判断文件：

    WORKDIR=/home/tmp
    LOCAL_LIST_FILE=$WORKDIR/local.list

    #if no local.txt file, generate a new one
    if [ ! -f  "$LOCAL_LIST_FILE" ]; then
      touch $LOCAL_LIST_FILE
    fi

    注意：一定要有空格，判断文件是否存在，如果不存在，就生成一个新的

    if [ ! -d /mnt ]        # be sure the directory /mnt exists
       then
           mkdir /mnt
    fi
    
    目录。/mnt是否存在，不存在就生成目录/mnt


The tests below are test conditions provided by the shell:

    * -b file = True if the file exists and is block special file.
    * -c file = True if the file exists and is character special file.
    * -d file = True if the file exists and is a directory.
    * -e file = True if the file exists.
    * -f file = True if the file exists and is a regular file
    * -g file = True if the file exists and the set-group-id bit is set.
    * -k file = True if the files' "sticky" bit is set.
    * -L file = True if the file exists and is a symbolic link.
    * -p file = True if the file exists and is a named pipe.
    * -r file = True if the file exists and is readable.
    * -s file = True if the file exists and its size is greater than zero.
    * -s file = True if the file exists and is a socket.
    * -t fd = True if the file descriptor is opened on a terminal.
    * -u file = True if the file exists and its set-user-id bit is set.
    * -w file = True if the file exists and is writable.
    * -x file = True if the file exists and is executable.
    * -O file = True if the file exists and is owned by the effective user id.
    * -G file = True if the file exists and is owned by the effective group id.
    * file1 –nt file2 = True if file1 is newer, by modification date, than file2.
    * file1 ot file2 = True if file1 is older than file2.
    * file1 ef file2 = True if file1 and file2 have the same device and inode numbers.
    * -z string = True if the length of the string is 0.
    * -n string = True if the length of the string is non-zero.
    * string1 = string2 = True if the strings are equal.
    * string1 != string2 = True if the strings are not equal.
    * !expr = True if the expr evaluates to false.
    * expr1 –a expr2 = True if both expr1 and expr2 are true.
    * expr1 –o expr2 = True is either expr1 or expr2 is true.

Shell中的数据问题

   ArrayName=("element 1" "element 2" "element 3")  #数组定义
   echo ${#ArrayName[@]}

  echo "The number of elements in the array is ${#ArrayName[*]}"     
    
    例子
    #!/bin/bash
    # define array
    # name server names FQDN
    NAMESERVERS=("ns1.nixcraft.net." "ns2.nixcraft.net." "ns3.nixcraft.net.")
 
    # get length of an array
     tLen=${#NAMESERVERS[@]}
 
    # use for loop read all nameservers
    for (( i=0; i<${tLen}; i++ ));
    do
      echo ${NAMESERVERS[$i]}
    done

   Sample output:

     ns1.nixcraft.net.
     ns2.nixcraft.net.
     ns3.nixcraft.net.