We know that the flag is in current directory by executing:

rbash-5.0$ echo *
flag-xEpAN7X3tGYjt4Y0p0FD.txt jail.sh

There was no file reading commands.

But luckily, we had echo and we can read the file by echo too.

rbash-5.0$ while read line; do echo $line; done < flag-xEpAN7X3tGYjt4Y0p0FD.txt 
sdctf{red1r3ct1ng_std1n_IS_p3rm1tt3d_1n_rb45h!}

Flag: sdctf{red1r3ct1ng_std1n_IS_p3rm1tt3d_1n_rb45h!}