New to shell scripting. Need help with conditions

koobi · 11-05-2007, 05:58 AM

Hi,
I just started learning it last this past friday.

I'm not sure what's wrong. Even if I hit enter, it says I've input a string.
I want to set a default value if nothing is input.

Also, on occasion, the -z switch does not seem to work. I'm on Bash 3.x

Any tips are welcome too.

Code:

#!/bin/bash
set -e


DOWNLOAD_DIR="/var/www/downloads"


make_main_dl_dir () {
    clear
    echo -n "Enter the path to download files\nDefault is ${DOWNLOAD_DIR} \n> "
    read DOWNLOAD_DIR_USER

    if [ -z DOWNLOAD_DIR_USER ]; then
        echo "No input. Default value used.\n"
    else
        echo "String entered.\n"
        if [ -d ${DOWNLOAD_DIR_USER} ]; then
            echo "${DOWNLOAD_DIR_USER} already exists but will be used anyway.\n"
        fi
        DOWNLOAD_DIR=${DOWNLOAD_DIR_USER}
    fi
    echo "\v\vDownload dir: ${DOWNLOAD_DIR}\n"

    exit 0
}



make_main_dl_dir;
exit 0

bigearsbilly · 11-05-2007, 06:02 AM

[ -z "$DOWNLOAD_DIR_USER" ]

Hobbletoe · 11-05-2007, 07:44 AM

Quote:

Originally Posted by koobi

I want to set a default value if nothing is input.

You can set a default value for a variable as such ...

Code:

${DOWNLOAD_DIR_USER:-value}

So say you want to set that variable to Daniel (:- is for if the variable is not set, or if it is empty), you could do your read and then

Code:

DOWNLOAD_DIR_USER=${DOWNLOAD_DIR_USER:-Daniel}

Check out Section 9.3 of the Advanced Bash-Scripting Guide for parameter substitutions.

rjlee · 11-05-2007, 07:53 AM

Quote:

Originally Posted by koobi

Code:

    if [ -z DOWNLOAD_DIR_USER ]; then

This won't work. First, $DOWNLOAD_DIR_USER needs a dollar sign to indicate that it's a variable. Secondly, it it's an emty string, then this will be interpreted as:

Code:

    if [ -z ]; then

which is a syntax error. So you need to enclose it in double-quotes.

Personally, I prefer to use if blocks to assign a default value to a variable, because this makes it easier to trap cases of non-empty but invalid input.

Hope that helps,

—Robert J Lee

bigearsbilly · 11-05-2007, 08:02 AM

Quote:

which is a syntax error

actually in this case it isn't,

though generally it's true for test statements.

you can do a default value like this:

: ${VARIABLE:=default}

koobi · 11-05-2007, 10:54 AM

Quote:

Originally Posted by bigearsbilly

[ -z "$DOWNLOAD_DIR_USER" ]

oops

silly of me for missing that one. thanks

Quote:

Originally Posted by Hobbletoe

You can set a default value for a variable as such ...

Code:

${DOWNLOAD_DIR_USER:-value}

So say you want to set that variable to Daniel (:- is for if the variable is not set, or if it is empty), you could do your read and then

Code:

DOWNLOAD_DIR_USER=${DOWNLOAD_DIR_USER:-Daniel}

Check out Section 9.3 of the Advanced Bash-Scripting Guide for parameter substitutions.

thanks for that explanation

i will be trying that out tomorrow and i will probably have some questions regarding this.

ok i've modified my script to this:

Code:

make_main_dl_dir () {
clear
    echo -n "Enter the path to download files\nDefault is ${DOWNLOAD_DIR} \n> "
    read DOWNLOAD_DIR_USER

    if [ -z ${DOWNLOAD_DIR_USER} ]; then
        echo "No input. Default value used.\n"
    else
        echo "String entered.\n"
        if [ -d ${DOWNLOAD_DIR_USER} ]; then
            echo "${DOWNLOAD_DIR_USER} already exists but will be used anyway.\n"
        fi
        DOWNLOAD_DIR=${DOWNLOAD_DIR_USER}
    fi
    echo "\v\vDownload dir: ${DOWNLOAD_DIR}\n"

    ALL_DL_DIRS="${DOWNLOAD_ATOMIC} ${DOWNLOAD_PGSQL}"
    for each_dir in ${ALL_DL_DIRS} ;
    do
        mkdir ${each_dir}
        if [ -d ${each_dir} ]; then
            echo "Directory ${each_dir} was created"
        else
            echo "Directory ${each_dir} could not be created. Please make sure you are the root user."
        fi
    done

    exit 0
}

does anything seem out of place here?
suppose the directory could not be created for some reason, will the script stop if i have 'set -e' at the beginning?

