Issue an error when using empty shell variables

Question

Sometimes I use, $PROJECT_HOME/* to delete all files in the project. When the environment variable, PROJECT_HOME is not set (because I did su and the new user doesn't have this environment variable set), it starts deleting all files from the root folder. This is apocalyptic.

How can I configure bash to throw error, when I use an undefined environment variable in the shell?

Answer 1

In POSIX shell, you can use set -u / set -o nounset, which will cause the shell to thrown an error when trying to expand an unset variable:

#!/bin/sh

set -u
: "${UNSET_VAR}"

E.g. in Bash, it will thrown an error like unset1.sh: line 3: UNSET_VAR: unbound variable

or using the ${var?message} Parameter Expansion, which will thrown an error with the given message if the variable is unset.

: "${UNSET_VAR?Unset variable}"

In your case, you should use :? instead of ? to also fail on set but empty variables:

rm -rf -- "${PROJECT_HOME:?PROJECT_HOME empty or unset}"/*

set -u doesn't have an equivalent for erroring on variables set to the empty string.

Answer 2

[ -z "$PROJECT_HOME" ] || rm -r "$PROJECT_HOME"/*

This will also catch the case where PROJECT_HOME is set but doesn't contain anything.

Example:

1) This will delete pretty much everything you can delete on your system (barring dotfiles inside / (there aren't usually any)):

set -u
PROJECT_HOME=
rm -r "$PROJECT_HOME"/*

2) This won't do anything:

PROJECT_HOME=
[ -z "$PROJECT_HOME" ] || rm -r "$PROJECT_HOME"/* 

---

Completely removing your project home and recreating it might be another option (if you want to get rid of dotfiles too):

#no apocalyptic threats in this scenario
rm -r "$PROJECT_HOME"
mkdir "$_" 

Answer 3

Another way to do this:

rm -r "${somevar:-/tmp/or_this_if_somevar_is_empty}"/*

There are many variable substitutions, the one above is when "somevar" is empty (and in that case it attempts to delete /tmp/or_this_if_somevar_is_empty/* )

Answer 4

FWIW, in ksh93 (the shell bash generally tries to imitate though it hasn't copied types, compound variables or disciplines yet), you can define a type of variable for which trying to expand it when empty causes a fatal error:

#! /bin/ksh93 -
progname=$0
typeset -T must_be_non_empty_upon_expansion=(
  function get {
    if [[ -z $_ ]]; then
      print -ru2 -- "$progname: line ${.sh.lineno}: ${.sh.name}: parameter empty"
      exit 1
    fi
  }
)
must_be_non_empty_upon_expansion foo bar
foo=non-empty
print -r "Fine: $foo"
bar=
print -r "Not fine: $bar"

$ ./that-script
Fine: non-empty
./that-script: line 15: bar: parameter empty