[SOLVED] Else if Statement

dmathwizard · 06-11-2011, 06:55 AM

Ok I am working on a bash script using else if statements, problem is I want an else if and the else to execute another else if and want to know if that is proper or what? I am coming from windows so I am learning this stuff and some things that are easy in windows just doesn't transfer.

Code:

read -p "Do you want to backup programs?(Y/n)" bck
if [ $bck = n -o $bck = N ]; then
  echo Programs will not be Saved
else
  echo Backing up Programs
fi

if [ -d /sdcard/apk ]; then
  echo 
else
  mkdir /sdcard/apk
fi

echo
cp /system/app/* /sdcard/apk

On the first else if statement if the user selects no then I want it bypass the second else if statement and continue with the script.

carltm · 06-11-2011, 07:07 AM

Yes, you can nest if statements. And I'm glad to see that you're
using indentation well. It becomes especially important once you
start nesting loops and if statements.

For example:

Code:

if [ sometest ]; then
  if [ secondtest ]; then
    echo "I passed both tests!"
  else
    echo "I passed the first test only"
  fi
else
  if [ `whoami` = root ]; then
    echo "Too bad"
  elif grep `whoami` /etc/group | grep ^sudo: > /dev/null; then
    echo "Try using sudo"
  else
    echo "Too bad"
  fi
fi

dmathwizard · 06-11-2011, 07:07 AM

My second question is how far can I take these else if statements. And whether they are proper once I get a good script going or would it be better to make other scripts and "call" these scripts based on the users choices?

grail · 06-11-2011, 07:12 AM

I think I must be missing something here? There are no else if (or elif) statements in your example.

Quote:

I am coming from windows so I am learning this stuff and some things that are easy in windows just doesn't transfer.

Maybe if you illustrate how you would 'easily' do this in Windows we can give you the equivalent?

carltm · 06-11-2011, 07:13 AM

I would congratulate you if you manually type (without copying
and pasting) enough nested if statements that it would break
anything. I think for most computers you would run out of
system memory before you'd reach any limit in the shell.

This is considered the right way to handle the recurring else
if statements.

carltm · 06-11-2011, 07:16 AM

Quote:

Originally Posted by grail

I think I must be missing something here? There are no else if (or elif) statements in your example.

Yes, the line you're missing is this:

elif grep `whoami` /etc/group | grep ^sudo: > /dev/null; then

Of course it is possible to add multiple elif statements in a row.
I just threw together a quick example to show nesting and one
instance of an elif statement.

colucix · 06-11-2011, 07:22 AM

Code:

if [ -d /sdcard/apk ]; then
  echo 
else
  mkdir /sdcard/apk
fi

This if/then construct can be condensed in a single statement:

Code:

mkdir -p /sdcard/apk

The -p option is necessary if both the directories /sdcard and /sdcard/apk don't exist. Also it prevents any error message if the directory already exists (no need to check for its existence).

dmathwizard · 06-11-2011, 07:26 AM

Awesome!! Thank you everyone. Everything works as planned and you guys are helpful.

@grail

Quote:

Maybe if you illustrate how you would 'easily' do this in Windows we can give you the equivalent?

In windows I can use GOTO LINE statements or for my later question I can make multiple ex1.cmd and ex2.cmd files. And when the user makes their choice ex.cmd will call ex1.cmd run its script and at the end ex1.cmd it will call ex.cmd which is the menu.

Btw I used cmd files instead of vbs because I never know what a user will be running and its just simpler.

dmathwizard · 06-11-2011, 07:30 AM

Quote:

Originally Posted by colucix

Code:

if [ -d /sdcard/apk ]; then
  echo 
else
  mkdir /sdcard/apk
fi

This if/then construct can be condensed in a single statement:

Code:

mkdir -p /sdcard/apk

The -p option is necessary if both the directories /sdcard and /sdcard/apk don't exist. Also it prevents any error message if the directory already exists (no need to check for its existence).

Thats awesome thank you. I still have a lot to learn. I know in Windows the less lines and if commands are abbreviated the script goes faster. Linux is already fast enough.

carltm · 06-11-2011, 07:37 AM

Quote:

Originally Posted by dmathwizard

I know in Windows the less lines and if commands are abbreviated the script goes faster. Linux is already fast enough.

It is possible to optimize scripts for speed, but generally I don't
bother doing that. The main exception to this is if a script takes
a long time to run and/or if some part of the command is repeated
thousands of times.

Most of my scripts are written for convenience, so my primary
concern is that they are easy to understand and easy to update
6 months from now when I don't remember the details!

grail · 06-12-2011, 03:37 AM

Quote:

for my later question I can make multiple ex1.cmd and ex2.cmd files. And when the user makes their choice ex.cmd will call ex1.cmd run its script and at the end ex1.cmd it will call ex.cmd which is the menu.

This is the same in linux in that you can create multiple scripts and then call the appropriate one based on user input.

As for GOTO ... I seem to remember in some dark recess an instructor at Uni failing any assignment using this construct

David the H. · 06-12-2011, 04:44 AM

Also consider using functions, particularly if you find yourself repeatedly duplicating the same commands. Of course, you'll still have to use if-else structures, but it will still help you keep your code clean and efficient.

http://tldp.org/LDP/Bash-Beginners-G...l/chap_11.html

MTK358 · 06-12-2011, 09:09 AM

Just remember that if statements work like any other command. This, for example is valid:

Code:

if if false; then true; else false; fi
then
    if true
    then
        echo "true"
    fi
else
    echo "false"
fi

It prints "false".

H_TeXMeX_H · 06-12-2011, 09:15 AM

If you end up making large trees of if else statements, maybe you're using the wrong structure or missing an easier way to do it. Remember there is also the case statement, which can help sometimes.