Explain me 2 lines of this shell script

问题

What does this mean?

if [ -f $2/$1 ]

and this line:

cp $1 $2/$1

Does $2/$1 represent a file, because it's associated with -f?

#!/bin/bash
if test $# -ne 2 
then
  echo "Numbers of argument invalid"
else
  if [ -f $2/$1 ]
  then
    echo "file already exist , replace ?"
    read return
    if test $return = 'y'
    then
      cp $1 $2/$1
    else
      exit
    fi

  else
    cp $1 $2/$1
  fi
fi

回答1:

./script.sh "tmp.txt" "/project"
echo $1 = tmp.txt
echo $2 = /project
$1 - file name
$2 - directory
if [ -f $2/$1 ] - checks if file exists, -f for files, -d for directory
cp $1 $2/$1 - copy file to directory/file_name
if [ -f /project/tmp.txt ]
cp tmp.txt /project/tmp.txt

回答2:

Given 2 arguments / names ($2 and $1), this is testing to ensure that a file under directory $2 with name $1 exists. If so, then it copies a local file indicated by $1 into directory $2 with the same name.

However, per your following example, it is put to use a little differently. If the file doesn't already exist in the destination, it immediately copies a file into a subdirectory with the name specified by $2. If the file does exist in the destination, it first prompts the user if it is o.k. to overwrite the existing file.

http://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html

来源：https://stackoverflow.com/questions/33331858/explain-me-2-lines-of-this-shell-script