Using sh -c if comparisons inside an SSH command

Question

I'm having some trouble with the != section of the if statement. Essentially this statement is valid as far as I'm aware, however executing this gives [: 1: !=: unexpected operator. I've tried executing using -n but for whatever reason, even if the output is blank, using -n still runs the echo command.

Any help on this is appreciated. I've attached the code snippet below.

#!/bin/sh

HOST=$1
USER="/scripts/whoowns $HOST | tr -d '\r'"

ssh -t $HOST -p 22 -l deehem "sh -c 'if [ "" != "\`$USER\`" ]; then echo "Username for $HOST: \`$USER\`"; fi' ; bash -login"

Ah okay, worked out that it's because "" is blank. Is there any viable alternative to "" != "$VAR"? — Dan Miller, Mar 20 '13 at 21:28

Karoly Horvath · Accepted Answer · 2013-03-20 21:51:42Z

As you realised, "bla "" bla" is just two strings concatenated ("bla bla").

You can escape the ": \", but the standard [ test tool has an option specifically for this task:

   -n STRING
          the length of STRING is nonzero

   -z STRING
          the length of STRING is zero

Note: why do you have \`? That way the string is never going to be empty....

Rublacava · Accepted Answer · 2022-02-08 03:19:27Z

In regards to your "however executing this gives [: 1: !=: unexpected operator" problem, here is a solution:

Use bash -c instead of sh -c. The bash program seems to be better than sh at handling square brackets. For example:

ubuntu@ubuntu:/$ echo "antipetalous" | if [[ "antipetalous" =~ "ti" ]]; then echo "match"; else echo "no"; fi
match
ubuntu@ubuntu:/$ echo "antepetalous" | if [[ "antepetalous" =~ "ti" ]]; then echo "match"; else echo "no"; fi
no
ubuntu@ubuntu:/$ sh -c 'echo "antipetalous" | if [[ "antipetalous" =~ "ti" ]]; then echo "match"; else echo "no"; fi'
sh: 1: [[: not found
no
ubuntu@ubuntu:/$ bash -c 'echo "antipetalous" | if [[ "antipetalous" =~ "ti" ]]; then echo "match"; else echo "no"; fi'
match
ubuntu@ubuntu:/$ bash -c 'echo "antepetalous" | if [[ "antepetalous" =~ "ti" ]]; then echo "match"; else echo "no"; fi'
no
ubuntu@ubuntu:/$ sh -c 'echo "antepetalous" | if [[ "antepetalous" =~ "ti" ]]; then echo "match"; else echo "no"; fi'
sh: 1: [[: not found
no
ubuntu@ubuntu:/$

So ssh -t $HOST -p 22 -l deehem "sh -c 'if [ "" != ... would become ssh -t $HOST -p 22 -l deehem "bash -c 'if [ "" != ....

Thanks to pLumo at https://askubuntu.com/questions/1310106/sh-c-sh-not-working-when-using-if-statement-and-having-in-the-filena ("command line - sh -c '...' sh {} not working when using if statement and having ' in the filename - Ask Ubuntu").

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