Ubuntu – lower-case ‘c’ key not working in bash

If the string is already stored in a variable you can use bash's parameter expansion, specifially ${parameter,,pattern} (available since bash 4.0), where parameter is the name of your variable and pattern is ommitted:

$ string="Hello, World!"
$ echo $string
Hello, World!
$ echo ${string,,}
hello, world!

Note that this does not change the value of the variable, only the output. To change the variable you have to assign the new value:

$ echo $string
Hello, World!
$ string=${string,,}
$ echo $string
hello, world!

The upper-case conversion works with ${parameter^^pattern}:

$ echo ${string^^}
HELLO, WORLD!

This works also with Unicode strings (at least with current bash versions, probably needs at least bash 4.3):

$ string='ἈΛΦΆβητος'
$ echo ${string,,}
ἀλφάβητος
$ echo ${string^^}
ἈΛΦΆΒΗΤΟΣ

If you are using zsh, you can use Parameter Expansion Flags (${(FLAGS)NAME}; available since zsh 2.5) to achieve the same results. The bash syntax does not work in zsh ¹⁾. The flag for lower case is L; for upper case it is U:

$ string="Hello, World!"
$ echo ${(L)string}
hello, world!
$ echo ${(U)string}
HELLO, WORLD!
$ echo $string
Hello, World!"

This also works with Unicode strings (at least since zsh 5.0; I did not try with earlier versions):

$ string='ἈΛΦΆβητος'
$ echo ${(L)string} 
ἀλφάβητος
$ echo ${(U)string}  
ἈΛΦΆΒΗΤΟΣ

¹⁾ Although, seeing that zsh had this for far longer, it should probably be: "the zsh syntax does not work in bash.

You cannot express number ranges easily in the case expressions - the pattern [1000-9999], for example, does not mean the numbers 1000 to 9999, but the characters 1, 0, 0, the range 0-9, the characters 9, 9, 9 - essentially all the digits. [1-85] does not mean the numbers 1 to 85, but the digits from 1 to 8, and 5, ... which are just the digits from 1 to 8. So [1-20]* means anything that begins with 1, 2, or 0 - so even 20000000 will match that. Use if/then/elif/else/fi instead:

if (( $SECURITY_PACKAGES == 0 ))
then
    echo "OK - not bad: There are a total of $TOTAL_PACKAGES packages to upgrade in this server, but none of them are security updates!"
    exit 0
elif (( $SECURITY_PACKAGES <= 20 ))
then
    echo "WARNING - $TOTAL_PACKAGES packages required to upgrade in this server, of which $SECURITY_PACKAGES are security updates"
    exit 1
elif (( $SECURITY_PACKAGES <= 9999 ))
then
    echo "CRITICAL -  $SECURITY_PACKAGES out of $TOTAL_PACKAGES are security     updates! Consider upgrading soon!"
    exit 2
else
    echo "UNKNOWN - I am not sure what's happening now, check later or check server: $TOTAL_PACKAGES to upgrade, $SECURITY_PACKAGES are security updates"
    exit 3
fi