Ubuntu – How to stop the bash script when a condition fails

bashcommand linescripts

Here it shows to use the || and && in a single line to concatenate the execution of commands: How can I check for apt-get errors in a bash script?

I am trying to stop a script execution if a certain condition fails,

e.g.

false || echo "Obvious error because its false on left" && exit

Here it prints Obvious error because its false on the left and exits the console which is what I wanted.

true || echo "This shouldn't print" && exit

Here, there's no echo print but the exit command runs as well, i.e., the console is closed, shouldn't the exit command not run because the echo command on the right was not executed?
Or is by default a statement considered false on the left hand side of an && operator?

Edit:
I should have mentioned it before, my aim was to echo the error and exit if it was not clear.
w.r.t to my specific case of catching errors when grouping conditions using && and ||, @bodhi.zazen answer solves the problem.

@takatakatek answer makes it more clear about the flow control and the bash guide links are excellent

@muru answer has good explanation of why not to use set -e if you want custom error messages to be thrown with alternatives of using perl and trap which I think is a more robust way and see myself using it from my second bash script onwards!

Best Answer

The problem is that || and && only skip one subsequent stanza of a command chain when the condition fails. If you write out a full block structure it makes perfect sense. What you wrote becomes this:

if ! true ; then
   echo "This shouldn't print"
else
   exit
fi

What you want is this:

if ! true ; then
   echo "This shouldn't print"
   exit
fi

When you are using Bash's shortcut conditional operators it is better to avoid mixing them. The logic is much easier to understand while you're writing it, and your readers will appreciate your good style.

Related Solutions

Ubuntu – Create bash script that allows you to choose multiple options instead of just one

From the discussion in the comments it is apparent that your main concern is to create as script with multiple selections, rather than focus on copying/moving files themselves, and the file operations are just an example. This functionality can be achieved via dialog command, which allows creating text user interfaces, with --checklist flag specifically; however there's nothing in the standard shell-only toolbox to achieve what you want. Hence, dialog is an appropriate tool for this job.

Below you will find an example script. While the script implements only 3 options that were discussed, it provides a decent starting point which users can extent further, and also addresses mutually exclusive options as mentioned in the comments. Particularly, the multiple selection is addressed in menu() function, which serves as a wrapper for dialog with --checklist option

To keep things simple, all you really need is this:

output=$(dialog --clear --backtitle "Backtitle. Use <SPACE> to select." --title "My Dialog" \
       --checklist "Select all that apply"  50 50 100 \
       checksum "SHA-256" off \
       copy "Copy only (exclusive with move)" off \
       move "Move only (exclusive with copy)" off 2>&1 > /dev/tty)

This saves the selection of multiple items to variable $output. Note that `2>&1 > /dev/tty) at the end are crucial to saving the return value into variable. But see the script below for more practical example:

#!/bin/bash

puke(){
    # function to exit with specific error message
    # analogous to 'die' in Perl
    printf ">>> Errors were encountered: %s\n" "$1" && exit
} > /dev/stderr

menu(){
    # dialog --help documents the option as follows:
    # --checklist    <text> <height> <width> <list height> <tag1> <item1> <status1>...
    # tags are what the output returns.
    # We can use word-splitting 
    # and iterate over output of this function in order. Of course first option
    # being checksum will always work and is not mutually exclusive with anything else
    dialog --clear --backtitle "Backtitle. Use <SPACE> to select." --title "My Dialog" \
           --checklist "Select all that apply"  50 50 100 \
           checksum "SHA-256" off \
           copy "Copy only (exclusive with move)" off \
           move "Move only (exclusive with copy)" off || puke

} 2>&1 1>/dev/tty

select_file(){
    dialog --backtitle "Choose file by typing or navigating and selecting with <SPACE>" --fselect "/etc/" \
           20 50  || puke
} 2>&1 1>/dev/tty


iter_actions(){
    # variables are available to child functions
    # Since we call iter_actions in main(), this
    # function also knows about main's variable $fselect

    for i ; do 
       case "$i" in
           checksum) sha256sum "$fselect" ;;
           copy) cp "$fselect" /tmp ;;
           move) mv "$fselect" /tmp ;;
       esac
    done
}

main(){
    # here I'm using /etc but you can use $PWD to default to user's 
    # current working directory, or accept positional parameters from command-line
    # as in $1, $2 and so forth
    fselect=$( select_file "/etc" )
    actions=$(menu) 
    printf "\r%b" "\033c" # this clear the screen

    case "$actions" in
       *copy*move|*move*copy) puke "Mutually exclusive options selected" ;;
        *) iter_actions $actions ;; # note here variable is unquoted on purpose
    esac
}

# script entry point
main "$@"

Best Answer

Related Solutions

Ubuntu – Create bash script that allows you to choose multiple options instead of just one

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