bash logic inside command substitution

问题

I have a little banner that I display at login

fignow() { figlet -w $(tput cols) $(date +"%b %d, Week %V"); figlet -w $(tput cols) -f univers $(date +"%H:%M"); }

This works fine if the additional fonts are installed to get univers, but that's not part of the default installation, so I can do the following:

fignow() { figlet -w $(tput cols) $(date +"%b %d, Week %V"); if [ -f /usr/share/figlet/univers.flf ]; then figlet -w $(tput cols) -f univers $(date +"%H:%M"); else figlet -w $(tput cols) $(date +"%H:%M"); fi; }

This also works fine, but I was curious if the if/else/fi could be removed and the logic could be done inside the command itself, i.e. something like:

fignow() { figlet -w $(tput cols) $(date +"%b %d, Week %V"); figlet -w $(tput cols) -f $(use-univers-or-failback-to-standard.flf-if-univers.flf-is-not-available) $(date +"%H:%M"); }

Is something like that possible?

回答1:

I suggest to use figlist to avoid hardcoding the themes path:

figlet -w "$(tput cols)" -f "$((figlist | /bin/grep -hw universe) || echo "standard")" "foo"

回答2:

I suggest:

fignow() {
  local opts
  figlet -w $(tput cols) $(date +"%b %d, Week %V")
  [ -f /usr/share/figlet/univers.flf ] && opts="-f univers"
  figlet -w $(tput cols) $opts $(date +"%H:%M")
}

Only if the file exists the variable $opts contains the necessary options.

回答3:

Checking for the file or parsing the output of figlist or showfigfonts is inherently fragile. Just call figlet to see if the option is available. Something like:

fignow() {
        local font=${1-universe}
        figlet -f "$font" a > /dev/null 2>&1 || font=
        figlet -w $(tput cols) $(date +"%b %d, Week %V")
        figlet -w $(tput cols) ${font:+-f "$font"} $(date +"%H:%M")
}

Note that this is one of those cases where you must not use double quotes around the variable, since you want ${font:+-f "$font"} to expand to the empty string when $font is empty. If you use "${font:+-f "$font"}", the semantics change and an empty string is passed to figlet.

来源：https://stackoverflow.com/questions/64941596/bash-logic-inside-command-substitution

标签

bash

command-substitution

figlet