2

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?

2

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 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 ~/.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).

  • This seems the most likely explanation. I will accept this answer. – Akilan Mar 18 at 8:52
  • Note: It will not set any /long/path/to/hell, because \W expands only to the basename of the CWD. In that case, only hell will be shown. – iBug Apr 25 at 9:33
  • @iBug thanks, fixed – mosvy Apr 25 at 9:39
2

(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.

1

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.

  • I understand that it is string comparison and && being used as a test. My question rather was about significance of that string and "why is PS1 set only if the test succeeds". – Akilan Aug 10 '17 at 11:45
  • The poster already acknowledged that && was testing for the success of the previous command and so the answer doesn't give the required response, – Raman Sailopal Aug 10 '17 at 12:52
  • I think you'd have to ask the maintainers of that file to ask why it is set that way. – Jeff Schaller Aug 10 '17 at 22:49
1

\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]\$ '
0

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\$
  • 1
    Bash unsets PS1 completely if the shell is non-interactive, so testing it against a non-empty string value doesn't really help there. Testing for empty $PS1 is done in some places, e.g. Debian's /etc/bash.bashrc has [ -z "$PS1" ] && return to exit if the shell is non-interactive. – ilkkachu Mar 15 at 23:39

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