[SOLVED] Two conditions in "if" statement: if first fails, does it check second condition?

ziphem · 03-04-2020, 06:09 PM

I hope I don't embarrass myself with the question, and I presume the answer, but wanted to verify. I tried to look this up on Google, but wasn't sure how to properly describe it, and so couldn't find anything that gave an answer on it.

Question: when there are two conditions in an if / then statement with certain required matches, and the first match is not met, does the script then make a conclusion at that point? Or does it always check both conditions regardless?

So, for example:

Code:

if [ "$item1" = 'A' ] && [ "$item2" = 'B' ]; then
    echo "Success"
else
    echo "Fail"
fi

and $item1 does not equal A, does the script stop without checking the other condition and put me to "else"? Or will it always check the second condition [ "$item2" = 'B' ]?

Maybe more detail than anyone wants, but my reason: $item1 = From my computer, I want to check whether my phone is connected to the network - and this can be any network:

Code:

 nmap -T4 -A -v -oG - `/sbin/ip -o -4 addr list ***MY WIRELESS DEVICE*** |  awk '{print $4}' | cut -d/ -f1 | grep -Po '.*(?=\.)'`.USUAL PORT RANGE -Pn -p PORT NUMBER WHERE PHONE WILL LISTEN | grep 'MY PHONE NAME' | grep -Pom 1 '[0-9.]{7,15}' | sort -u

$item 2 = If it is, I verify with a test SSH. If $item1 fails, I don't want to send a test login to some random IP address on the network. For reference that test login:

Code:

sshpass -p 'PASSWORD' ssh $PHONE -p PHONE'S SSH PORT -o ConnectTimeout=10 'exit 0' 2>&1 | grep 'TEXT OUTPUT WHEN SSHED IN'

(I have a bunch of other protections in place - I only turn on my phone's Wifi and SSH server, check if I'm on a non-standard IP, check if on VPN, etc. etc. My script works now, just trying to clean up and simplify. I'll probably post the final code in case it benefits someone else.)

Thanks a ton!

astrogeek · 03-04-2020, 06:40 PM

Quote:

Originally Posted by ziphem

Question: when there are two conditions in an if / then statement with certain required matches, and the first match is not met, does the script then make a conclusion at that point?

Yes, if the first condition fails it is done...

michaelk · 03-04-2020, 06:44 PM

Furthermore, an if statements evaluates to either true or false and multiple conditions follows binary math i.e.

Code:

false && false = false
true  && false = false
false && true  = false
true  && true  = true

if both are true then echo success otherwise echo fail.

MadeInGermany · 03-04-2020, 11:44 PM

After the if normal shell code follows until it reaches a then, at that point the $? (last exit status) decides if the following normal shell code up to a fi or else is run.
The following demonstrates it (I hope)

Code:

if
    echo "now running test command(s)"
    [ "$item1" = 'A' ] && [ "$item2" = 'B' ]
then
    echo "Success"
else
    echo "Fail"
fi

So, the question is about normal shell code rather than a specific if-then behavior.
Indeed && and || are logical operators.
If the $? is not 0 (false) then a && command is skipped.
If the $? is 0 (true) then a || command is skipped.

MadeInGermany · 03-04-2020, 11:48 PM

After the if normal shell code follows until it reaches a related then, at that point the $? (last exit status) decides if the following normal shell code up to a related fi or else is run.
The following demonstrates it (I hope)

Code:

if
    echo "now running test command(s)"
    [ "$item1" = 'A' ] && [ "$item2" = 'B' ]
then
    echo "Success"
else
    echo "Fail"
fi

So, the question is about normal shell code rather than a specific if-then behavior.
Indeed && and || are logical operators.
If the $? is not 0 (false) then a && command is skipped.
If the $? is 0 (true) then a || command is skipped.

Note the "inverse" values for true and false, this is specific to the shell.
Other programming languages have non-0 for true and 0 for false

pan64 · 03-05-2020, 03:29 AM

with other words: the conditional expression will be evaluated, and if the result was found the evaluation is stopped.

Code:

if [[ conditionA ]] && [[ conditionB ]]

if conditionA is false, the result will be false anyway (regardless of conditionB), so the second part will not be evaluated.

