Bash Open Questions

If you want to help with the BashFAQ, you could try to answer one of the following questions. Just answer it, copy it to a new subpage of the BashFAQ page (e.g. [:BashFAQ/101:]), and remove the question here.

• 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#faq45:Bash FAQ #45] now.

  • It's also covered in ProcessManagement.

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

  • getopts, or a while/shift loop (see [:BashFAQ#faq35:Bash FAQ #35])

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

  • Answer: eval -- see also [:BashFAQ#faq6:Bash FAQ #6] and the warnings in [:BashFAQ#faq48:#48].

  • If othervar is an integer, use an [:BashFAQ#faq5:array] instead.

• How can I make bash set the xterm title to the command it is currently executing?
  • This can't be done in any straightforward manner because there's no hook in Bash to execute arbitrary code after a user presses Enter but before a command is executed. The DEBUG trap is triggered after each command, and the PS1 variable is triggered when a prompt is displayed, which is also after the command, rather than before.
• How to determine Yesterday's date?
  • date -d'yesterday' # GNU date
  • date -r $(( $(date +%s) - 86400 )) # OpenBSD date
• How to convert Unix time stamps to a date (and vice versa)?
  • [:BashFAQ#faq70:FAQ #70]
• How to redirect stderr to a pipeline? Now [:BashFAQ#faq47:Bash FAQ #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

    • Commands in a process substitution ( >(cmd) or <(cmd) ) are definitely run in a subshell. I'm not sure I understand what this question is really asking. Maybe it's related to [:BashFAQ#faq84:FAQ 84].