Issue controlling script flow - linux

Question

I'm new to shell scripting, my script appears to be okay, but its the flow that I'm having an issue controlling. Could someone point out what silly mistake I've made please.
#! /bin/sh
echo "Are you sure youx want to delete $1? Answer y or n"
read ans
echo $ans
if $ans = "y"|"Y"
then
mv $1 /home/parallels/dustbin
echo "File $1 has been deleted"
else echo "File $1 has not been deleted"
fi

Answer 1

Make your if condition like this:
if [ "$ans" = "y" -o "$ans" = "Y" ]

Answer 2

There are a few things wrong with your script. Some are serious, some are less so.
First, the serious problems.
As guru suggested, you need to use square brackets to surround your if condition. This is because if only tests for the output of a condition, it doesn't perform actual string comparisons. Traditionally, a program called /bin/test, which was also called /bin/[ took care of that. These days, that functionality is built in to the shell, but /bin/sh still behaves as if it's a separate program.
In fact, you can do interesting things with if when you don't use square brackets for your condition. For example, if grep -q 'RE' /path/to/file; then is quite common. The grep -q command issues no output, but simply returns a "success" or "fail" that is detected by if.
Second serious problem is that you are echoing a status that may or may not be true. I call this a serious problem because ... well, log messages simply shouldn't make false claims. If the permissions are wrong for the file in $1, or the filename contains a space, then your mv command will fail, but the message will claim that it did not. More on this later.
Next, the less serious problems.
These are mostly style and optimization things.
First off, read on most platforms includes a -p option that lets you specify a prompt. Use this, and you don't need to include an echo command.
Second, your indenting makes it hard to see what the if construct is wrapping. This isn't a huge problem in a program this small, but as you grow, you REALLY want to follow consistent standards.
Third, you can probably get more flexibility in multiple-choice questions like this if you use case statements instead of if.
After all that, here's how I'd write this script:
#!/bin/sh
if [ "$1" = "-y" ]; then
ans=y
shift
elif [ -t 0 ]; then
read -p "Are you sure you want to delete '$1' (y/N) ? " ans
fi
case "$ans" in
Y*|y*)
retval=0
if [ -z "$1" ]; then
retval=64
echo "ERROR: you didn't specify a filename." >&2
if [ ! -f "$1" ]; then
retval=66
echo "ERROR: file '$1' not found!" >&2
elif mv "$1" /home/parallels/dustbin/; then
echo "File '$1' has been deleted" >&2
else
retval=$?
echo "ERROR: file '$1' could not be deleted!" >&2
fi
;;
*)
echo "ABORT: file '$1' has not been deleted" >&2
retval=4
;;
esac
exit $retval
Aside from what's mentioned above, here are some things in this code snippet:
[ "$1" = "-y" ] - if the user specifies a -y option, then we behave as if the question was answered with a "yes".
[ -t 0 ] - this tests whether we are on an interactive terminal. If we are, then it makes sense to ask questions with read.
Y*|y*) - in a case statement, this matches any string that begins with an upper or lower case "y". Valid affirmative responses would therefore be "Y", "yes", "yellow", etc.
[ ! -f "$1" ] - this tests whether the file exists. You can man test or man sh to see the various tests available in shell. (-f may not be the most appropriate for you.)
>&2 - at the end of a line, sends its output to "standard error" instead of "standard out". This changes how output will be handled by pipes, cron, etc. Errors and log data are often sent to stderr, so that stdout can be dedicated to a program's actual output.
mv "$1" ... - The filename is in quotes. This protects you in case the filename has special characters like spaces in it.
$retval - the values for this came from a best guess of the closest item in man sysexits.
retval=$? - this is the exit status of the most recently executed command. In this case, that means we're assigning mv's exit status to the variable $retval, so that if mv failed, the whole script reports the reason for the fail, as far as mv is concerned.

Answer 3

You can also convert the user response to either case and just check it for respective case like
read ans
ans=${ans,,} # make 'ans' lowercase, or use ${ans^^} for making it uppercase
if [ "$ans" = "y" ]
then
....
fi

Answer 4

Below is the perfect code with error handling included
#!/bin/sh
echo "Are you sure you want to delete $1? Answer y or n"
read ans
echo $ans
if [ $ans == "y" ] || [ $ans == "Y" ]
then
if [ -f $1 ]
then
mv $1 /home/parallels/dustbin
echo "File $1 has been deleted"
else
echo " File $1 is not found"
fi
else
echo "File $1 has not been deleted"
fi