You can simply check how does it work:

Code:

{ echo 1; false; } || { echo 2; false; } && { echo 3; true; }

themanwhowas · 03-07-2020, 12:09 PM

&& is a conditional and or a short circuit and. The 2nd part will not be evaluated if the first fails.

Using & instead of && will evaluate both parts.

Code:

a = 0;
if(a!=0 && b%a)

will work fine as if a !=0 the second test won't run. If however you have

Code:

a = 0;
if(a!=0 & b%a)

you will get a divide by zero.

Same with or ||

ziphem · 12-06-2023, 09:35 PM

Guys, I never did say - thank you for the feedback! It was helpful. I appreciate it.

astrogeek · 12-07-2023, 02:34 AM

Quote:

Originally Posted by ziphem

Guys, I never did say - thank you for the feedback! It was helpful. I appreciate it.

You are welcome! We appreciate the return and feedback!

Turbocapitalist · 12-07-2023, 02:45 AM

Quote:

Originally Posted by ziphem

If $item1 fails, I don't want to send a test login to some random IP address on the network.

Speaking of feedback, in that case you really ought to use an SSH key for authentication instead of sending out passwords for harvesting if you somehow manage to contact the wrong server. The key can be locked down on the server end so that it does nothing:

Code:

command="true" ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHjK/gl+HLaqkwoUCxksnqueQmETAz6GwSMYsQD36VV6 network test

Then it will return success but disconnect immediately if it gets through.

GazL · 12-07-2023, 03:25 AM

I didn't see it mentioned above, but just to add a little for anyone returning to this thread...

This will check for the existence of both files, even if the first doesn't exist:
if [ -f file1 -a -f file2 ] ...

These won't:
if [ -f file1 ] && [ -f file2 ] ...
if [[ -f file1 && -f file2 ]] ...

For this reason most shell-scripting "style guides" will advise you not to use the '-a' option in a 'test'.

MadeInGermany · 12-07-2023, 05:12 AM

And you can prove it with

Code:

$ strace -f sh -c '[ -f file1 -a -f file2 ]' 2>&1 | grep -w 'file[12]'
execve("/bin/sh", ["sh", "-c", "[ -f file1 -a -f file2 ]"], 0x3fffdff618 /* 23 vars */) = 0
newfstatat(AT_FDCWD, "file1", 0x3fffa79d78, 0) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "file2", 0x3fffa79d68, 0) = -1 ENOENT (No such file or directory)

$ strace -f sh -c '[ -f file1 ] && [ -f file2 ]' 2>&1 | grep -w 'file[12]'
execve("/bin/sh", ["sh", "-c", "[ -f file1 ] && [ -f file2 ]"], 0x3fffbee618 /* 23 vars */) = 0
newfstatat(AT_FDCWD, "file1", 0x3fffd09c48, 0) = -1 ENOENT (No such file or directory)

$ strace -f bash -c '[[ -f file1 && -f file2 ]]' 2>&1 | grep -w 'file[12]'
execve("/bin/bash", ["bash", "-c", "[[ -f file1 && -f file2 ]]"], 0x3ffffc1618 /* 23 vars */) = 0
newfstatat(AT_FDCWD, "file1", 0x3fffe21f18, 0) = -1 ENOENT (No such file or directory)

$

sundialsvcs · 12-07-2023, 08:00 AM

The official name for this idea is "short-circuit" expression evaluation – routinely used for if statements and other conditionals. As soon as it can be determined that the branch will or will not be taken, further evaluation is not performed. This very expressive notion is used a lot, because it makes "clean" code.

Contrast this to bitwise evaluation within, say, an assignment statement.

pan64 · 12-08-2023, 12:07 AM

Quote:

Originally Posted by sundialsvcs

The official name for this idea is "short-circuit" expression evaluation – routinely used for if statements and other conditionals. As soon as it can be determined that the branch will or will not be taken, further evaluation is not performed. This very expressive notion is used a lot, because it makes "clean" code.

The problem is that many people don't know about it, so they don't use it properly (that's why we have this thread too).