How to extract a string into a variable

ButterSideUp · 09-23-2012, 02:40 PM

Although i can do a lot of the things i need to do, I'm new to bash, so please bear with me.

I have a script, in which I need to extract just the the UUID for my iscsi device partition;

blkid /dev/sdb1:

UUID="5a34315a-e9e9-4d01-823b-d07b8c6195d8" TYPE="ext4"

And i want to store this in a variable so i can update the fstab file later in the script.

I realise there's probably a few ways to do this, so please feel free to offer alternatives,but i'd be grateful if you could offer an exaplanation of how each option works.

For example, one thing I'm struggling with is where the output of blkid /dev/sdb1 gets stored when it is run within my script. Is it $?

many thanks,

pixellany · 09-23-2012, 02:50 PM

The output of any command goes to "standard output"---unless you tell the system otherwise.

To store something in a variable, simply set the variable using "=". If the value to be set is from another command, then this can be captured using "$(<command>)".

Try this in a terminal:

Code:

[mherring@herring_lap ~]$ a=23
[mherring@herring_lap ~]$ echo $a
23
[mherring@herring_lap ~]$ a="the quick brown fox"
[mherring@herring_lap ~]$ echo $a
the quick brown fox
[mherring@herring_lap ~]$ b=$(echo $a)
[mherring@herring_lap ~]$ echo $b
the quick brown fox
[mherring@herring_lap ~]$

ButterSideUp · 09-23-2012, 03:00 PM

Thanks for that. I maybe didn't write my post as accurately as i should have.

My question is:

1. In a script, how do i extract just the UUID string from the blkid /dev/sdb1 command?
2. I asked about assigning the output to a variable because i don't understand where information gets stored as it is derived within the script.

many thanks

suicidaleggroll · 09-23-2012, 03:06 PM

Commands in a script work exactly the same as on the command line. Meaning if you run "blkid /dev/sdb1" in the script, output will be sent to the screen, just as it would if you ran it from the command line. As pixellany said, if you want to capture this output you'll need to send it to a variable:

Code:

output=$(blkid /dev/sdb1)

You can then use any number of commands to extract a piece of this (just the UUID).

pixellany · 09-23-2012, 03:16 PM

Quote:

Originally Posted by ButterSideUp

Thanks for that. I maybe didn't write my post as accurately as i should have.

My question is:

1. In a script, how do i extract just the UUID string from the blkid /dev/sdb1 command?
2. I asked about assigning the output to a variable because i don't understand where information gets stored as it is derived within the script.

many thanks

2. It goes to standard output!! If you don't direct it somewhere, think of it as going to the terminal screen that noone is looking at.....

1. Extract using grep:

Code:

blkid /dev/sda1 | egrep -o 'UUID[^ ]*'

many other ways to do this--eg SED. (The regex reads "UUID, followed by any number of characters that are not spaces")

ButterSideUp · 09-23-2012, 03:24 PM

Ok that's great, thanks!, but i'm still being stupid here.

I kinda understand what you are saying, but how do i access the standard output value within the script?

Do i assign the entire command to a variable, if so maybe it would be easier to just see the syntx. Sorry, i'm being dumb!!!

I tried to use "$(<command>)" as you suggested, but can't quite get it working.

pixellany · 09-23-2012, 03:40 PM

I recommend running some experiments in the terminal---before trying to make a script work.

We've seen the assignment to a variable already. Another method is called "redirection"---here's an example:

Code:

[mherring@herring_lap ~]$ x="she sells seashells by the seashore"
[mherring@herring_lap ~]$ echo $x
she sells seashells by the seashore
[mherring@herring_lap ~]$ echo $x > sea_file
[mherring@herring_lap ~]$ more sea_file
she sells seashells by the seashore
[mherring@herring_lap ~]$ echo "saltwater is not good to drink" >> sea_file
[mherring@herring_lap ~]$ more sea_file
she sells seashells by the seashore
saltwater is not good to drink
[mherring@herring_lap ~]$

So---">" means "send output to <filename>"
">>" means the same thing, but the data is appended to what is already there

ButterSideUp · 09-23-2012, 03:51 PM

Thanks that's really helpful. So, i have this:

