bash script if / else case insensitive answers

Question

I have an if/else statement in a bash script that's asking if an EBS volume is going to be created from a snapshot id.

I'd like to create a line like: if [ "$from_snapshot" -eq "Y|y" ] but I don't think that will work.

Here's what I have:

  if [ "$from_snapshot" -eq "Y" ]
    then
      echo "Enter Snapshot ID: "
      read -r snapshot_id
   elif "$from_snapshot" -eq "y"
      echo "Enter Snapshot ID: "
      read -r snapshot_id
   else 
      echo "No Snapshot Required"
   fi

If there any way I can state that more succinctly?

Consider case $from_snapshot in [Yy]) echo "yes code goes here";; *) echo "other code goes here";; esac for a baseline-POSIX approach. — Charles Duffy
– Charles Duffy, Commented May 21, 2018 at 19:31
Alternatively, you could to change the case of your parameter: [[ ${from_snapshot^} = Y ]] or [[ ${from_snapshot,} = y ]] — Abelisto
– Abelisto, Commented May 21, 2018 at 22:04

rici · Accepted Answer · 2018-05-21 21:12:06Z

5

The -eq operator in test (aka [) is used for arithmetic comparisons. That's certainly not what you want; you're trying to do a string comparison, which is the = operator. (Bash lets you type that as ==, but = is what the standard requires.)

With bash, you are much better off using the [[ conditional command, which allows for "glob"-style pattern comparisons (and also regular expressions if you need that). So you could use

if [[ "$from_snapshot" = [Yy] ]]; then

Note: Do not put quotes around [Yy]. Quoting it will make it into a literal string instead of a pattern.

edited May 21, 2018 at 21:12

answered May 21, 2018 at 19:07

rici

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dbush · Accepted Answer · 2018-05-21 19:27:47Z

2

In the more general case, you can use the -o option which performs a logical OR:

if [ \( "$from_snapshot" = "Y" \) -o \( "$from_snapshot" = "y" \) ]
then
  echo "Enter Snapshot ID: "
  read -r snapshot_id
else 
  echo "No Snapshot Required"
fi

Or you can use multiple tests with || between them:

if [ "$from_snapshot" = "Y" ] || [ "$from_snapshot" = "y" ]
then
  echo "Enter Snapshot ID: "
  read -r snapshot_id
else 
  echo "No Snapshot Required"
fi

answered May 21, 2018 at 19:27

dbush

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1 Comment

Charles Duffy Over a year ago

Note the OB flags in the POSIX test spec -- passing more than four arguments to a single test command, or using -o, -a or parens, is obsolescent behavior (and an XSI extension, rather than baseline POSIX, even prior to that designation). The core issue is that it's ambiguous -- test can't tell if parens are intended to be literal or syntactic; is [ '(' = ')' ] supposed to be asking whether = is an empty string, with that test inside a group, or checking whether ( and ) are the same character?

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