bash scripting testing for file

exvor · 08-07-2007, 07:07 PM

I am trying to make a boot script for my wireless card damon that sometimes when shutting the computer down properly doesn't remove the /var/run/damon file. Ive been reading some bash stuff but its getting rather confusing. Below is what I think you can do but im not sure.

Code:

 
if [ cat /var/run/damon ] 
then rm /var/run/damon

sirclif · 08-07-2007, 07:28 PM

Quote:

if [ cat /var/run/damon ]
then rm /var/run/damon

yea, this is a confusing issue. the if operator in bash checks the return value of the command following it. the [ ] part is actually a command. now, i'm not an expert on the history here or anything, but basically the command is called test, and [ is an abbreviation of it. bash may have a build in [ ] operator by now, but at any rate, the functionality is the same.

for the case above to work, you would need to remove the []'s

Code:

if cat /var/run/damon
then
rm /var/run/damon

this relies on the cat command returning 0 if the file you are trying to cat exists. what you probably would rather do is

Code:

if [ -e /var/run/damon ]
then
rm /var/run/damon

which will actually do the same thing as

Code:

if test -e /var/run/damon
then
rm /var/run/damon

the test command, or [ for short, is checking if the file exists, and will return 0 if it does (which is the unix "there were no errors" return code)

sirclif · 08-07-2007, 07:30 PM

and for some reason i forget the fi on all the if statments

Code:

if [ -e /var/run/damon ]
then
rm /var/run/damon
fi

exvor · 08-07-2007, 07:51 PM

Ahh ok the [] is a actually command not an enclosure bracket. That is where I was getting confused.

Thanks for the help this helps alot.

Now I just need to figure out why the deamon does not exit properly when I shut the system down.

fvu · 08-08-2007, 03:42 PM

You can also use the `&&' operator to write a concise one-liner:

# If file exists, remove it
[ -e /var/run/damon ] && rm /var/run/damon

If you want to make sure code is executed on exit, you might try setting a bash trap, see template below.
For more information see: http://fvue.nl/wiki/Bash:_Error_handling

Code:

#!/bin/bash
# Example script for handling bash errors: Exit on error and trap exit.
# This script is supposed to run in a subshell.
# See also: http://fvue.nl/wiki/Bash:_Error_handling
    
set -o errtrace           # Let shell functions inherit ERR trap.  Same as `set -E'.
set -o nounset            # Trigger error when expanding unset variables.  Same as `set -u'.
trap onexit 1 2 3 15 ERR  # Trap non-normal exit signals: 1/HUP, 2/INT, 3/QUIT, 15/TERM, ERR

# Process `exit' event
# @param $1 integer  (optional) Exit status.  If not set, use `$?' (exit status previous command)
function onexit() {
    local exit_status=${1:-$?}
    # myscript here...
    echo Exiting $0 with $exit_status
    exit $exit_status
}

# myscript here...

onexit  # Allways call `onexit' at end of script

Hope this helps,
Freddy
http://fvue.nl/wiki/Bash