[root@qradar tim]# blkid /dev/sdb1 | egrep -o 'UUID[^ ]*'
UUID="5a34315a-e9e9-4d01-823b-d07b8c6195d8"

I'm trying to assign the command to a variable as below but not working:

[root@qradar tim]# <variable>= blkid /dev/sdb1 | egrep -o 'UUID[^ ]*'

Also, i don't need "UUID" just the number.

suicidaleggroll · 09-23-2012, 03:53 PM

Quote:

Originally Posted by ButterSideUp

Thanks that's really helpful. So, i have this:

[root@qradar tim]# blkid /dev/sdb1 | egrep -o 'UUID[^ ]*'
UUID="5a34315a-e9e9-4d01-823b-d07b8c6195d8"

I'm trying to assign the command to a variable as below but not working:

[root@qradar tim]# <variable>= blkid /dev/sdb1 | egrep -o 'UUID[^ ]*'

Also, i don't need "UUID" just the number.

You missed the $()

Code:

<variable>=$(blkid /dev/sdb1 | egrep -o 'UUID[^ ]*')

ButterSideUp · 09-23-2012, 04:07 PM

Ah brilliant thanks, i get it now.

My script will do 2 things to the fstab file:

1 change /store to /store_old
2.set the UUID and file options for /store

The value derived into the variable in your comment above:

[root@qradar tim]# echo $a
UUID="5a34315a-e9e9-4d01-823b-d07b8c6195d8"

is different from the fstab entry because it includes " "

UUID=240ecbe0-6e44-4e0d-8d5d-efd8ba4f867a /store ext4 defaults,noatime,nobarrier 1 2

##### 6: Edit the /etc/fstab file. ##########################################################################

echo "%%%%% Step6: Edit the etc/fstab file. %%%%%"

sed -i 's/LABEL=\/store.*\/store /LABEL=\/store \/store_old/' /etc/fstab
echo "/dev/$DEVICE /store ext4 noatime,nobarrier 0 0" >> /etc/fstab

suicidaleggroll · 09-23-2012, 04:11 PM

You can remove the quotes with a simple bash string replace

Code:

$ a='UUID="5a34315a-e9e9-4d01-823b-d07b8c6195d8"'
$ echo $a
UUID="5a34315a-e9e9-4d01-823b-d07b8c6195d8"
$ echo ${a//\"/}
UUID=5a34315a-e9e9-4d01-823b-d07b8c6195d8

ButterSideUp · 09-23-2012, 04:55 PM

Again, very helpful indeed!

My final question is about this line:

echo "/dev/$DEVICE /store ext4 noatime,nobarrier 0 0" >> /etc/fstab

I've realised that /store already exists in my fstab file and that if i use this
line it will obviously append a new entry for /store to the file, which will mean
i have 2 /store entries.

So, i need to either edit my fstab entry for /store with this:

$a /store ext4 noatime,noauto,nobarrier 0 0

or,

append the new /store line and then delete the old one.

Stuck again!!!

pixellany · 09-23-2012, 05:53 PM

It's time for this:
http://tldp.org/LDP/Bash-Beginners-G...tml/index.html

To answer your specific question:

You can of course edit the fstab file directly...

To do a replacement, look at SED also---for example, to edit a file "in place", replacing "fred" with "ralph", you would do this:

Code:

sed -i 's/fred/ralph/g' filename

The ultimate SED tutorial here:
http://www.grymoire.com/Unix/Sed.html

chrism01 · 09-23-2012, 05:58 PM

Use this to create a new version without the store line, then edit the new one as above

Code:

grep -v store /etc/fstab >/etc/fstab.new

then check the new one by eye.
If its all good, you can overwrite the original with the new.
In any case, you should always backup a critical file like this before messing with it eg

Code:

cp /etc/fstab /etc/fstab.<YYYYMMDD>

AnanthaP · 09-23-2012, 07:42 PM

for

Quote:

cp /etc/fstab /etc/fstab.<YYYYMMDD>

I prefer

Quote:

mv /etc/fstab /etc/fstab.<YYYYMMDD>
cp /etc/fstab.<YYYYMMDD> /etc/fstab

since it retains the original time stamp on the backed up - original - file (/etc/fstab.<YYYYMMDD>) and is useful to restore back if needed.

OK