bash - check if file exists, then perform action if not

marchelloUA · 01-23-2017, 07:54 AM

Hi all,
I use command below

Code:

if [ -f /mnt/wd/file.txt ]; then echo 'file exists'; else echo 'file does not exist'; fi

and it does work fine except those cases when there is input/output error when reading from usb drive. I tried to catch that i/o error using some construction like

Code:

if ls /mnt/wd/ | grep -i "error" = error ; then echo 'there is i/o error'; else echo 'there is no i/o error'; fi

but need your help to figure out because of my poor knowledge.
Please advise.

pan64 · 01-23-2017, 08:04 AM

there is a stdout and a stderr. stdout is standard out, all the normal messages sent to it, stderr is standard error, all the error messages are sent to here. https://en.wikipedia.org/wiki/Standa...t_.28stdout.29
if you want to grep on stderr you need to redirect it:
ls /mnt/wd 2>&1 | grep -i error
also if you want to check if stderr contains the word error:
if ls /mnt/wd 2>&1 | grep -i error; then ....
do not need == error.

grail · 01-23-2017, 08:06 AM

As usual, ls is a poor command to utilise for this type of thing. Why not use the mount command to test if the drive is mounted or not and then continue with file testing if that is the case.

szboardstretcher · 01-23-2017, 08:11 AM

I like using the proc directory.

Code:

if grep -q '/mnt/wd' /proc/mounts; then
 if [ -f /mnt/wd/file.txt ]; then echo 'file exists'; else echo 'file does not exist'; fi
else
 echo "Not Checking -- /mnt/wd not mounted."
fi

marchelloUA · 01-23-2017, 10:29 AM

Something is missing probably.

I just got

Code:

$ ls /mnt/wd
ls: reading folder /mnt/wd: Input/output error

But when I run

Code:

if grep -q '/mnt/wd' /proc/mounts; then if [ -f /mnt/wd/file.txt ]; then echo 'file exists'; else mount -U 84B46F79B46F6C9C /mnt/wd; fi else mount -U 84B46F79B46F6C9C /mnt/wd; fi

it says "file exists".

Please help to correct the command. I'd like it to perform mount -U 84B46F79B46F6C9C /mnt/wd instead.
Thx ahead.

grail · 01-23-2017, 11:16 AM

Sheesh, nested if's on a single line ... are you trying to make it hard to read or you just like the challenge??

I am not really following what you are trying to do? You perform the mount command whether it is mounted or not and also if the file does not exist. Why would you need to remount something
if a file does not exist??

It is starting to read like xy problem, ie. you are telling us something you want to do when really there is a bigger picture you are not telling us about and what you really want is something else.

Maybe ignore the code for a moment and explain what it is you are trying to achieve?

michaelk · 01-23-2017, 11:18 AM

It appears that you have an underlying filesystem problem. What filesystem is on the USB drive and is it a mechanical or flash/SSD drive?

marchelloUA · 01-23-2017, 12:24 PM

>>> grail
>>> Sheesh, nested if's on a single line ... are you trying to make it hard to read or you just like the challenge??

It will go to crontab, that's why it is on a single line. But you're right, it can be in script and readable.

>>> I am not really following what you are trying to do? You perform the mount command whether it is mounted or not and also if the file does not exist. Why would you need to remount something
if a file does not exist??

Well, I created that file for testing purpose. In my understanding, it should not exist if my drive is not mounted..

>>> It is starting to read like xy problem, ie. you are telling us something you want to do when really there is a bigger picture you are not telling us about and what you really want is something else.

I just need to ensure my drive is mounted and test file exists, otherwise I run mount command.

>>> Maybe ignore the code for a moment and explain what it is you are trying to achieve?

Good thing, I just explained it above and hope it is ok now. Sorry if it's still not.

>>> michaelk
>>> It appears that you have an underlying filesystem problem. What filesystem is on the USB drive and is it a mechanical or flash/SSD drive?

It is ntfs, mechanical USB western digital drive.

michaelk · 01-23-2017, 12:41 PM

Do you also connect it to Windows and if so does it work?
Look at the output of the dmesg command and see what errors are associated with the drive if any.

marchelloUA · 01-23-2017, 12:54 PM

>>> michaelk
>>> Do you also connect it to Windows and if so does it work?

I do not connect it to Windows and can't check it because I got no Windows near by.

>>> Look at the output of the dmesg command and see what errors are associated with the drive if any.

Will do and will update, thx.

Habitual · 01-23-2017, 01:17 PM

Quote:

Originally Posted by marchelloUA

it says "file exists".

Please help to correct the command. I'd like it to perform mount -U 84B46F79B46F6C9C /mnt/wd instead.
Thx ahead.

Code:

if $(grep -q /dev/sdc1 /proc/mounts); then something; else something mount -U 84B46F79B46F6C9C /mnt/wd; fi

See also https://help.ubuntu.com/community/UsingUUID
http://www.linuxquestions.org/questi...8/#post3066998
https://duckduckgo.com/?q=test+for+m...estions&ia=web

michaelk · 01-23-2017, 01:21 PM

If nothing on the drive is of any importance and you do not connect it to Windows then I would reformat it using an ext4 filesystem assuming the drive is still functional. If there is then we need to look into trying to recover your files.

marchelloUA · 01-23-2017, 04:15 PM

>>> Habitual

Quote:

$ ls /mnt/wd
ls: reading folder /mnt/wd: Input/output error

Quote:

$ if $(grep -q /dev/sdc1 /proc/mounts); then echo 'ok'; else mount -U 84B46F79B46F6C9C /mnt/wd; fi
ok

I'm afraid it doesn't work.. Just tested two commands below one by one, first shows that /mnt/wd is not mounted and the second should mount my device, but returns "ok".
How do I perform

Quote:

mount -U 84B46F79B46F6C9C /mnt/wd

if ls /mnt/wd returns ls: reading folder /mnt/wd: Input/output error ?

Habitual · 01-23-2017, 04:41 PM

Quote:

Originally Posted by marchelloUA

I'm afraid it doesn't work.. Just tested two commands below one by one, first shows that /mnt/wd is not mounted and the second should mount my device, but returns "ok".
How do I perform if ls /mnt/wd returns ls: reading folder /mnt/wd: Input/output error ?

You didn't get "ls" from me. Try again.

You want to mount this the same, every time? You will need to use the UUID parameter to do so,
https://help.ubuntu.com/community/UsingUUID explains how.
First you ID the drive.

/dev/sdc1 is my mount.
yours is "/mnt/wd" whatever that is and you haven't specified.
Do you know how to find it?

http://xyproblem.info/
and/or
https://www.linuxquestions.org/quest...llected-35954/

grail · 01-23-2017, 09:29 PM

If the grep is returning that it is mounted, assuming you are putting the correct drive name in, then maybe you could show us the output of the grep?