i1-article-athas-shell-redirections.md - 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
i1-article-athas-shell-redirections.md (2125B)	Err	bitreich.org	70
i---	Err	bitreich.org	70
i     1 # Shell Redirections by athas	Err	bitreich.org	70
i     2 	Err	bitreich.org	70
i     3 Newcomers to the Unix shell quickly encounter handy tools such as	Err	bitreich.org	70
i     4 sed(1) and sort(1).  This command prints the lines of the given file	Err	bitreich.org	70
i     5 to stdout, in sorted order:	Err	bitreich.org	70
i     6 	Err	bitreich.org	70
i     7         $ sort numbers	Err	bitreich.org	70
i     8 	Err	bitreich.org	70
i     9 Soon after, newcomers will also encounter shell redirection, by which	Err	bitreich.org	70
i    10 the output of these tools can conveniently be read from or stored in	Err	bitreich.org	70
i    11 files:	Err	bitreich.org	70
i    12 	Err	bitreich.org	70
i    13         $ sort < numbers > numbers_sorted	Err	bitreich.org	70
i    14 	Err	bitreich.org	70
i    15 Our new user, fascinated by the modularity of the Unix shell, may then	Err	bitreich.org	70
i    16 try the rather obvious possibility of having the input and output file	Err	bitreich.org	70
i    17 be the same:	Err	bitreich.org	70
i    18 	Err	bitreich.org	70
i    19         $ sort < numbers > numbers	Err	bitreich.org	70
i    20 	Err	bitreich.org	70
i    21 But disaster strikes: the file is empty!  The user has lost their	Err	bitreich.org	70
i    22 precious collection of numbers - let's hope they had a backup.  Losing	Err	bitreich.org	70
i    23 data this way is almost a rite of passage for Unix users, but let us	Err	bitreich.org	70
i    24 spell out the reason for those who have yet to hurt themselves this	Err	bitreich.org	70
i    25 way.	Err	bitreich.org	70
i    26 	Err	bitreich.org	70
i    27 When the Unix shell evaluates a command, it starts by processing the	Err	bitreich.org	70
i    28 redirection operators - that's the '>' and '<' above.  While '<' just	Err	bitreich.org	70
i    29 opens the file, '>' *truncates* the file in-place as it is opened for	Err	bitreich.org	70
i    30 reading!  This means that the 'sort' process will dutifully read an	Err	bitreich.org	70
i    31 empty file, sort its non-existent lines, and correctly produce empty	Err	bitreich.org	70
i    32 output.	Err	bitreich.org	70
i    33 	Err	bitreich.org	70
i    34 Some programs can be asked to write their output directly to files	Err	bitreich.org	70
i    35 instead of using shell redirection (sed(1) has '-i', and for sort(1)	Err	bitreich.org	70
i    36 we can use '-o'), but this is not a general solution, and does not	Err	bitreich.org	70
i    37 work for pipelines.  Another solution is to use the sponge(1) tool	Err	bitreich.org	70
i    38 from the "moreutils" project, which stores its standard input in	Err	bitreich.org	70
i    39 memory before finally writing it to a file:	Err	bitreich.org	70
i    40 	Err	bitreich.org	70
i    41         $ sort < numbers | sponge numbers	Err	bitreich.org	70
i    42 	Err	bitreich.org	70
i    43 The most interesting solution is to take advantage of subshells, the	Err	bitreich.org	70
i    44 shell evaluation order, and Unix file systems semantics.  When we	Err	bitreich.org	70
i    45 delete a file in Unix, it is removed from the file system, but any	Err	bitreich.org	70
i    46 file descriptors referencing the file remain valid.  We can exploit	Err	bitreich.org	70
i    47 this behaviour to delete the input file *after* directing the input,	Err	bitreich.org	70
i    48 but *before* redirecting the output:	Err	bitreich.org	70
i    49 	Err	bitreich.org	70
i    50         $ (rm numbers && sort > numbers) < numbers	Err	bitreich.org	70
i    51 	Err	bitreich.org	70
i    52 This approach requires no dependencies and will work in any Unix	Err	bitreich.org	70
i    53 shell.	Err	bitreich.org	70
.