Re: [Help-bash] Help With School Assignment, Learning Bash Scripts

[Eduardo A . Bustamante López]

Hey Nick :) You forgot to CC the list, I'll include a copy of your
email:

> I asked my teacher and all he states is read the power points.
> Furthermore I will paste all the steps below. If you have any 
> other questions please let me known.
> Nick 
> 1.Start this step by just adding the correct "echo" piped into the correct
> "grep", without redirecting the output, and make sure the script acts
> appropriately, for example:
> 
> add2 4 -3 12 9
> 22
> 
> add2 4 -3 twelve nine
> twelve
> nine
> 2.Now add an "if" structure right after the "echo ... | grep ...", testing the
> exit status of the "grep".  If the "grep" was successful, then the appropriate
> message should be displayed, and the script terminated.  Make sure the script
> acts appropriately, for example:
> 
> add2 4 -3 12 9
> 22
> 
> add2 4 -3 twelve nine
> twelve
> Sorry, 'twelve' is not a number
> 
> 3.Finally, redirect the output of the grep to "/dev/null".  Make sure the
> script acts correctly, for example:
> 
> add2 4 -3 12 9
> 22
> 
> add2 4 -3 twelve nine
> Sorry, 'twelve' is not a number

Yeah, I know teachers like these! I'm sorry about that, and about my
response :)

Ok, so you hve to do a script that adds numbers, and it should
complain if these are not really numbers. Then, it asks you to use
something that we consider "bad practice" (the, '"if" structure right
after the ...' part, that's wrong, but, ok, we'll do it as the
teacher says).

First step: 

address@hidden ~/t % ./step1 9 -2 4
11
address@hidden ~/t % ./step1 9 -2 four
four
7
address@hidden ~/t % cat step1
#!/bin/bash

sum=0
for number in "$@"; do
  echo "$number" | grep '[^0-9+-]'

  sum=$((sum + number))
done

echo "$sum"



Then, teacher asks us to 'test' if grep was successful:

address@hidden ~/t % ./step2 9 -2 4
11
address@hidden ~/t % ./step2 9 -2 four
four
Sorry, 'four' is not a number
address@hidden ~/t % cat ./step2
#!/bin/bash

sum=0
for number in "$@"; do
  echo "$number" | grep '[^0-9+-]'
  if [ $? -eq 0 ]; then
    echo "Sorry, '$number' is not a number"
    exit 1
  fi

  sum=$((sum + number))
done

echo "$sum"



Then, as a last step, it asks us to 'mute' the output of grep:

address@hidden ~/t % ./step3 9 -2 4
11
address@hidden ~/t % ./step3 9 -2 four
Sorry, 'four' is not a number
address@hidden ~/t % cat ./step3
#!/bin/bash

sum=0
for number in "$@"; do
  echo "$number" | grep '[^0-9+-]' >/dev/null
  if [ $? -eq 0 ]; then
    echo "Sorry, '$number' is not a number"
    exit 1
  fi

  sum=$((sum + number))
done

echo "$sum"



Now, this is what the teacher *asked for*. But, I don't like it, I
would've done it like this:

address@hidden ~/t % ./correct 9 -2 4
11
address@hidden ~/t % ./correct 9 -2 four
Sorry, 'four' is not a number
address@hidden ~/t % cat correct
#!/bin/bash

shopt -s extglob

sum=0
for number in "$@"; do
  if [[ $number != ?([+-])+([[:digit:]]) ]]; then
    echo "Sorry, '$number' is not a number"
    exit 1
  fi

  sum=$((sum + number))
done

echo "$sum"



I don't want to make this explanation long, so I'll just give you
some links you can read on why I said that:

- http://mywiki.wooledge.org/BashGuide/TestsAndConditionals#Conditional_Blocks
- http://wiki.bash-hackers.org/syntax/ccmd/if_clause
- http://mywiki.wooledge.org/glob#extglob 

That should explain why:
- avoid using grep when you can do it inside the shell
- avoid doing: command; if [ $? -eq 0 ]; then.... when you can test
  directly: if command; then ...

So, that's it. If you have questions on that, please ask.