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