Bash: Some issue when using read <<<“$VARIABLE” on a read-only root partition. Any known workarounds

bashhere-documentreadreadonlytmp

Just by coincidence I had to use my ATA-ID-to-device-name script (found here: https://serverfault.com/questions/244944/linux-ata-errors-translating-to-a-device-name/426561#426561) on a read-only / partition. In case you're curious, it was an Ubuntu recovery console which will let you access your / partition, but will mount it read-only by default. I am glad about that, because otherwise I would probably never have found out that my script behaves strangely on a R/O system due to a specific line, this one:

IFS=: read HostMain HostMid HostSub <<< "$HostFull"

This does not work if there is no write permission. I wouldn't have assumed it would fail, though. But apparently the <<< operator does require to write some temporary file to somewhere.

But is there any way to circumvent the creation of a temporary file, or, is there any way to specify where the file is written to? In the Ubuntu recovery console, there is—oddly enough—write permission on the /run directory, so that would do, if I could somehow "tell" read to write the temp file to somewhere else than usual.

Best Answer

An array could make the string parsing without the need for a temporal file. Don't forget to turn off globbing.

set -f
IFS=: Hosts=($HostFull)
HostMain=${Hosts[0]}
HostMid=${Hosts[1]}
HostSub=${Hosts[2]}
set +f

Related Solutions

Linux – read only root filesystem

The default behaviour for most Linux file systems is to safeguard your data. When the kernel detects an error in the storage subsystem it will make the filesystem read-only to prevent (further) data corruption.

You can tune this somewhat with the mount option errors={continue|remount-ro|panic} which are documented in the system manual (man mount).

When your root file-system encounters such an error, most of the time the error won't be recorded in your log-files, as they will now be read-only too. Fortunately since it is a kernel action the original error message is recorded in memory first, in the kernel ring buffer. Unless already flushed from memory you can display the contents of the ring buffer with the dmesg command. .

Most real hard disks support SMART and you can use smartctl to try and diagnose the disk health.

Depending on the error messages, you could decide it is still safe to use file-system and return it read-write condition with mount -o remount,rw /

In general though, disk errors are a precursor to complete disk failure. Now is the time to create a back-up of your data or to confirm the status of your existing back-ups.

Bash – How to get bash tab completion to work with read only file system

The first test you can do to see where is your tmp folder and if it is working is executing this command:

tempfile

The output should be something like this:

/tmp/fileupz962

If the folder /tmp exists and is working (has write permissions) but the temp folder is pointed to other directory like /var/tmp you can try to set the temp folder to point to /tmp using:

export TMPDIR=/tmp

If bash is saving the temporary file in /tmp and if for some reason the /tmp folder is not working you can try to unmount it (in case it is mounted). It may happen that the /tmp is mounted as read only. In both cases should be useful to try:

sudo umount /tmp

Then you can try using RAM storage for the /tmp directory:

sudo mount -t tmpfs -o size=128M tmpfs /tmp/ram/

You can set the amount of RAM (-o size=128M) to whatever value you want.

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