如何在Unix上创建一个驻留在文件中的Shell脚本？

Question

I'm trying to create a shell script called sv that will prepend to a file, but the solution I'm using seems to only be good for one use, and them the temporary file is deleted. Is there a way I can make a shell script that will be go to use over and over?

Here's the questions: "Suppose we wish to maintain a list of all the dates when we logged on to our UNIX system. It would be easy to do this by adding the following to the .login file: date >> logdates Unfortunately, the latest date comes at the end of file logdates. I want it at the front; that is, the file should contain login dates from latest to earliest. Write a C shell script sv that will be used as follows: date | sv logdates (This way, the script is quite general, and I can use it for other cases when I want to add things to the front of a file.)"

Here's the script I've come up with:

"#!/bin/sh
cat - logdates  /tmp/out && mv /tmp/out logdates"

This will work once, when I try again the system tells me that /tmp/out doesn't exist.

Does anyone have any suggestions?

Thank you!

Answer 1

Using sponge utility :

#!/bin/sh
cat - "$1" | sponge "$1"

Answer 2

Simply use tac ( reverse of cat ) to output a file in reverse

#!/bin/sh
tac logdates > /tmp/out && mv /tmp/out logdates

Answer 3

Your shell script sv could contain the following, where newline is your data read from the | and $1 is the filename passed to sv :

#!/bin/sh
read newline
(echo "$newline"; cat $1) > tmp; mv tmp $1

And then you could use it like:

$ date > logdates
$ date >> logdates
$ cat logdates
Tue Mar 11 22:14:34 CDT 2014
Tue Mar 11 22:14:37 CDT 2014
$ date | ./sv logdates
$ cat logdates
Tue Mar 11 22:14:50 CDT 2014
Tue Mar 11 22:14:34 CDT 2014
Tue Mar 11 22:14:37 CDT 2014

This will only work for one-line appends though, as read is terminated by the newline (/n) character.