Shell脚本变量替换

Question

I am puzzled about the variable substitution in shell scripting. Here is the deal: I have following script.

if [ -d ~someone/Desktop ]
then
    echo exist
fi

which would determine whether user "someone" has "Desktop" directory under his home directory. However, if I substitute the someone by other variable, it will not be correct. See below,

var=someone
if [ -d ~${var}/Desktop ]
then
    echo exist
fi

Although the user "someone" has Desktop directory, it will not print exist in the output. Can someone tell me why this happened?

Answer 1

var=someone
if [ -d $(eval echo ~${var})/Desktop ]
then
    echo exist
fi

Answer 2

~user is a special expression that is interpreted by the shell (for explanation, see man bash -> Tilde Expansion). In your case the tilde isn't followed by a user name, so normal variable expansion takes place, ~$var expands to a literal ~someone , not to the usual /home/of/someone .

The quickest way of getting the user's home directory would be to grep it from /etc/passwd :

grep "^$var:" /etc/passwd | cut -d: -f6