The following article describes some basics regarding first contact with Synology DSM shell.
I used DSM 6.1.3 out of the box, running in a Virtual machine (using Virtual Machine Manager) on DS1817+ hardware.
First, enable the SSH service DSM.
Control Panel > Terminal & SNMP > Enable SSH service
Try not to use Telnet, not even locally (not secure).
PuTTY is a good Windows SSH client.
My test DSM server is called neelix_vm.
My test user is called geohei.
Login using the name and password of the user created during the initial DSM installation process.
login as: geohei geohei@192.168.1.162's password: Could not chdir to home directory /var/services/homes/geohei: No such file or directory geohei@neelix_vm:/$ ls -l /var/services/ total 0 lrwxrwxrwx 1 root root 24 Aug 15 13:41 pgsql -> /volume1/@database/pgsql lrwxrwxrwx 1 root root 13 Aug 15 13:42 tmp -> /volume1/@tmp geohei@neelix_vm:/$ pwd / geohei@neelix_vm:/$ cat /etc/passwd admin:x:1024:100:System default user:/var/services/homes/admin:/bin/sh anonymous:x:21:21::/nonexist:/usr/bin/nologin avahi:x:84:84::/:/bin/false bind:x:53:53::/:/usr/bin/nologin daemon:x:2:2::/:/bin/sh dbus:x:81:81::/:/usr/bin/nologin dovecot:x:143:143::/dev/null:/usr/bin/nologin ftp:x:21:21::/nonexist:/usr/bin/nologin geohei:x:1026:100::/var/services/homes/geohei:/bin/sh guest:x:1025:100:Guest:/nonexist:/usr/bin/nologin http:x:1023:1023::/var/services/web:/bin/false ldap:x:439:439::/var/lib/openldap:/usr/bin/nologin lp:x:7:7::/var/spool/lpd:/bin/sh mysql:x:66:66::/var/services/mysql:/bin/false nobody:x:99:99::/:/bin/false ntp:x:87:87::/var/lib/ntp:/bin/false postfix:x:125:125::/dev/null:/usr/bin/nologin postgres:x:55:55::/var/services/pgsql:/bin/sh root:x:0:0::/root:/bin/ash rpc:x:32:32::/dev/null:/bin/false system:x:1:1::/usr/syno/synoman:/usr/bin/nologin
/var/services/homes/geohei is supposed to be geohei's home directory. However it was not created during initial DSM installation user setup, since not relevant for DSM. However while logging into the shell, it attempts to change to the directory (chdir), which doesn't exist, and complains. The present working directory (pwd) will therefore default to /.
/etc/passwd defines the user's home directory, which the shell is used (sh for user geohei), and much more.
Next ... sometimes, you might need root access. For instance, I cannot see the content of the file /etc/shadow as user geohei, since this file holds hashed passwords. I do need root access for this. The shell user right and permission management has nothing to do with the DSM user and permission management. User geohei might be administrator in DSM, but is only user in the shell.
geohei@neelix_vm:/$ cat /etc/shadow cat: /etc/shadow: Permission denied geohei@neelix_vm:/$ ls -l /etc/shadow -rw------- 1 root root 902 Aug 15 16:03 /etc/shadow geohei@neelix_vm:/$ sudo -i Password: root@neelix_vm:~# cat /etc/shadow admin:$6$761Y.ISJWdMwjz2SdxGFghwFfgdEdztDWW34fvDFbcjGFhfNuiwebGZFTZvHFTZ5lhg7chgDDDhjkHKGjzmIFqfhxfelkRa163kR5/:17393:0:99999:7::1: anonymous:*:10933:0:99999:7::: avahi:*:10933:0:99999:7::: bind:*:10933:0:99999:7::: daemon:*:10933:0:99999:7::: dbus:*:10933:0:99999:7::: dovecot:*:10933:0:99999:7::: ftp:*:10933:0:99999:7::: geohei:$6$6ckppkOxsKj4MWy$8bjkdew./hbduzef5gve32rvj8/gdhwevd5/szu23e5mdtTFLuEDQl52zSA/.xAYgztw5RRdWcjF.tT.6OETc1:17393:0:99999:7::: guest:$1$synergy$O31nVEghZgsd4Edrts8MK.:10933:0:99999:7::1: http:*:10933:0:99999:7::: ldap:*:10933:0:99999:7::: lp:*:10933:0:99999:7::: mysql:*:10933:0:99999:7::: nobody:*:10933:0:99999:7::: ntp:*:10933:0:99999:7::: postfix:*:10933:0:99999:7::: postgres:*:10933:0:99999:7::: root:*:10933:0:99999:7::: rpc:*:10933:0:99999:7::: system:*:10933:0:99999:7:::
"sudo -i" gives root access to a non-root user. The password required is the password of the user, not the root password (which is not set as seen in /etc/shadow above).
/etc/shadow also shows that user admin, although disabled in DSM (default), has an (unknown) password stored.