We can see a shell
```
$ cat with | vim
/bin/bash: line 1: vim: command not found
```
And apparently the shell is stuck in a mode, where each command is prefixed by a 'cat' command of some file, which is piped to the command we are entering. For example, the command 'more' will result in the piped text to be printed on the screen:
```
$ cat with | more
A cantilever is a rigid structural element that extends horizontally...
```

We can also transform this text with 'base64'
```
$ cat with | base64
QSBjYW50aWxldmVyIGlzIGEgcmlnaWQgc3RydWN0dXJhbCBlbGVtZW50IHRoYXQgZXh0ZW5kcyBo
b3Jpem9udGFsbHkgYW5kIGlzIHVuc3VwcG9ydGVkIGF0IG9uZSBlbmQuIFR5cGljYWxseSBpdCBl
...
```
And 'ls' appears to be executable too
```
$ cat with | ls
flag.txt
run
with
```
Some characters seem to be disallowed

```
$ cat with | -
disallowed: -
$ cat with | ;
disallowed: ;
$ cat with | flag
disallowed: flag

Also disallowed are ", ', $, \, -, & and flag
```
We might be able to circumvent the forbidden 'flag' keyword with some command line tools:
```
$ cat with | echo fuag.txt | tr u l
flag.txt
$ cat with | echo fuag.txt | tr u l | cat
/bin/bash: fork: retry: Resource temporarily unavailable
flag.txt
```
Another approach
```
printf %s fl a g . t x t
```

Solution:

```
$ cat with | printf %s fl a g . t x t | xargs cat
```

Flag:
```
bctf{owwwww_th4t_hurt}
```