Catching syntax error when trying to write to and call an alias in .bashrc

Question

Catching syntax error when trying to write to and call an alias in .bashrc

I’m trying to create an alias for determining whether or not I need to reboot my server.

Here’s what I’m doing:

$ echo "alias rcr='if [ -f /var/run/reboot-required ]; then read -p "Continue? (Y/N): " confirm && [[ $confirm == [yY] || $confirm == [yY][eE][sS] ]] || exit 1 && reboot; else echo No reboot needed; fi'" >> ~/.bashrc
$ source ~/.bashrc
$ rcr

But this throws an error:

bash: conditional binary operator expected
bash: syntax error near `[yY]'

I’ve tried every different combination that I could think of, from escaping characters to prefixing variables. I’m a little new to bash (not Linux in general), so I’m just trying to get the hang of this.

What am I doing wrong here? Here’s what gets added to .bashrc:

alias rcr='if [ -f /var/run/reboot-required ]; then read -p "Continue? (Y/N): " confirm && [[  == [yY] ||  == [yY][eE][sS] ]] || exit 1 && reboot; else echo No reboot needed; fi'

Asked By: Mike K

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Source

Answer 1

The outer double quotes allow your interactive shell to expand $confirm – presumably to an empty value – so that what you actually write to the .bashrc file is

alias rcr='if [ -f /var/run/reboot-required ]; then read -p "Continue? (Y/N): " confirm && [[  == [yY] ||  == [yY][eE][sS] ]] || exit 1 && reboot; else echo No reboot needed; fi'

You can prevent that by escaping the $ (which is an an additional form of quoting):

$ echo "alias rcr='if [ -f /var/run/reboot-required ]; then read -p "Continue? (Y/N): " confirm && [[ $confirm == [yY] || $confirm == [yY][eE][sS] ]] || exit 1 && reboot; else echo No reboot needed; fi'"
alias rcr='if [ -f /var/run/reboot-required ]; then read -p "Continue? (Y/N): " confirm && [[ $confirm == [yY] || $confirm == [yY][eE][sS] ]] || exit 1 && reboot; else echo No reboot needed; fi'

However IMHO it would be simpler and clearer to use a quoted here document to prevent expansion altogether:

cat >> ~/.bashrc << 'EOF'

alias rcr='
  if [ -f /var/run/reboot-required ]; then 
    read -p "Continue? (Y/N): " confirm
    case $confirm in 
      ([yY]|[yY][eE][sS]) reboot ;; 
    esac
  else
    echo "No reboot needed"
  fi
'
EOF

I also took the liberty of converting your conditionals to a case statement. You could also consider using a shell function instead of an alias here.

Answered By: steeldriver