Could you explain the syntax of math in a bash shell?

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for i in {1..99}
do
  if ([ $((i % 2)) -eq 1 ])
  then
      echo $i
  fi
done

I'm learning bash, and I'm trying to better understand line 3. Why does $((i % 2)) have to be double wrapped in parenthesis, and why can't I put the $ symbole inside next to the i like:

([ (($i % 2)) -eq 1 ])

or

([ ($(i % 2)) -eq 1 ])

?

Everything inside $((...)) is treated as an arithmetic expression. You can use parameter expansion inside an arithmetic expression, but a bare string is interpreted as a variable whose (integer) value is used. You can write

if [ $(( i % 2 )) -eq 1 ]

to check if i is odd. You can also check for equality inside the expression, as $(( x == y )) evaluates to 1 if x == y and 0 otherwise, but you would still have to compare that value to something.

In bash, you can use the arithmetic command, which has an exit status of 0 if the resulting value is non-zero, and 1 otherwise. This lets you write

if (( i % 2 == 1 )); then

Math Arithmetic: How To Do Calculation in Bash?, How to solve the syntax error: invalid arithmetic operator? Before we get into the details on how to do Math in Bash, remember that an integer is a whole number that is not a What are the Bash Arithmetic Operators? In this article, we will be discussing about variables in bash shell scripting. This article is part of the tutorial series about "bash shell scripting". If you are a beginner, I would then recommend reading the below two articles before proceeding further with this one. Read: An Introduction To Bash Shell Scripting

$(( expression )) is the syntax for evaluating an arithmetic expression, and replacing this syntax with the result of that expression. It's documented in the Bash Manual here;

The syntax of arithmetic expressions is described here. Putting $ before variable names is optional, so you can also write it as $(($i % 2)).

You have to wrap it in two parentheses because $(...) already has a meaning, it's used for command substitution: $(some command) executes some command and is then replaced with the output of the command.

You don't need parentheses around [ ... ]. The normal way to write your if statement would be

if [ $((i % 2)) -eq 1 ]

You can also write it as

if (( i % 2 == 1 ))

(( expression )) evaluatees the arithmetic expression, and then sets its exit status depending on whether the result is zero or non-zero.

Arithmetic, When we do this we don't need the $ sign preceding the brackets. Line 19 - This is a slightly different form of the previous example. Here the value of the variable b� You can perform math operations on Bash shell variables. The bash shell has built-in arithmetic option. The bash shell has built-in arithmetic option. You can also use external command such as expr and bc calculator .

Since you specify bash, simplest is

for i in {1..99}
do  if ((i % 2)) 
    then echo $i
    fi
done

the ((i % 2)) will return i mod 2, which will always be zero or one. This particular construct behaves like a C-style boolean, so zero is false and anything else is true (the opposite behavior from [[ ... ]] which uses return code of zero to mean true/ok).

How to do math on the Linux command line, How to do math on the Linux command line Let's look at some very useful commands and syntax for command line math. Notice that you have to use a \ character in front of * to keep the shell from interpreting the asterisk� This explains both of the bash for loop methods, and provides 12 different examples on how to use the bash for loop in your shell scripts. Bookmark this article for future reference, as this is the only article you would ever need to refer on how to use bash for loops with examples.

You can also use expr:

for i in {1..99}
do
  num=`expr i % 2`
  if (( num == 1 ))
  then
      echo $i
  fi
done

How to do Basic Math in Linux Command Line, In this section, we will describe the syntax for performing the above mentioned calculations and also present how you can use the expr� 3 Basic Shell Features. Bash is an acronym for ‘Bourne-Again SHell’.The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, The rules for evaluation and quoting are taken from the POSIX specification for the ‘standard’ Unix shell.

Math Commands, Bash can't handle floating point calculations, and it lacks operators for certain important mathematical functions. It has a syntax vaguely resembling C. 3) If you are really ambitious, #+ expand this script to print complete amortization tables. Bash Variable. Bash Variable in bash shell scripting is a memory location that is used to contain a number, a character, a string, an array of strings, etc.. Some important points to remember about variables in bash scripting

5.9. Math in Shell Scripts — Introduction to Unix Study Guide, So what do you get by declaring a variable to be an integer? The following example illustrates that a declared integer is not treated as a string. $ n=6/3 $ echo $n 6� Thanks. Backtick is old shell syntax. BASH supports new $(command) syntax for command substitution. Also since BASH support arithmetic operations in $(( )) it is better to not to use an external utility expr – anubhava Feb 15 '14 at 10:53

Perform arithmetic operations - Linux Shell Scripting Tutorial, You can perform math operations on Bash shell variables. The bash shell has built-in arithmetic option. You can also use external command such as expr and�

Comments
  • Single parentheses are used to execute commands in a subshell.
  • Using if ([ ... ]) is very odd. The outer parentheses are not needed.
  • As a general rule, shell syntax is extremely context-dependent. The syntax you can use inside $(( )) (and a few other other "arithmetic" contexts) is very different from what you can use elsewhere.
  • As Barmar said single parentheses are used to execute commands in a subshell and usefull to store output of commands in vars like so var=$(ls /home) And if you want to do some math you should use double $((a+b)) or square $[a+b] You can actualy put $ inside but it's redundant here. Test statements after if should be in single or double(better) square parentheses.
  • So to directly answer the question, you can't use (($i % 2)) or ($(i % 2)) to do math because neither of them are on the form $((...)).
  • Well, you can use (( $i % 2 )), but it's a command, not an expression. The two are just used in different contexts.
  • @thatotherguy For example, ((++i)) to increment i works fine.
  • Not in the context where OP is using it
  • You can even write ((i%2))||printf '%d is even\n' "$i"
  • You could also write it as (( i % 2 == 1 )) && echo $i if you wanted to do away with the if altogether...
  • Yes, but I'm a big believer in if, so I rarely mention that myself.
  • I write makefiles, where if's are a pain, but I can see your point.