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Top Forums Shell Programming and Scripting What's going on in my scripts? Post 302188355 by era on Wednesday 23rd of April 2008 08:42:43 AM
Old 04-23-2008
The problem is that you have two things which want to read standard input. Your while loop reads one line at a time, but the dbSomething (or something else in that script) slurps in as much as it can get, the first time it runs.

So we make them read from different file descriptors. That's what the exec does: it connects the input file to file descriptor 3 (instead of 0, standard input; 1 and 2 are standard out and standard error, respectively) and then we modify the while loop to expect its input from that file descriptor. Then at the end we close fd3 when we're done with it, with another exec.

exec, like set, has two different uses. One is to replace the current shell with another process (exec programname); the other is to manipulate the current shell's file descriptors. We use the latter functionality here.

Probably you could redirect dbSomething to read from /dev/null instead -- that would be a simpler change, all right, but I wasn't particularly in the mood to speculate on whether that will work with that particular program or not.
 

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exec(1)                                                            User Commands                                                           exec(1)

NAME
exec, eval, source - shell built-in functions to execute other commands SYNOPSIS
sh exec [argument...] eval [argument...] csh exec command eval argument... source [-h] name ksh *exec [arg...] *eval [arg...] DESCRIPTION
sh The exec command specified by the arguments is executed in place of this shell without creating a new process. Input/output arguments may appear and, if no other arguments are given, cause the shell input/output to be modified. The arguments to the eval built-in are read as input to the shell and the resulting command(s) executed. csh exec executes command in place of the current shell, which terminates. eval reads its arguments as input to the shell and executes the resulting command(s). This is usually used to execute commands generated as the result of command or variable substitution. source reads commands from name. source commands may be nested, but if they are nested too deeply the shell may run out of file descrip- tors. An error in a sourced file at any level terminates all nested source commands. -h Place commands from the file name on the history list without executing them. ksh With the exec built-in, if arg is given, the command specified by the arguments is executed in place of this shell without creating a new process. Input/output arguments may appear and affect the current process. If no arguments are given the effect of this command is to mod- ify file descriptors as prescribed by the input/output redirection list. In this case, any file descriptor numbers greater than 2 that are opened with this mechanism are closed when invoking another program. The arguments to eval are read as input to the shell and the resulting command(s) executed. On this man page, ksh(1) commands that are preceded by one or two * (asterisks) are treated specially in the following ways: 1. Variable assignment lists preceding the command remain in effect when the command completes. 2. I/O redirections are processed after variable assignments. 3. Errors cause a script that contains them to abort. 4. Words, following a command preceded by ** that are in the format of a variable assignment, are expanded with the same rules as a vari- able assignment. This means that tilde substitution is performed after the = sign and word splitting and file name generation are not performed. EXIT STATUS
For ksh: If command is not found, the exit status is 127. If command is found, but is not an executable utility, the exit status is 126. If a redi- rection error occurs, the shell exits with a value in the range 1-125. Otherwise, exec returns a zero exit status. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Availability |SUNWcsu | +-----------------------------+-----------------------------+ SEE ALSO
csh(1), ksh(1), sh(1), attributes(5) SunOS 5.10 17 Jul 2002 exec(1)
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