How to verify correct directory before running command

[Mike Keys]

I have a script that I am trying to make bulletproof. I have a very dangerous find/remove command that I execute from a particualr directory. This runs as root. If the directory that the script runs from does not exist, then the command runs in the current directory, which can cause damage.

I cd into a directory and I want to add something to the script that verifies that I am in the correct directory before running the find/remove command.

Any suggestions?

[James R. Ferguson · Accepted solution]

Hi Mike:

# cd /path || { echo "STOP!"; exit 1; }

...will exit your script if '/path' isn't valid.

Another safeguard in scripts is to use 'set -u' so that unset variables are treated as errors:

# cat ./thisfails
#!/usr/bin/sh
set -u
cd ${DIR}
exit 0

Regards!

...JRF...

[Patrick Wallek]

Why cd into the directory first? Why not let find do all the work for you.

Instead of this:

cd /dir
find . -type f -exec rm

Do this:

find /dir -type f -exec rm

Now if /dir does not exist find will error.

Here is an example (where the /tmp/pww1 directory does not exist):

# find /tmp/pww1 -type f -exec rm {} \+
find: cannot stat /tmp/pww1

[Mike Keys]

James,

I entered that code and have the following for my script (I am running from the /home/ops directory):

#! /sbin/sh
cd /home/ops/mike || { echo "Can't find directory"; exit 1; }
echo "You are in the $PWD directory"

After testing to make sure that if I could cd to the directory, that the command would fail, I added the echo command to make sure that I was in the correct directory after the script ran. However, after the script ran and I issued a 'll' command I was still in the /home/ops directory. The script didn't go into the directory.

Do I need to issue another cd command if the check passes?

[Mike Keys]

Patrick,

My original script looked like this (the directory is not important for this example):

#! /sbin/sh
cd /home/ops/mike
#find $PWD * -mtime +30 -exec rm {} \;

so, instead of using the variable $PWD, I should jus hardcode the directory in the find command?

[James R. Ferguson]

Hi Mike:

If you pass the 'cd' syntax I gave, you are where you should be. You can play with this:

#!/usr/bin/sh
set -u
DIR=/home
cd ${DIR} 2> /dev/null && echo "I am in $(pwd)" || { echo "Can't 'cd' to ${DIR}"; exit 1; }
echo "...continuing"

Regards!

...JRF...

[Mike Keys]

O.K. I see, the cd in the script is only valid while in the script. Once the script is done, the pwd is returned to what it was originally. For example, if I started in the /home/ops directory and executed the script that runs a command in the /home/ops/mike dir, then while the script is running it should be in the /home/ops/mike dir. Once the script is done and I do a 'pwd' I would see /home/ops.

[James R. Ferguson]

Hi Mike:

> I see, the cd in the script is only valid while in the script. Once the script is done, the pwd is returned to what it was originally.

Yes, that is true. A child process (your script) cannot alter the environment of its parent (the shell). After all, children should obey their parents :-)

Regards!

...JRF...

[James R. Ferguson]

Hi (again) Mike:

One more thing you should understand is the difference between parentheses and curly braces in a shell script:

( list ) executes in a separate environment.

{ list ; } executes in the *same* environment.

For example, consider:

# ./notewell
#!/usr/bin/sh
set -u
DIR1=/home
DIR2=/var/tmp
{ cd ${DIR1} && ls -ld . || exit 1; }
echo "I am now in $PWD"
( cd ${DIR2} && ls -ld . || exit 2 )
echo "I am STILL in $PWD"

Regards!

...JRF...

[Dennis Handly]

>Mike: the cd in the script is only valid while in the script. Once the script is done, the pwd is returned to what it was originally.

Right. If you want a script fragment to actually cd, you need to source it:
. check_directory

[Rasheed Tamton]

>so, instead of using the variable $PWD, I should jus hardcode the directory in the find command?

Yes. It is better - if you have few dirs in the script. In that case, you can use as Patrick said.


find $PWD -type f -mtime +30 -exec ls {} \+

find /home/mike -type f -mtime +30 -exec ls {} \+

replace ls with rm -i while you test the script.

rgds.