bash programming using sed

[JMK8]

I have a relatively straight-forward task to do in a bash script using sed. I'm not familiar with bash scripting and even less so with sed. I have a text file like this:

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 random text PARTUUID=a5d5af73-01 random text

Obviously I have no control over the hex number. What I want to do is substitute PARTUUID=a5d5af73-01 with /dev/mmcblk0p1 I have gone around in circles trying to do this with sed but no joy -maybe I'm too thick to get my head around it

Can anyone help or give me an alternative method?

-JohnK

[B.Goode]

Maybe awk or perl would provide an alternative way of doing the substitution?

[Paeryn]

I have a relatively straight-forward task to do in a bash script using sed. I'm not familiar with bash scripting and even less so with sed. I have a text file like this:

Code: Select all

 random text PARTUUID=a5d5af73-01 random text

Obviously I have no control over the hex number. What I want to do is substitute PARTUUID=a5d5af73-01 with /dev/mmcblk0p1 I have gone around in circles trying to do this with sed but no joy -maybe I'm too thick to get my head around it

Can anyone help or give me an alternative method?

-JohnK

This should change PARTUUID=... (including all characters from the = up to the first space) into /dev/mmcblk0p1 in the file filename, and backs up the original to filename.bak If you don't want the original file backing up don't pass a string to --in-place. And if you don't want to actually alter the file don't pass --in-place, then the output is sent to stdout rather than rewriting the file.

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sed --in-place=".bak" "s,PARTUUID=[^ ]*,/dev/mmcblk0p1," filename

Edit: Just in case there are tabs as well as spaces in there you might want to add \t between the square brackets, and if the PARTUUID= string appears more than once on a line add a g to the end of the search/replace string (otherwise it only alters the first occurrence on a line).

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sed --in-place=".bak" "s,PARTUUID=[^ \t]*,/dev/mmcblk0p1,g" filename

[JMK8]

Thanks guys, I will try this when I get home next week.

-JohnK

[DougieLawson]

. What I want to do is substitute PARTUUID=a5d5af73-01 with /dev/mmcblk0p1 I have gone around in circles trying to do this with sed but no joy -maybe I'm too thick to get my head around it

Why? PARTUUID works and is agnostic to the device you're booting. That's why it's in Raspbian Jessie so that the same image works on an SDCard or a bootable USB device (on RPi3).

[JMK8]

Dougie - OK here's the long story.

About two years ago I wrote a bash script to clone the Raspberry Pi SD card on a running system. As I said previously I am not much of bash scripter but my script does what I want it to. Except -- when I try to boot the cloned SD card I found it would hang. I found that if edit /boot/cmdline.txt and /etc/fstab on the cloned SD card to replace PARTUUID=a5d.....entries with /dev/mmcblk0pX as appropriate everything works fine when booting the cloned SD card. This is a minor inconvenience since I clone SD cards quite infrequently.

I never bothered to try and fix this until a few days ago when I had an hour or two to spare and I decided to go for the blunt instrument fix of automating the editing of the two files. Hence the request in this forum

-JohnK

[DougieLawson]

Use blkid to get the PARTUUID and run with them, it will be 10,000,000 times easier in future.

You can stick PARTUUIDs in /boot/cmdline.txt and /etc/fstab and they "just work".