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Rules with using double parentheses in Bash

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# 1  
Old 02-09-2011
Rules with using double parentheses in Bash
Hi.could you explain me what are the rules when we are using double parentesis in if statement,if I put [ $# > 0 ],the code is working ,with (( is not

Code:
 
#!/bin/bash
if (($# > 0))
then
 if ((! -d "$1"))  
then
 echo "Directory $1 not found"
fi
else
 echo Problem
fi

# 2  
Old 02-09-2011
This is probably what you're looking for:

Test Constructs
# 3  
Old 02-09-2011
Quote:
Originally Posted by lio123
Hi.could you explain me what are the rules when we are using double parentesis in if statement,if I put [ $# > 0 ],the code is working

That doesn't do what you think it does. (I made the same mistake when I first started shell scripting.)

The greater-than symbol, >, is a redirection operator, not a comparison operator (unless it is escaped, in which case it is string comparison, not numerical). What you need is:
Code:
[ $# -gt 0 ]

(I never use the non-standard (( ... )) syntax.)
This User Gave Thanks to cfajohnson For This Post:
lio123
# 4  
Old 02-09-2011
Some ideas:

Code:
#!/bin/bash
# scriptname : A script to do something with a directory
#
# Parameter validation
if [ ! $# -eq 1 ]
then
       echo "Usage: scriptname directory"
       exit 1
fi
# Save supplied parameter
mydir="$1"
# Does the directory exist?
if [ ! -d "${mydir}" ]
then
        echo "Directory $1 not found"
        exit 1
fi
# Processing continues here

# 5  
Old 02-09-2011
Quote:
Originally Posted by methyl
Some ideas:

Code:
#!/bin/bash
# scriptname : A script to do something with a directory
#
# Parameter validation
if [ ! $# -eq 1 ]


Code:
if [ $# -ne 1 ]

Quote:
Code:
then
       echo "Usage: scriptname directory"
       exit 1
fi
# Save supplied parameter
mydir="$1"


Code:
mydir=$1

Quote:
Code:
# Does the directory exist?
if [ ! -d "${mydir}" ]


Code:
if [ ! -d "$mydir" ]

Quote:
Code:
then
        echo "Directory $1 not found"
        exit 1
fi
# Processing continues here

# 6  
Old 02-09-2011
@cfajohnson
I only agree with the simplification of the "not" condition in the first command you post. The rest is pedantic and naive and reduces the resiliance of the script to duff parameters and similar environment variable names.

If I thought that there were lots of parameters I would have posted:
Code:
mydir="${1}"

When you get into 5,000 line Shell scripts you take all precautions.
Last edited by methyl; 02-09-2011 at 07:03 PM..
# 7  
Old 02-09-2011
Quote:
Originally Posted by methyl
@cfajohnson
I only agree with the simplification of the "not" condition in the first command you post. The rest is pedantic and naive and reduces the resiliance of the script to duff parameters and similar environment variable names.

If you write your scripts correctly, you don't need that "protection".
Quote:
If I thought that there were lots of parameters I would have posted:
Code:
mydir="${1}"


That is the same as:
Code:
mydir=$1

The braces and quotes are unnecessary in any circumstances.

If you don't know what is required and what is not, how do you know what difference extra "precautions" will make?
Last edited by cfajohnson; 02-09-2011 at 07:57 PM..
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