mosemann@sardion.unl.edu (Russell Mosemann) (03/01/91)
In the Bourne shell, why does this not work
echo words | read W
but this does (if I put the words in the file)
read W < file
?
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University of Nebraska - Lincoln Voice: (402) 472-5930 Fax: 472-5280pfalstad@phoenix.Princeton.EDU (Paul Falstad) (03/01/91)
mosemann@sardion.unl.edu (Russell Mosemann) wrote: > In the Bourne shell, why does this not work > echo words | read W > but this does (if I put the words in the file) > read W < file This is in the FAQ, although somewhat buried. Basically, the pipe forces read into a subshell in the first case. sh spawns a subshell to simplify file descriptor handling for the pipe; the subshell then reads its input into the variable W, and then exits. The parent shell's W is not changed. In the second (simpler) case, read is executed in the main shell. In some versions of sh (evidently not in SunOS), as well as in ksh (I think) and in another shell whose name escapes me at the moment, "echo words | read W" will return the expected result. I don't know of any nice way around this. If you can, redirect the output to a file, and then do "read W <file". If not, you could do something gross like create a named pipe, etc. -- Paul Falstad, pfalstad@phoenix.princeton.edu PLink:HYPNOS GEnie:P.FALSTAD How DO you delete a file called "-"? For viewers at home, the answer is coming up on your screen. For those of you who wish to play it the hard way, stand upside down with your head in a bucket of piranha fish.
mike (03/01/91)
In an article, sardion.unl.edu!mosemann (Russell Mosemann) writes: > > In the Bourne shell, why does this not work > > echo words | read W > > but this does (if I put the words in the file) > > read W < file The read is in a different instance of the shell. The shell forks the read, and proceeds to create/modify the environment variable. When the child dies, so does its environment. -- Michael Stefanik, MGI Inc., Los Angeles| Opinions stated are not even my own. Title of the week: Systems Engineer | UUCP: ...!uunet!bria!mike ------------------------------------------------------------------------------- Remember folks: If you can't flame MS-DOS, then what _can_ you flame?
sadkins@oucsace.cs.OHIOU.EDU (Scott W. Adkins) (03/01/91)
In article <1991Feb28.160948.24987@hoss.unl.edu> mosemann@sardion.unl.edu (Russell Mosemann) writes: > > In the Bourne shell, why does this not work > > echo words | read W > > but this does (if I put the words in the file) > > read W < file Well, it could be a matter of where things are coming from and where they are going to. In the first case, echo words is sending the stuff to stdout, not stdin. The read W is waiting for input from stdin, not stdout. Result? The words will be echoed to the screen and then the script will wait for keyboard input. In the second example, the '<' symbol literally means redirect the output of this file to this other file. In other words, file is redirected to stdin of read W. I hope this helps. If I am wrong on these accounts, I will be watching for the correct solution. Scott Adkins sadkins@oucsace.cs.ohiou.edu