TIL - read-only variables in Linux

 A co-worker was seeing an exception: -

 line 8: TMOUT: readonly variable

when trying to SCP a file from a remote Linux box.

I did some digging and found a RedHat article: -

Why does it prompt "bash: TMOUT: readonly variable" when sudo'ing or ssh'ing to the system? 

that said, in part: -

The TMOUT variable is usually defined read-only to avoid users from unsetting or modifying its value. Due to this it's not possible to set it twice.

I reproduced the problem on my Ubuntu 18.04 box: -

-bash: TMOUT: readonly variable

with a pair of scripts: -

The first script: -

cat /etc/profile.d/a.sh

readonly TMOUT=500; export TMOUT

sets TMOUT as readonly

The second script: -

cat /etc/profile.d/b.sh

TMOUT=600; export TMOUT

then tries to override it

which I validated: -

fgrep -R TMOUT /etc/profile.d/

/etc/profile.d/a.sh:readonly TMOUT=500; export TMOUT

/etc/profile.d/b.sh:TMOUT=600; export TMOUT

I left my colleague to dig into /etc etc. and see what was going on, but TIL about read-only variables