also, is there a place where i can look up best practiced in bash scripting? the reason i'm asking you instead of looking on google is that, i was looking up creating functions in bash and quite a few sites had something similar to this example:

Code:

function foo
{
    echo 'something'
}

but this did not work for me. i'm guessing it's a version related issue?
however, after much trial and error, i found that this worked for me:

Code:

foo ()  {
    echo 'something'
}

thanks for everything so far, it's been very useful

PS:
which is the 'right way' to assign default values?

Code:

${DOWNLOAD_DIR_USER:-value}

or

Code:

: ${VARIABLE:=default}

Hobbletoe · 11-05-2007, 11:04 AM

That really depends on what you are doing. The := will use your default value if your variable is not set. The :- will use the default value if the value is empty or if it is not set. I can't say that I can figure out how a variable is set without a value, but the two seem to be pretty much the same.

It should be noted that you can also use ${var-value} and ${var=value} to set default values as well. I'd really suggest reading through the Advanced Bash-Scripting Guide that I mentioned above. I have found it to be a VERY good source of information, and has a whole section dealing with parameter substitution.

bigearsbilly · 11-06-2007, 02:14 AM

the way I did it

: ${VAR:=thing}

is actually a cheat, so that I can define all my default variables in the same place
if you have it alone on the line you get...

Code:

dysp0024:primalA$ ${VAR:=thang}
ksh: thing:  not found

the colon makes it a label, so I can do this...

Code:

: ${VAR:=thing}

it will be treated as a command
and the shell will try and run it. The colon makes it a label so it gets set but is not evaled as a
command.
as for := and :- it's all in man ksh or bash

:= actually sets it if null
:- just uses it if null without setting e.g:

Code:

dysp0024:$ echo ${THIS}         

dysp0024$ echo ${THIS:-or that}
or that
dysp0024$ echo ${THIS}

koobi · 11-08-2007, 11:35 PM

Great, thanks everyone

I'm making progress with my scrip thanks to this forum.

I want to create a nested menu though, what's the best way to do this?
For example, I have this menu:

Quote:

MAIN MENU
1 - Manage HTTPD
2 - Manage Database

0 - Exit

Enter choice:

upon entering 1, for example, I would like to show:

Quote:

MAIN MENU
1 - Get HTTPD status
2 - Start HTTPD
3 - Stop HTTPD
4 - Restart HTTPD
5 - Reload HTTPD
6 - Graceful HTTPD
7 - Configtest HTTPD
8 - Fullstatus HTTPD

0 - Exit

Enter choice:

I can create simple menu's with CASE but what about nested menu's? Have the nested blocks in separate functions and then call them once an option is selected?

Thanks.

Disillusionist · 11-09-2007, 01:57 AM

I would normally create a seperate function for sub menus.

Code:

f_sub_menu_a()
{
   echo "Sub Menu"

   ## Rest of sub_menu_a goes here (include case statement)



}



## Further down the code (where you have your case statement for your menu:
   1) f_sub_menu_a;;

The important thing is to have the function before the part of the script that calls it otherwise your script will error as the function hasnt been defined at that point.

bigearsbilly · 11-09-2007, 02:16 AM

you do menus with select

Code:

$ select choice in *; do 
     echo you picked $choice; 
     break; 
done

1) Proxy.tgz                        9) plan9.iso.bz2
2) Text-Indent-0.1.tar.gz          10) sol-nv-b70b-x86-dvd-iso-a.zip
3) ade.ftp                         11) sol-nv-b70b-x86-dvd-iso-b.zip
4) dtpdev.ftp                      12) sol-nv-b70b-x86-dvd-iso-c.zip
5) ftp2ade.bat                     13) sol_sums
6) ftp2dtpdev.bat                  14) suns_own_sums
7) ftp2vca.bat                     15) vca.ftp
8) md5sum_dvd_x86.list
#? 15
you picked vca.ftp

Disillusionist · 11-09-2007, 02:24 AM

Billy is right select is the way do go! much cleaner.

Hobbletoe · 11-09-2007, 07:28 AM

Select is coved in section 10.4 of the Advanced Bash-Scripting Guide. If you want to change the prompt that is used for select, you will need to set the PS3 variable to what you would like it to be.