[SOLVED] Creating a temperature conversion POSIX compliant script without "if" statements

Hello

I am trying to write a POSIX compliant shell script to convert temperatures using rofi, without "if" statements.

In the first part I prompt for scale. I want a notification to be sent in case of invalid input and a silent exit in case of empty string.
This is what I've got so far, but it's not working.

TO=$(printf "Celsius\\nFahrenheit" | rofi -dmenu -p "Convert to which scale?")
(echo "$TO" | grep -Evq '^(Celsius|Fahrenheit)$') && \
        ( ( (echo "$TO" | grep -Evq '^$') && notify-send "Please choose either Celsius or Fahrenheit" && exit 1) || exit 0)

Any one have suggestions?

Here is the full script. Notice that it will exit with error message on first promt even in case of empty string.

#!/bin/sh

# Determine what scale to convert to
TO=$(printf "Celsius\nFahrenheit" | rofi -dmenu -p "Convert to which scale?")
(echo "$TO" | grep -Evq '^(Celsius|Fahrenheit)$') && \
        (echo "$TO" | grep -Evq '^$') && notify-send "Please choose either Celsius or Fahrenheit" && exit 1

# Determine what temperature to convert from
TEMP=$(rofi -dmenu -p "Please input integer temperature")
(echo "$TEMP" | grep -Eq '(^$|[^0-9])') && notify-send "Temperature must be an integer" && exit 1

# Compute and send notification
case "$TO" in
        Fahrenheit) notify-send "$TEMP Cº is equal to $(echo "scale=2; $TEMP * 1.8 + 32" | bc) Fº";;
        Celsius) notify-send "$TEMP Fº is equal to $(echo "scale=2; ($TEMP - 32) / 1.8" | bc) Cº";;
        *) notify-send "Unexpected case. Trying to convert to $TO";;
esac

you should find out what your default /bin/sh actually is:

ls -l /bin/sh

for me it links to bash.
if i want to make sure i write simpler scripts i sometimes use

#!/bin/dash

which is "more" POSIX-compliant than bash.
AFAIK, no single shell is 100% POSIX compliant, but dash comes pretty close.

then i strongly question the requirement to not use "if" statements. why on earth?

I had already done as you suggested and determined my default /bin/sh. I am on Ubuntu Mate 19.04 and the default is set to dash, or rather, it is a link to dash.

You can think of the requirement of no "if" statements as a personal challenge and/or style preference. I achieved the objective by using "if" statements at first, however, it seems to me that I can achieve it without using them and so I want to find out how to do it.

I wouldn't use those

(echo ... | grep ... )

structures, instead, I would try test (see man test) and case/esac.
In general you need not use () (the only exception is $(command) - use {} instead, if you need.

Solved it. Here is the final script.

EDIT: Applied pan64's suggestion not to use (echo ... | grep ...)

#!/bin/sh

# Determine what scale to convert to
TO=$(printf "Celsius\nFahrenheit" | rofi -dmenu -p "Convert to which scale?")
test "$TO" = "" && exit 0
test "$TO" != "Celsius" -a "$TO" != "Fahrenheit" && notify-send "Please choose either Celsius or Fahrenheit" && exit 1

# Determine what temperature to convert from
TEMP=$(rofi -dmenu -p "Please input integer temperature")
(echo "$TEMP" | grep -Eq '(^$|[^0-9])') && notify-send "Temperature must be an integer" && exit 1

# Compute and send notification
case "$TO" in
        Fahrenheit) notify-send "$TEMP Cº is equal to $(echo "scale=2; $TEMP * 1.8 + 32" | bc) Fº";;
        Celsius) notify-send "$TEMP Fº is equal to $(echo "scale=2; ($TEMP - 32) / 1.8" | bc) Cº";;
        *) notify-send "Unexpected case. Trying to convert to $TO";;
esac

The problem had something to do with the script not exiting when encountering "exit" statements. From what I've read in other places it seems to me like it had something to do with the "exit" statements being within parentheses.

I am marking this thread as solved, but if anyone wishes to explain this last point I would appreciate it.
Also, tips that would result in simplification or improvement of speed (even though that would be hard at this point) will be appreciated.

Thank you for the help.

Originally Posted by Johnny_Metal EDIT: Applied pan64's suggestion not to use (echo ... | grep ...)

not really, I can still see ....

Originally Posted by Johnny_Metal #!/bin/sh # Determine what scale to convert to TO=$(printf "Celsius\nFahrenheit" | rofi -dmenu -p "Convert to which scale?") test "$TO" = "" && exit 0 test "$TO" != "Celsius" -a "$TO" != "Fahrenheit" && notify-send "Please choose either Celsius or Fahrenheit" && exit 1 # Determine what temperature to convert from TEMP=$(rofi -dmenu -p "Please input integer temperature") (echo "$TEMP" | grep -Eq '(^$|[^0-9])') && notify-send "Temperature must be an integer" && exit 1 # Compute and send notification case "$TO" in Fahrenheit) notify-send "$TEMP Cº is equal to $(echo "scale=2; $TEMP * 1.8 + 32" | bc) Fº";; Celsius) notify-send "$TEMP Fº is equal to $(echo "scale=2; ($TEMP - 32) / 1.8" | bc) Cº";; *) notify-send "Unexpected case. Trying to convert to $TO";; esac

Originally Posted by Johnny_Metal The problem had something to do with the script not exiting when encountering "exit" statements. From what I've read in other places it seems to me like it had something to do with the "exit" statements being within parentheses.

That is exactly happened because of ( commands ) - which actually means a new shell will be used to execute those commands. And an exit [inside] will only terminate that shell, not the one you started.

