65d7roiv6bfj7d652fid.onion/tgtimes	bitreich.org	70
1Log	/scm/tgtimes/log.gph	bitreich.org	70
1Files	/scm/tgtimes/files.gph	bitreich.org	70
1Refs	/scm/tgtimes/refs.gph	bitreich.org	70
1Tags	/scm/tgtimes/tag	bitreich.org	70
1README	/scm/tgtimes/file/README.md.gph	bitreich.org	70
i---	Err	bitreich.org	70
1commit c8279e8b137436c61ad76e077afe0cec09ed9260	/scm/tgtimes/commit/c8279e8b137436c61ad76e077afe0cec09ed9260.gph	bitreich.org	70
1parent 9c6672aaa881709d5e0139128423ad83000b557f	/scm/tgtimes/commit/9c6672aaa881709d5e0139128423ad83000b557f.gph	bitreich.org	70
hAuthor: Josuah Demangeon <me@josuah.net>	URL:mailto:me@josuah.net	bitreich.org	70
iDate:   Sun, 16 Oct 2022 02:09:47 +0200	Err	bitreich.org	70
i	Err	bitreich.org	70
iopus7: submit a new article from Athas	Err	bitreich.org	70
i	Err	bitreich.org	70
iDiffstat:	Err	bitreich.org	70
i  A opus7/article-athas-shell-redirect… |      73 +++++++++++++++++++++++++++++++	Err	bitreich.org	70
i	Err	bitreich.org	70
i1 file changed, 73 insertions(+), 0 deletions(-)	Err	bitreich.org	70
i---	Err	bitreich.org	70
1diff --git a/opus7/article-athas-shell-redirections.mw b/opus7/article-athas-shell-redirections.mw	/scm/tgtimes/file/opus7/article-athas-shell-redirections.mw.gph	bitreich.org	70
i@@ -0,0 +1,73 @@	Err	bitreich.org	70
i+.SH athas	Err	bitreich.org	70
i+Shell Redirections	Err	bitreich.org	70
i+.2C 60	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.PP	Err	bitreich.org	70
i+Newcomers to the Unix shell quickly encounter handy tools such as	Err	bitreich.org	70
i+sed(1) and sort(1).  This command prints the lines of the given file	Err	bitreich.org	70
i+to stdout, in sorted order:	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.DS	Err	bitreich.org	70
i+$ sort numbers	Err	bitreich.org	70
i+.DE	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.PP	Err	bitreich.org	70
i+Soon after, newcomers will also encounter shell redirection, by which	Err	bitreich.org	70
i+the output of these tools can conveniently be read from or stored in	Err	bitreich.org	70
i+files:	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.DS	Err	bitreich.org	70
i+$ sort < numbers > numbers_sorted	Err	bitreich.org	70
i+.DE	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.PP	Err	bitreich.org	70
i+Our new user, fascinated by the modularity of the Unix shell, may then	Err	bitreich.org	70
i+try the rather obvious possibility of having the input and output file	Err	bitreich.org	70
i+be the same:	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.DS	Err	bitreich.org	70
i+$ sort < numbers > numbers	Err	bitreich.org	70
i+.DE	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.PP	Err	bitreich.org	70
i+But disaster strikes: the file is empty!  The user has lost their	Err	bitreich.org	70
i+precious collection of numbers - let's hope they had a backup.  Losing	Err	bitreich.org	70
i+data this way is almost a rite of passage for Unix users, but let us	Err	bitreich.org	70
i+spell out the reason for those who have yet to hurt themselves this	Err	bitreich.org	70
i+way.	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.PP	Err	bitreich.org	70
i+When the Unix shell evaluates a command, it starts by processing the	Err	bitreich.org	70
i+redirection operators - that's the '>' and '<' above.  While '<' just	Err	bitreich.org	70
i+opens the file, '>' *truncates* the file in-place as it is opened for	Err	bitreich.org	70
i+reading!  This means that the 'sort' process will dutifully read an	Err	bitreich.org	70
i+empty file, sort its non-existent lines, and correctly produce empty	Err	bitreich.org	70
i+output.	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.PP	Err	bitreich.org	70
i+Some programs can be asked to write their output directly to files	Err	bitreich.org	70
i+instead of using shell redirection (sed(1) has '-i', and for sort(1)	Err	bitreich.org	70
i+we can use '-o'), but this is not a general solution, and does not	Err	bitreich.org	70
i+work for pipelines.  Another solution is to use the sponge(1) tool	Err	bitreich.org	70
i+from the "moreutils" project, which stores its standard input in	Err	bitreich.org	70
i+memory before finally writing it to a file:	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.DS	Err	bitreich.org	70
i+$ sort < numbers | sponge numbers	Err	bitreich.org	70
i+.DE	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.PP	Err	bitreich.org	70
i+The most interesting solution is to take advantage of subshells, the	Err	bitreich.org	70
i+shell evaluation order, and Unix file systems semantics.  When we	Err	bitreich.org	70
i+delete a file in Unix, it is removed from the file system, but any	Err	bitreich.org	70
i+file descriptors referencing the file remain valid.  We can exploit	Err	bitreich.org	70
i+this behaviour to delete the input file *after* directing the input,	Err	bitreich.org	70
i+but *before* redirecting the output:	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.DS	Err	bitreich.org	70
i+$ (rm numbers && sort > numbers) < numbers	Err	bitreich.org	70
i+.DE	Err	bitreich.org	70
i+.	Err	bitreich.org	70
i+.PP	Err	bitreich.org	70
i+This approach requires no dependencies and will work in any Unix	Err	bitreich.org	70
i+shell.	Err	bitreich.org	70
.