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stringshellsubstitution

shell string bad substitution

I'm new to shell programming. I intend to get directory name after zip file was extracted. The print statement of it is

$test.sh helloworld.zip
helloworld

Let's take a look at test.sh:

#! /bin/sh
length=echo `expr index "$1" .zip`
a=$1    
echo $(a:0:length}

However I got the Bad substitution error from the compiler.

And when I mention about 'shell'.I just talking about shell for I don't know the difference between bash or the others.I just using Ubuntu 10.04 and using the terminal. (I am using bash.)

Solution

  • If your shell is a sufficiently recent version of bash, that parameter expansion notation should work.

    In many other shells, it will not work, and a bad substitution error is the way the shell says 'You asked for a parameter substitution but it does not make sense to me'.

    Also, given the script:

    #! /bin/sh
length=echo `expr index "$1" .zip`
a=$1    
echo $(a:0:length}

    The second line exports variable length with value echo for the command that is generated by running expr index "$1" .zip. It does not assign to length. That should be just:

    length=$(expr index "${1:?}" .zip)

    where the ${1:?} notation generates an error if $1 is not set (if the script is invoked with no arguments).

    The last line should be:

    echo ${a:0:$length}

    Note that if $1 holds filename.zip, the output of expr index $1 .zip is 2, because the letter i appears at index 2 in filename.zip. If the intention is to get the base name of the file without the .zip extension, then the classic way to do it is:

    base=$(basename $1 .zip)

    and the more modern way is:

    base=${1%.zip}

    There is a difference; if the name is /path/to/filename.zip, the classic output is filename and the modern one is /path/to/filename. You can get the classic output with:

    base=${1%.zip}
base=${base##*/}

    Or, in the classic version, you can get the path with:

    base=$(dirname $1)/$(basename $1 .zip)

    If the file names can contain spaces, you need to think about using double quotes, especially in the invocations of basename and dirname.