Bash Open Questions

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• How can I redirect the output of the script to both standard output and a log file?
  • Duplicating fds is not sufficient, because it duplicates the file descriptor number, not the data.

  • But you can still use tee: exec > >(tee log)

• Somewhere the content of variables in my script lose whitespace.

  • echo "$var" - quoting. See BashPitfalls for more.

• How can I ensure that only once instance of my script is running at a time?

  • Answer: e.g. lockfile. This should be answered in the UnixFaq

  • It's also BashFaq #45 now.

• How can I handle command line arguments in a shell script?

  • getopts, or a while/shift loop (see BashFaq #35)

• How can I have variable variables, e.g. myvar=prefix$othervar?

  • Answer: eval

  • If othervar is an integer, use an array instead.

• How can I make bash set the xterm title to the command it is currently executing?
• How to determine Yesterday's date?
  • date -d'yesterday' # requires GNU date
• How to convert Unix time stamps to a date (and vice versa)?
  • gdate -d "1970-01-01 UTC + $time seconds"
  • something involving awk and strftime()
  • perl -e "print scalar localtime $time, \"\n\""

• How to redirect stderr to a pipeline? Now BashFaq #47

  • command 2>&1 | command

  • To discard stdout entirely: command 2>&1 >/dev/null | command

• How can I redirect file names using sequential numbers to avoid overwriting existing ones?
  • mv --backup=numbered # requires GNU mv
• How can I set the output of a command to a variable without executing a subshell (i.e. $() ) or writing to a file and reading it back?

  • i think this does the job, have not had time to test: http://tldp.org/LDP/abs/html/process-sub.html