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In /etc/bashrc file in Fedora/Red Hat I see following line:

[ "$PS1" = "\s-\v\$ " ] && PS1="[u@h W]\$ "

What is the check being done in [ "$PS1" = "\s-\v\$ " ] and why is PS1 set only if the test succeeds?

(I debated whether to make this an answer since it's such a swag but here ya go...)

Just a theory but the first form (e.g. bash-3.2.1$) looks like the prompt I often see when logged in as root or other "non-user" account . The second, overriding form (e.g. [joeblow@myhost /tmp]$) is more user-centric. So maybe this is to detect when going from a system account to user account...then, and only then, change to a more appropriate prompt. Otherwise assume someone would want to preserve the current prompt.

The check being done is a string comparison =. It's comparing the value of $PS1 to the string s-v$, where the backslashes need to be escaped in the string so that they get compared to actual backslashes instead of attempting to escape the following character.

The && syntax is the part that sets PS1 only if the preceding test succeeds.

The overall logic here is apparently to update PS1 only if it was previously set to a specific value.

s-v$␣ is the default prompt string, the one Bash sets when starting interactively with PS1 unset. It shows the basename of the shell process (usually bash or sh), the version and a dollar sign, e.g. bash-4.4$. (Or a # instead of $ if running as root.)

The idea of appears to be to set a more useful prompt, but not if some other startup file has already set it.

The backslashes are doubled in "\s-\v\$ " since within a double-quoted string x has a special meaning for some values of x. \ unambiguously represents a literal backslash. (Though for some reason they haven't done that on the assignment side).

Using single quoted strings would make the backslashes a bit easier to read:

[ "$PS1" = 's-v$ ' ] && PS1='[u@h W]$ '

I believe the comparison is checking to see if the shell is an interactive session. s-v$ as a $PS1 var evaluates to bash-5.0$ on my machine. See the differences here:

Jonathans-Air:~ lirum$ bash

bash-5.0$ echo "$PS1"

s-v$

bash-5.0$ exit

exit

Jonathans-Air:~ lirum$ echo "$PS1"

h:W u$

It's an awkward way to check if PS1 is bash's default, or if it was customized by the user in ~/.bashrc (or in any of the /etc/profile, /etc/profile.d/* or ~/.bash_profile files in the case of a login shell).

In the former case, PS1 will be set to the ugly and annoying [user@host /long/path/to/hell]$; in the latter case, it will be left as is.

Notice that on RedHat /etc/bashrc is sourced explicitly from the default ~/.bashrc and ~/.bash_profile installed from /etc/skel; do not confuse /etc/bashrc with /etc/bash.bashrc which is sourced before ~/.bashrc on some systems (eg. Debian).