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Full Discussion: bash dropping SIGHUPs
Top Forums Shell Programming and Scripting bash dropping SIGHUPs Post 302548477 by Corona688 on Thursday 18th of August 2011 01:18:56 PM
Old 08-18-2011
I don't think it's ever going to miss a HUP, it will still set the variable. The loop may just detect it 30 seconds late.

There's no point backgrounding sleep 30 when all you're going to do is wait for it. You catch the circumstance when HUP comes in the instant between running wait and waiting for wait, and open 3 other circumstances where the HUP could creep in before you're ready.

A C application would use SIGMASK to temporarily stop a signal but shell doesn't have this... hmm...

How about a pipe?

Code:
#!/bin/sh

trap "rm -f /tmp/$$" HUP

echo "pid is $$"
exec 5>&2               # Save stderr
exec  2> /dev/null      # stop error spew

while true
do
        mkfifo /tmp/$$  # Recreate FIFO

        # wait for fifo to be deleted, which will make read fail
        while read X < /tmp/$$ ; do true ; done

        echo "read loop broken"
done

exec 2>&5       # restore stderr
exec 5>&-       # close fd 5

The shell will create a FIFO and try to read from it. Since there's no other process writing to it, it will block until something does something to it. A HUP signal will delete the FIFO, causing the read to fail. Once it's ready, the shell will create a new FIFO and start over.

Redirecting stderr is necessary since deleting the FIFO causes a bit of error spam. You can redirect into >&5 if you need to write to stderr.

Tested this in lots of shells and two OSes so it looks decently portable.

Last edited by Corona688; 08-18-2011 at 02:24 PM..
 

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mkfifo(1)						      General Commands Manual							 mkfifo(1)

NAME
mkfifo - Makes FIFO special files SYNOPSIS
mkfifo [-m mode] file... The mkfifo utility creates FIFO special files in the order specified. STANDARDS
Interfaces documented on this reference page conform to industry standards as follows: mkfifo: XCU5.0 Refer to the standards(5) reference page for more information about industry standards and associated tags. OPTIONS
Sets the file permission bits of the new file to the specified mode value, after creating the FIFO special file. The mode argument is a symbolic mode string (see chmod), in which the operator characters + (plus sign) and - (minus) are interpreted relative to the default file mode for that file type. The + character adds permissions to the default mode, and - deletes permissions from the default mode. [Tru64 UNIX] The default mode is a=rw (permissions of rw-rw-rw) as modified by the current file mode creation mask (umask). OPERANDS
The path name of a FIFO special file to be created. DESCRIPTION
For each file argument, mkfifo performs actions equivalent to the mkfifo() call with the following arguments: The file argument is used as the pathname argument. If the -m option is not used, the value of the bitwise inclusive OR of S_IRUSR, S_IWUSR, S_IRGRP, S_IWGRP, S_IROTH, and S_IWOTH is used as the mode argument. EXIT STATUS
The mkfifo utility exits with one of the following values: Indicates that all the specified FIFO special files were created successfully. Indicates that an error occurred. EXAMPLES
To create a FIFO special file with permissions prw-r--r--, enter: mkfifo -m 644 /tmp/myfifo The command creates the /tmp/myfifo file with read/write permissions for the owner and read permission for the group and for others. To create a FIFO special file using the - (minus) operand to set permissions of prw-r-----, enter: mkfifo -m g-w,o-rw /tmp/fifo2 The command creates the /tmp/fifo2 file, removing write permission for the group and all permissions for others. ENVIRONMENT VARIABLES
The following environment variables affect the execution of mkfifo: Provides a default value for the internationalization variables that are unset or null. If LANG is unset or null, the corresponding value from the default locale is used. If any of the internationalization variables contain an invalid setting, the utility behaves as if none of the variables had been defined. If set to a non-empty string value, overrides the values of all the other internationalization variables. Determines the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multibyte characters in arguments). Determines the locale for the format and contents of diagnostic messages written to standard error. Determines the location of message catalogues for the processing of LC_MESSAGES. SEE ALSO
Commands: mkdir(1), mknod(8) Functions: chmod(2), mkdir(2), mkfifo(3), mknod(2), umask(2) Standards: standards(5) mkfifo(1)
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