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 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.

  • In bash 4, the DEBUG trap is triggered before every command, according to help trap. This is a change from bash 3. So this should be possible in bash 4. Someone just needs to write it up....

• How to determine Yesterday's date?
  • date -d'yesterday' # GNU date (sufficiently new versions)
  • date -d '1 day ago' # GNU date (all versions I have available to test)
  • date -r $(( $(date +%s) - 86400 )) # OpenBSD date
• 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 (see also ProcessSubstitution on this site)

    • Commands in a process substitution ( >(cmd) or <(cmd) ) are definitely run in a subshell (or rather a full-blown background process). I'm not sure I understand what this question is really asking. Maybe it's related to FAQ 84.

• Creating temporary files securely and portably (BashFAQ/062 has a partial answer)

• Which shebang — #!/bin/bash or #!/usr/bin/env bash — should be used or in which circumstance? Is one preferred over the other?