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
[ "$PS1" = "\\s-\\v\\\$ " ] and why is PS1 set only if the test succeeds?
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It's an awkward way to check if
bash's default, or if it was customized by the user in
~/.bashrc (or in any of the
~/.bash_profile files in the case of a login shell).
In the former case,
PS1 will be set to the ugly and annoying
[user@host last_path_elem]$; in the latter case, it will be left as is.
Notice that on RedHat
/etc/bashrc is sourced explicitly from the default
~/.bash_profile installed from
/etc/skel; do not confuse
/etc/bash.bashrc which is sourced before
~/.bashrc on some systems (eg. Debian).
(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 $PS1 to the string
\s-\v\$, where the backslashes need to be escaped in the string so that they get compared to actual backslashes instead of attempting to escape the following character.
&& syntax is the part that sets PS1 only if the preceding test succeeds.
The overall logic here is apparently to update PS1 only if it was previously set to a specific value.
\s-\v\$␣ is the default prompt string, the one Bash sets when starting interactively with
PS1 unset. It shows the basename of the shell process (usually
sh), the version and a dollar sign, e.g.
bash-4.4$. (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
\\ 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]\$ '