Avoid expansion of * in bash builtin function let

问题

I have a problem with a bash script. I have to use the operator * to multiplicate. Instead the script bugs me with expansion and using as operator the name of the script itself. I tried with single quotes but it doesn't work :( Here's the code

#!/bin/bash -x

# Bash script that calculates an arithmetic expression
# NO PRECEDENCE FOR OPERATORS
# Operators: + - * 

if [ "$#" -lt "3" ]
then 
    echo "Usage: ./calcola.scr <num> <op> <num> ..."
    exit 1
fi

result=0
op=+
j=0

for i in "$@"
do
    if [ "$j" -eq "0" ]
    then
        # first try
        #result=$(( $result $op $i )) 

        # second try
        let "result$op=$i"

        j=1
    else
        op=$i
        j=0
    fi
done

echo "Result is $result"

exit 0

回答1:

If "op" is "*", it will be expanded by the shell before your script even sees it. You need to choose something else for your multiplication operator, like "x", or force your users to escape it by putting it in single quotes or preceeding it with a backslash.

If the terms of the exercise allow it, maybe you should try using "read" to get the expression from standard input instead of getting them from the command line.

回答2:

If you don't need "* expansion" (referred as "globbing" in general) at all for your script, just start it with "-f"; you can also change it during run time:

mat@owiowi:/tmp/test$ echo *
A B
mat@owiowi:/tmp/test$ set -f
mat@owiowi:/tmp/test$ echo *
*
mat@owiowi:/tmp/test$ set +f
mat@owiowi:/tmp/test$ echo *
A B

回答3:

It works, you're just not escaping the * correctly. Try using the backslash:

$ ./calcola.scr 2 \* 3
+ '[' 3 -lt 3 ']'
+ result=0
+ op=+
+ j=0
+ for i in '"$@"'
+ '[' 0 -eq 0 ']'
+ let result+=2
+ j=1
+ for i in '"$@"'
+ '[' 1 -eq 0 ']'
+ op='*'
+ j=0
+ for i in '"$@"'
+ '[' 0 -eq 0 ']'
+ let 'result*=3'
+ j=1
+ echo 'Result is 6'
Result is 6
+ exit 0
$

Although, as Paul Tomblin mentioned, it would probably be better to use x as the multiplication operator instead.

来源：https://stackoverflow.com/questions/372807/avoid-expansion-of-in-bash-builtin-function-let

标签

bash

scripting

expansion

let