Bash Scripting – Add or Subtract Two Numbers

bashscriptingvariable

I can read the numbers and operation in with:

echo "First number please"
read num1
echo "Second number please"
read num2
echo "Operation?"
read op

but then all my attempts to add the numbers fail:

case "$op" in
  "+")
    echo num1+num2;;
  "-")
    echo `num1-num2`;;
esac

Run:

First number please
1
Second mumber please
2
Operation?
+

Output:

num1+num2

…or…

echo $num1+$num2;;

# results in: 1+2

…or…

echo `$num1`+`$num2`;;

# results in: ...line 9: 1: command not found

Seems like I'm getting strings still perhaps when I try add add ("2+2" instead of "4").

Best Answer

Arithmetic in POSIX shells is done with $ and double parentheses (( )):

echo "$(($num1+$num2))"

You can assign from that (sans echo):

num1="$(($num1+$num2))"

There is also expr:

expr $num1 + $num2

In scripting $(()) is preferable since it avoids a fork/execute for the expr command.

Related Solutions

Bash – Replace multiple strings in a single pass

OK, a general solution. The following bash function requires 2k arguments; each pair consists of a placeholder and a replacement. It's up to you to quote the strings appropriately to pass them into the function. If the number of arguments is odd, an implicit empty argument will be added, which will effectively delete occurrences of the last placeholder.

Neither placeholders nor replacements may contain NUL characters, but you may use standard C \-escapes such as \0 if you need NULs (and consequently you are required to write \\ if you want a \).

It requires the standard build tools which should be present on a posix-like system (lex and cc).

replaceholder() {
  local dir=$(mktemp -d)
  ( cd "$dir"
    { printf %s\\n "%option 8bit noyywrap nounput" "%%"
      printf '"%s" {fputs("%s", yyout);}\n' "${@//\"/\\\"}"
      printf %s\\n "%%" "int main(int argc, char** argv) { return yylex(); }"
    } | lex && cc lex.yy.c
  ) && "$dir"/a.out
  rm -fR "$dir"
}

We assume that \ is already escaped if necessary in the arguments but we need to escape double quotes, if present. That's what the second argument to the second printf does. Since the lex default action is ECHO, we don't need to worry about it.

Example run (with timings for the skeptical; it's just a cheap-o commodity laptop):

$ time echo AB | replaceholder A B B A
BA

real    0m0.128s
user    0m0.106s
sys     0m0.042s
$ time printf %s\\n AB{0000..9999} | replaceholder A B B A > /dev/null

real    0m0.118s
user    0m0.117s
sys     0m0.043s

For larger inputs it might be useful to provide an optimization flag to cc, and for current Posix compatibility, it would be better to use c99. An even more ambitious implementation might try to cache the generated executables instead of generating them each time, but they're not exactly expensive to generate.

Edit

If you have tcc, you can avoid the hassle of creating a temporary directory, and enjoy the faster compile time which will help on normal sized inputs:

treplaceholder () { 
  tcc -run <(
  {
    printf %s\\n "%option 8bit noyywrap nounput" "%%"
    printf '"%s" {fputs("%s", yyout);}\n' "${@//\"/\\\"}"
    printf %s\\n "%%" "int main(int argc, char** argv) { return yylex(); }"
  } | lex -t)
}

$ time printf %s\\n AB{0000..9999} | treplaceholder A B B A > /dev/null

real    0m0.039s
user    0m0.041s
sys     0m0.031s

Bash – Command line method to find repeat-word typos, with line numbers

Edited: added install and demo

You need to take care of at least some edge cases, like

repeated words at the end (and beginning) of the line.
search should be case insensitive, because of frequent errors like The the apple.
probably you want to restrict search only to word constituent to not match something like ( ( a + b) + c ) (repeated opening parentheses.
only full words should match to eliminate the thesis
When it comes to human language Unicode characters inside words should properly interpreted

All in all I recommend pcregrep solution:

pcregrep -Min --color=auto '\b([^[:space:]]+)[[:space:]]+\1\b' file

Obviously color and line number (n option) is optional, but usually nice to have.

Install

On Debian-based distributions you can install via:

$ sudo apt-get install pcregrep

Example

Run the command on jefferson_typo.txt to see:

$ pcregrep -Min --color=auto '\b([^[:space:]]+)[[:space:]]+\1\b' jefferson_typo.txt
1:He has has refused his Assent to Laws, the most wholesome and necessary
3:He has forbidden his Governors to pass Laws of immediate and
and pressing importance, unless suspended in their operation till his
5:Assent should be be obtained; and when so suspended, he has utterly

The above is just a text capture, but on a color-supported terminal, matches are colorized:

has has
and
and
be be