Any suggestions on how to get rid of the last (echo ... | grep ...)?

grep ^$ means empty lines, you need to find the test which will check if TEMP is empty.
grep [^0-9] will check if TEMP contains non-digit characters (although it is not really correct). https://stackoverflow.com/questions/...number-in-bash

The case (keyword) takes a glob as an expression to compare with.
A glob is anchored; a floating match must begin and end with * (any number of any characters).
The glob takes ! as a negation of a [character set]. Some shells support ^ as a synonym.
The case takes | as an OR.
The case takes (expression) or expression).

case $TEMP in (""|*[!0-9]*) notify-send ...; esac

Unlike [ ] or test, the case does no further expansions on $TEMP, so quotes can be omitted. (Actually the case behaves like the == in a non-Posix [[ ]].)
In a case the branch code must end with ;; but can be omitted before the esac. A ; or newline is required then.

I actually changed the script a little further to account for the sign (+ or -). The grep part looks like:

(echo "$TEMP" | grep -Eq '^$') && exit || (echo "$TEMP" | grep -Evq '^[+-]?[0-9]+$') && notify-send "Temperature must be an integer" && exit 1

Trying to use a case as suggested, but have not been successfull thus far...

EDIT:

Remove "+" sign if it exists. Limit minimum temperature to nearest integer values to absolute zero. Can't find a way to apply the regular expression with a "case" though...

#!/bin/sh

# Determine what scale to convert to
TO=$(printf "Celsius\\nFahrenheit" | rofi -dmenu -p "Convert to which scale?")
test "$TO" = "" || (test "$TO" != "Celsius" && test "$TO" != "Fahrenheit" && notify-send "Please choose either Celsius or Fahrenheit") && exit


# Determine what temperature to convert from
TEMP=$(rofi -dmenu -p "Please input integer temperature")
case $TEMP in [!0-9+-]*) notify-send "Temperature must be an integer" && exit 1; esac

# Remove "+" sign if it is present
oldIFS=$IFS && IFS=+ && case $TEMP in +*) set -- $TEMP && TEMP=$2; esac; IFS=$oldIFS

# Compute and send notification
case "$TO" in
        Fahrenheit) (test "$TEMP" -lt 273 && notify-send "Temperature in Celsius must be at least -273º" && exit) || notify-send -- "$TEMP Cº is equal to $(echo "scale=2; $TEMP * 1.8 + 32" | bc) Fº";;
        Celsius) (test "$TEMP" -lt 459 && notify-send "Temperature in Fahrenheit must be at least -459º") || notify-send -- "$TEMP Fº is equal to $(echo "scale=2; ($TEMP - 32) / 1.8" | bc) Cº";;
        *) notify-send "Unexpected case. Trying to convert to $TO";;
esac

This is overcomplicated.

case $TEMP in
    "" ) <do something if TEMP is empty> ;;
    *[!0-9]*) ) <do something if TEMP contains non-number chars> ;;
esac

# do something is some like this:
notify-send -- <message> && exit 1
# without any ( )

don't use () but {}.

add set -xv at the beginning of your script to see what's happening.

All that's left is to take into account the sign. Here's a few examples of how it should behave:

• 5 -> pass
• -5 -> pass
• +5 -> pass
• "" -> exit silently
• d5 -> exit with error message
• 5d -> exit with error message

yes, and now you can try to write your own test commands. or a case-esac list if you wish.

With a case-esac (glob) it is a bit tricky.
As a function

is_num(){
  case $1 in
  ("")
    return 1
  ;;
    (?*[!0-9]*|[!-+0-9]*)
    return 2
  ;;
  esac 
  return 0
}

The function sets exit status 0,1,2, that can be queried by $? or tested for 0=success like this interactive example

until
  echo "enter a number"; read var; is_num "$var"
do
  echo "not a number"
done

The until takes the exit status of the last command before the do,

Managed to finally do the check using case/esac. Also added "-no-custom" option to rofi, which cuts need for check of $TO.

#!/bin/sh

MIN_CELSIUS=-237
MIN_FAHR=-459

is_num(){
	case $1 in
		("") return 1;;
		(?*[!0-9]*|[!-+0-9]*) return 2;;
	esac
	return 0
}

# Determine what scale to convert to
TO=$(printf "Celsius\\nFahrenheit" | rofi -dmenu -no-custom -p "Convert to which scale?")

until
	TEMP=$(rofi -dmenu -p "Please input integer temperature"); is_num "$TEMP"
do
	result=$?
	test "$result" = 1 || (test "$result" = 2 && notify-send "Temperature must be an integer") && exit
done

# Remove "+" sign if it is present
oldIFS=$IFS && IFS=+ && case $TEMP in +*) set -- "$TEMP" && TEMP=$2; esac; IFS=$oldIFS

# Compute and send notification
case "$TO" in
	Fahrenheit) (test "$TEMP" -lt "$MIN_CELSIUS" && notify-send "Temperature in Celsius must be at least $MIN_CELSIUSº" && exit) || notify-send -- "$TEMP Cº is equal to $(echo "scale=2; $TEMP * 1.8 + 32" | bc) Fº";;
	Celsius) (test "$TEMP" -lt "$MIN_FAHR" && notify-send "Temperature in Fahrenheit must be at least $MIN_FAHRº") || notify-send -- "$TEMP Fº is equal to $(echo "scale=2; ($TEMP - 32) / 1.8" | bc) Cº";;
	*) notify-send "Unexpected case. Trying to convert to $TO";;
esac

There is just one more thing that I noticed. rofi is exhibiting a behaviour I do not fully understand. If I type a dash followed by any character that is not part of one of rofi's options it ignores it and accepts whatever comes after it.

For example, if I type "-i", it is recognized as invalid input and does not allow me to proceed. If I type something like "-5" it ignores it and minds only what I type after. Anyone know why?