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64 lines
1.7 KiB
64 lines
1.7 KiB
#!/bin/sh
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#
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# This script takes a filename as input and writes the current date to the
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# file. If the file already exists, it will not be overwritten unless the '-f'
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# (or '--force') flag is given.
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#
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# This script demonstrates several types of shFlags functionality.
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# - declaration of the FLAGS_HELP variable to customize the help output
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# - direct calling of the flags_help() function for script controlled usage
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# output
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# - handling of non-flag type command-line arguments that follow the flags
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#
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# Try the following:
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# $ ./write_date.sh now.out
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# $ cat now.out
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#
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# $ ./write_date.sh now.out
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# $ cat now.out
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#
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# $ ./write_date.sh -f now.out
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# $ cat now.out
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# Source shFlags.
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. ../shflags
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# Configure shFlags.
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DEFINE_boolean 'force' false 'force overwriting' 'f'
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FLAGS_HELP="USAGE: $0 [flags] filename"
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die() {
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[ $# -gt 0 ] && echo "error: $@"
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flags_help
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exit 1
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}
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# Parse the command-line.
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FLAGS "$@" || exit 1
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eval set -- "${FLAGS_ARGV}"
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# Check for filename on command-line.
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[ $# -gt 0 ] || die 'filename missing.'
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filename=$1
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# Redirect STDOUT to the file ($1). This seemingly complicated method using exec
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# is used so that a potential race condition between checking for the presence
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# of the file and writing to the file is mitigated.
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if [ ${FLAGS_force} -eq ${FLAGS_FALSE} ] ; then
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[ ! -f "${filename}" ] || die "file \"${filename}\" already exists."
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# Set noclobber, redirect STDOUT to the file, first saving STDOUT to fd 4.
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set -C
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exec 4>&1 >"${filename}" # This fails if the file exists.
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else
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# Forcefully overwrite (clobber) the file.
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exec 4>&1 >|"${filename}"
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fi
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# What time is it?
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date
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# Restore STDOUT from file descriptor 4, and close fd 4.
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exec 1>&4 4>&-
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echo "The current date was written to \"${filename}\"."
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