How the Shell determines whether the file exists

method One:

Locate_library () {[!] "$1*" = "' echo $1* '"]; }

Usage:

# Look for PAM (do weirdly due to distribution bugs (e.g. Debian) or the
# Crypt library.
If Find_func Pam_start SYSDEPUTIL.O; Then
  locate_library/lib/libpam.so.0 && echo "/lib/libpam.so.0";
  Locate_library/usr/lib/libpam.so && echo "-lpam";
  Locate_library/usr/lib64/libpam.so && echo "-lpam";
  # HP-UX ends shared libraries with. SL
  locate_library/usr/lib/libpam.sl && echo "-lpam";
  # AIX ends shared libraries with. A
  locate_library/usr/lib/libpam.a && echo "-lpam";
else
  locate_library/lib/libcrypt.so && echo "-lcrypt";
  Locate_library/usr/lib/libcrypt.so && echo "-lcrypt";
  Locate_library/usr/lib64/libcrypt.so && echo "-lcrypt";
Fi

Method Two:

#!/bin/sh  

mypath= "/var/log/httpd/"  
myfile= "/var/log/httpd/access.log"  

#这里的-x parameter judgment $  MyPath exists and has executable permissions if  
[!-x $myPath]; then  
mkdir "$myPath"  
fi  

#这里的-D parameters to determine if $mypath exists if  
[!-D "$myPath"]; Then  
mkdir "$myPath"  
fi  

#这里的-F parameter to determine if there is an  
if [!-F "$myFile"]; then touch  
"$myFile"  
Fi  

#其他参数还有-n,-n is to determine whether a variable has a value of if  
[!-n "$myVar"]; then  
echo "$myVar is empty"  
exit 0  
fi
  
    #两个变量判断是否相等  
if ["$var 1" = "$var 2"]; then  
echo ' $var 1 eq $var 2 '  
else  
Echo ' $var 1 don't eq $var 2 ' 
   fi  
  

The identification and meaning of various judgments are as follows:

-A file exists.
-B file exists and is a block special file.
-c file exists and is a character special file.
-D file exists and is a directory.
-e file exists (just the same as-a).
-F file exists and is a regular file.
-G file exists and has its setgid (2) bit set.
-G file exists and has the same group ID as this process.
-K file exists and has its sticky bit set.
-l file exists and is a symbolic link.
-N string length is not zero.
-o Named option is set on.
-O file exists and is owned by the user ID of this process.
-P file exists and is a-a-a-in-a-a-i-Special file or
Named pipe.
-R file exists and is readable by the current process.
-S file exists and has a size greater than zero.
-s file exists and is a socket.
-T file descriptor number Fildes is open and associated with a
Terminal device.
-U file exists and has its setuid (2) bit set.
-W file exists and is writable by the current process.
-X file exists and is executable by the current process.
-Z string length is zero.