21.3 Unique Names for Temporary Files

All users share /tmp ( 21.2 ) , so you should make unique filenames there. The best way to do this is by putting $$ in the filename. For example:

% 

vi /tmp/jerry.$$


"/tmp/jerry.12345" [New file]
% 

lpr /tmp/jerry.$$


% 

rm /tmp/jerry.$$

The shell replaces $$ with the shell's PID number ( 38.3 ) (in this case, 12345 ).

If you use a subshell ( 38.4 ) , or have more than one login session or window, and want to share the same temp file, $$ won't work for you. In that case, just pick a unique name. You could use today's date instead.

To give yourself both options with a minimum of work, here are lines for your shell setup files ( 2.2 ) . The left column has lines for csh -like shells, and the right is for sh -like shells.

`...`
 

[n]
 

.cshrc:
		
.profile
:

set tf=/tmp/jp$$	tf=/tmp/jp$$


.login:
		export TF
set date = (`date`)	set `date`
setenv TF /tmp/jp$date[4]	TF=/tmp/jp$4

(The last two lines grab the fourth word - the current time - from the output of the date ( 51.10 ) command.) When I want a temporary file in my current shell, I type:

*
 

% 

grep foo bar > $tf-1


% 

grep wheeze bar > $tf-2


% 

more $tf-*

The shell expands the shell variable ( 6.8 ) $tf-1 into a filename like /tmp/jp2345-1 , and $tf-* expands into all my temporary files in this shell. Usually, that's great. But if I go to a subshell, do a shell escape, and so on, the temporary files I make with $tf won't be the same as the ones I make in my login shell because the PIDs are different. If I need them to be the same, I use $TF , the environment variable ( 6.1 ) . It's set to the time I logged in. And because environment variables are passed to child shells, the name (like /tmp/jp09:34:56 ) will be the same in subshells:

[..]
 

% 

someprog > $TF-1


...
% 

otherprog > $TF-6


% 

sh


$ 

head $TF-[16]

If I'll be using a file for more than a minute or two, I might forget what's in which file. So I leave myself a note in shell variables named xf n and environment variables named XF n -where "xf" means "explain file" and n is 1 , 2 , etc. to correspond to the variable. If I don't remember which have what, I get a list by piping the output of set (for shell variables) or printenv or env ( 6.1 ) (for environment variables) through grep . For example:

% 

sort -t: +2 $tf-2 > $tf-3


% 

set xf3='sorted list of chapter 21 files'


   
...later...

% 

set | grep xf


xf1     sorted list of chapter 20 files
xf3     sorted list of chapter 21 files
% 

lpr $tf-3

nonomatch
 

-d
 
|&
 





$<
 
=~
 

# CLEAN FILES (IF ANY) OUT OF /tmp:
set nonomatch
set tmpf="\`ls -d $tf-* $TF-* |& grep -v ' not found'\`"
if ( "$tmpf" =~ ?* ) then
    echo; echo "Your files in /tmp:"
    ls -d $tmpf
    echo -n "'rm -rf' them? [ny](n) "
    if ( "$<" =~ y* ) rm -rf $tmpf
endif

If I made any temporary files from my login shell or any subshells, I get this message when I log out:

% 

logout



Your files in /tmp:
/tmp/jp2345-1   /tmp/jp2345-2  /tmp/jp2748-1  /tmp/09:23:45-1
'rm -rf' them? 

y

Another way to do this is with a script like del ( 23.6 ) .