Why Can't Bash Recognize The Existence of a Socket File

Why can't bash recognize the existence of a socket file

http://www.tldp.org/LDP/abs/html/fto.html

Use -S to test if its a socket. -f is for regular files.

See man 1 test:

   -e FILE
          FILE exists
   -f FILE
          FILE exists and is a regular file
   ...
   ...
   -S FILE
          FILE exists and is a socket

How do I tell if a file does not exist in Bash?

The test command (written as [ here) has a "not" logical operator, ! (exclamation mark):

if [ ! -f /tmp/foo.txt ]; then
    echo "File not found!"
fi

Shell prompt does not shows on socket

man bash:

An interactive shell is one started without non-option arguments (unless -s is specified) and without the -c option whose standard input and
error are both connected to terminals (as determined by isatty(3)), or one started with the -i option.

A non-interactive shell normally won't print the prompt at all.

Always gives false even though file exists and is not empty

It is enough to check for -s, because it says:

FILE exists and has a size greater than zero

http://unixhelp.ed.ac.uk/CGI/man-cgi?test

also your output is switched, so it outputs does not exists when a file exists, because -s will give TRUE if file exists AND has a size > 0.

So correctly you should use:

echo " enter file name "
read file
if [ -s "$file" ]
then 
   echo " file exists and is not empty "
else
   echo " file does not exist, or is empty "
fi

This will give you the expected output.

Also it should be

read file

instead of

read $file

If you want further informations, I recommand reading man test and man read

How can I check a file exists and execute a command if not?

[ -f /tmp/filename.pid ] || python daemon.py restart

-f checks if the given path exists and is a regular file (just -e checks if the path exists)

the [] perform the test and returns 0 on success, 1 otherwise

the || is a C-like or, so if the command on the left fails, execute the command on the right.

So the final statement says, if /tmp/filename.pid does NOT exist then start the daemon.

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