Bash – Arithmetic syntax in bash

arithmeticbashshell-script

I am getting the below error on arithmetic values

#!/bin/bash
n=0
line_count=9
line_count=$(line_count)/3
echo $line_count
exit 0

expected result is 3

[]$ ./test.sh
./test.sh: line 4: line_count: command not found
/3
[]$ more test.sh

Best Answer

To complement @Kusalananda's answer, in addition to the standard sh syntax:

line_count=$((line_count / 3))

In bash you can also use these syntaxes inherited from ksh (also work in zsh):

```
((line_count = line_count / 3))
```
```
((line_count /= 3))
```
```
let line_count/=3
```

typeset -i line_count; line_count=line_count/3

bash (and zsh) also support:

```
line_count=$[line_count/3]
```

For old pre-POSIX Bourne/Almquist sh:

line_count=`expr "$line_count" / 3`

Related Solutions

Bash – How to make bash abort the execution of a script on syntax error

Wrapping the whole into a function seems to do the trick:

#!/bin/bash -e

main () {
readonly a=(1 2)
    # A syntax error is here:
    if (( "${a[#]}" == 2 )); then
        echo ok
    else
        echo not ok
    fi
    echo status $?
    echo 'Bad: has not aborted execution on syntax error!'
}

main "$@"

Result:

$ ./sh-on-syntax-err 
$ ./sh-on-syntax-err line 6: #: syntax error: operand expected (error token is "#")
$

Though I have no clue why - maybe someone else can explain?

Bash – Why does arithmetic expansion work in Bash but not in Zsh

Try doing unsetopt equals and try again. Or put quotes on the first equals character.

If you use [ for conditions, all arguments are evaluated normally and = at the start of a word is a useful zsh shorthand to get the location of a command. e.g. =ls expands to /bin/ls. Given == zsh looks for an = command and doesn't find one.

Zsh in posix mode (sh emulation) will be a lot more compatible with bash. Many things are compatible and most learnt knowledge of bash is still useful but there are certain things to be aware of if you want to write portable scripts.

Unlike [ which is a builtin, [[ is a reserved word understood by the parser and will give you fewer issues. However in this case you're using arithmetic mode so you simplify it:

    (( RANDOM % 6 == 0 )) && echo "Boom" || echo "Click"

In fact the comparison to zero is even superfluous because, the default return status reflects whether the value is zero, though the sense is reversed:

    (( RANDOM % 6 )) && echo "Click" || echo "Boom"

Best Answer

Related Solutions

Bash – How to make bash abort the execution of a script on syntax error

Bash – Why does arithmetic expansion work in Bash but not in Zsh

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