Explain the Linux OS Return Codes

When computer programs are executed, the operating system creates an abstract entity called a process in which the book-keeping for that program is maintained. In multitasking operating systems such as Unix or Linux, new processes can be created by active processes.

The process that spawns another is called a parent process, while those created are child processes. Child processes run concurrently with the parent process.

 The technique of spawning child processes is used to delegate some work to a child process when there is no reason to stop the execution of the parent. When the child finishes executing, it exits by calling theexit system call. This system call facilitates passing the exit status code back to the parent, which can retrieve this value using the wait system call.

There is no  straight way to get return code when it come to Linux/AIX operating systems.I found indirect method. Always 0 = Success anything else is an error.
Return Codes:

The exit status or return code of a process in computer programming is a small number passed from a child process (or callee) to a parent process (or caller) when it has finished executing a specific procedure or delegated task. In DOS, this may be referred to as an errorlevel.

0
1       Operation not permitted
2       No such file or directory
3       No such process
4       Interrupted system call
5       Input/output error
6       No such device or address
7       Argument list too long
8       Exec format error
9       Bad file descriptor
10      No child processes
11      Resource temporarily unavailable
12      Cannot allocate memory
13      Permission denied
14      Bad address
15      Block device required
16      Device or resource busy
17      File exists
18      Invalid cross-device link
19      No such device
20      Not a directory
21      Is a directory
22      Invalid argument
23      Too many open files in system
24      Too many open files
25      Inappropriate ioctl for device
26      Text file busy
27      File too large
28      No space left on device
29      Illegal seek
30      Read-only file system
31      Too many links
32      Broken pipe
33      Numerical argument out of domain
34      Numerical result out of range
35      Resource deadlock avoided
36      File name too long
37      No locks available
38      Function not implemented
39      Directory not empty
40      Too many levels of symbolic links
41      Unknown error 41
42      No message of desired type
43      Identifier removed
44      Channel number out of range
45      Level 2 not synchronized
46      Level 3 halted
47      Level 3 reset
48      Link number out of range
49      Protocol driver not attached
50      No CSI structure available
51      Level 2 halted
52      Invalid exchange
53      Invalid request descriptor
54      Exchange full
55      No anode
56      Invalid request code
57      Invalid slot
58      Unknown error 58
59      Bad font file format
60      Device not a stream
61      No data available
62      Timer expired
63      Out of streams resources
64      Machine is not on the network
65      Package not installed
66      Object is remote
67      Link has been severed
68      Advertise error
69      Srmount error
70      Communication error on send
71      Protocol error
72      Multihop attempted
73      RFS specific error
74      Bad message
75      Value too large for defined data type
76      Name not unique on network
77      File descriptor in bad state
78      Remote address changed
79      Can not access a needed shared library
80      Accessing a corrupted shared library
81      .lib section in a.out corrupted
82      Attempting to link in too many shared libraries
83      Cannot exec a shared library directly
84      Invalid or incomplete multibyte or wide character
85      Interrupted system call should be restarted
86      Streams pipe error
87      Too many users
88      Socket operation on non-socket
89      Destination address required
90      Message too long
91      Protocol wrong type for socket
92      Protocol not available
93      Protocol not supported
94      Socket type not supported
95      Operation not supported
96      Protocol family not supported
97      Address family not supported by protocol
98      Address already in use
99      Cannot assign requested address
100     Network is down
101     Network is unreachable
102     Network dropped connection on reset
103     Software caused connection abort
104     Connection reset by peer
105     No buffer space available
106     Transport endpoint is already connected
107     Transport endpoint is not connected
108     Cannot send after transport endpoint shutdown
109     Too many references: cannot splice
110     Connection timed out
111     Connection refused
112     Host is down
113     No route to host
114     Operation already in progress
115     Operation now in progress
116     Stale NFS file handle
117     Structure needs cleaning
118     Not a XENIX named type file
119     No XENIX semaphores available
120     Is a named type file
121     Remote I/O error
122     Disk quota exceeded
123     No medium found
124     Wrong medium type
125     Operation canceled
126     Required key not available
127     Key has expired

Hope it is useful.