"less file" prints nothing and causes a messy terminal

1 What happened?

Today when I tried to less a file, it printed nothing and caused the terminal a mess.

$ cat t.txt

HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH

$ cat t.txt | less # here the less works normally

HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH

(END)

$ less t.txt # nothing output here!!!

$            # the shell is dead now

2 Why?

$ cat /etc/passwd | grep user01

user01:x:1001:1001::/home/user01:/bin/csh

$ cat ~/.cshrc

/bin/bash -l

The account "user01" has /bin/csh as its login shell. But it wants to use /bin/bash instead, so it invokes a bash automatically by taking advantage of .cshrc.

As a result, every time a "csh" is executed, whether it's login-shell or not, the .cshrc would be run automatically.

The "less t.txt" command uses $SHELL to run some work under the hood. Here $SHELL is /bin/csh which for sure will load .cshrc.

The .cshrc intercepts the csh script and run a login bash instead. 

less t.txt --> /bin/csh --> ~/.cshrc --> /bin/bash -l

3 Solution

Only login csh should invoke a login bash.

$ cat ~/.cshrc

if (x$0 == 'x-csh') then

        /bin/bash -l

endif

4 Conclusion

Run an infinite loop in a script is not good. Invoking an interactive shell is one kind of infinite loop.
Actually, staring another interactive shell in an existing shell's RC file can cause lots of surprises. Another common problem caused by the same reason can be found at https://www.linuxexam.net/2020/03/ansible-is-stuck-ssh-command-is-stuck.html.