Linux get return code

Linux bash exit status and how to set exit status in bash

C an you explain bash exit status code? How do I set bash exit status in my Linux shell scripts?

Each Linux or Unix command returns a status when it terminates normally or abnormally. You can use value of exit status in the shell script to display an error message or run commands. For example, if tar command is unsuccessful, it returns a code which tells the shell script to send an e-mail to sysadmins.

Tutorial details
Difficulty level	Easy
Root privileges	No
Requirements	Bash running on Linux, macOS or Unix
Est. reading time	3 minutes

More on Linux bash shell exit status codes

1. Every Linux or Unix command executed by the shell script or user, has an exit status.
2. The exit status is an integer number.
3. For the bash shell’s purposes, a command which exits with a zero (0) exit status has succeeded.
4. A non-zero (1-255) exit status indicates failure.
5. If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.
6. All of the Bash builtins return exit status of zero if they succeed and a non-zero status on failure.

How do I display the exit status of shell command?

You can use special shell variable called $? to get the exit status of the previously executed command. To print $? variable use the echo command/printf command. The syntax is:
command
echo $?
OR
/path/to/script.sh
command
date
echo $?
## OR use the printf command ##
printf «%d\n» $?
## run non-existence command ##
foobar13535
## display status code ##
echo $?

How to store the exit status of the command in a shell variable

Assign $? to a shell variable. The syntax is:

Linux exit status and the conditional/list constructs

A simple shell script to locate host name (findhost.sh)

How to use the && and || operators with exit codes

If a dir named “/tmp/foo” not found create it:
[ ! -d «/tmp/foo» ] && mkdir -p «/tmp/foo»
For example, show usage syntax when filename not passed as the command line arg:

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Here is another shell script that shows usage:

List of common exit codes for GNU/Linux

Exit Code	Description
0	Success
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
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
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
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 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
125	Operation canceled
126	Required key not available
127	Key has expired
128	Key has been revoked
129	Key was rejected by service
130	Owner died
131	State not recoverable
132	Operation not possible due to RF-kill
133	Memory page has hardware error

The perror command explain error codes which is part of MySQL/MariaDB package:
perror 0
perror 1

Conclusion

This page explained bash exit status and related commands. For more info see bash shell man page here.

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Bash получить код завершения команды в Linux / Unix

Я новый пользователь системы Linux. Как мне получить код завершения команды?

Как получить код вывода или статус команды оболочки Linux или Unix и сохранить его в переменной оболочки?

Введение. Каждая команда оболочки Linux или Unix возвращает состояние, когда она завершается нормально или ненормально.

Например, если скрипт backup.sh не выполнен, и он возвращает код, который сообщает скрипту оболочки отправить электронное письмо админу.

Что такое код вывода в оболочке bash?

Каждая команда Linux или Unix, выполняемая скриптом оболочки или пользователем, имеет статус вывода.

Статус вывода – это целое число.

0 состояние выхода означает, что команда была успешной без каких-либо ошибок.

Ненулевое (1-255 значений) состояние выхода означает, что команда была неудачной.

Как узнать код вывода команды

Вам нужно использовать определенную переменную оболочки с именем $? чтобы получить статус вывода из ранее выполненной команды.

Выведем $? переменной используя команду echo или команду printf:

Из приведенных выше выводов ясно, что код вывода 0 означает, что команда date была успешной.

Кроме того, код вывода – 127 (не ноль), так как команда nonexistant не была успешной.

Bash как получить код завершения команды – Как использовать коды вывода в скриптах оболочки

Итак, как вы сохранить статус вывода команды в переменной оболочки?

Просто назначьте $? в переменную оболочки. Синтаксис:

Как мне установить код вывода для моих собственных скриптов оболочки?

Команда exit вызывает обычное завершение скриптов оболочки.

Вывод из оболочки со статусом N. Синтаксис:

Пример скрипта оболочки для получения кода завершения команды

Заключение

На этой странице показано, как использовать коды вывода в системах на основе Linux или Unix и как получить статус вывода / код команды.

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1. Invoke shell commands

In general, there are three common methods for calling shell commands using C programs in Linux systems: system (), popen (), and exec series functions.

• Using system () does not require the user to create a process because it is already encapsulated, just add the shell command directly;
• Use popen () to execute shell commands, the cost is less than system ();
• Exec requires the user to fork / vfork the process, and then exec the shell commands required.

1.1 system()

Function prototype

Function description
system () will call fork () to generate a subprocess, and the subprocess will call / bin / sh -c string to execute the command represented by the parameter string, and the command will be executed immediately after execution Return to the original calling process. During the call to system (), the SIGCHLD signal will be temporarily suspended, and the SIGINT and SIGQUIT signals will be ignored.

return value
If system () fails when calling / bin / sh, it returns 127, and other failure reasons return -1. If the parameter string is a null pointer (NULL), a non-zero value is returned. If the system () call is successful, the return value after executing the shell command is finally returned, but this return value may also be 127 returned by the failure of the system () call / bin / sh, so it is best to check errno again to confirm the successful execution .

Additional information
Do not use system () when writing a program with SUID / SGID authority, because system () inherits environment variables, which may cause system security problems.

1.2 popen()

Function prototype

Function description
popen () will call fork () to create a child process, and then call / bin / sh -c from the child process to execute the command command parameter. The parameter type can use «r» for reading and «w» for writing. According to this type value, popen () will establish a pipe to the child’s standard output device or standard input device, and then return a file pointer. Then the process can use this file pointer to read the output device of the child process or write to the standard input device of the child process. In addition, except for fclose (), all other functions that use the file pointer (FILE *) operation can also be used.

return value
If successful, the file pointer is returned, otherwise NULL is returned, and the cause of the error is stored in errno.

Precautions
When writing a program with SUID / SGID authority, please avoid using popen () as much as possible, because popen () inherits environment variables, which may cause system security problems.

Examples

1.3 Exec function cluster

Function prototype

Examples
Use vfork () to create a new child process, and then call the exec function family.

2 Get the return result

Above we introduced several methods of calling shell commands in a C program. Among them, we found a problem-although we can know whether the shell command has been executed, sometimes we cannot get the information returned by it. Then, at this time, you can consider the following methods.

2.1 Using temporary files

The easiest way to think of it first should be to redirect the shell command output to a temporary file and read this temporary file in our application to obtain the results of the external command execution.
The code is as follows:

This use uses a temporary file as a link between the application and external commands. In the application, you need to read the file and then delete the temporary file, which is cumbersome. The advantage is that it is simple to implement and easy to understand.

2.2 Use anonymous pipes

In the book «Advanced Programming for UNIX Environment» (APUE), an example of outputting program results to a paging program through anonymous pipes is given. Therefore, we can also use pipes to connect the results of external commands to applications stand up. The method is to fork a child process and create an anonymous pipe, execute shell commands in the child process, and dup its standard output to the input end of the anonymous pipe. The parent process reads from the pipe to obtain the output of the shell command.
The code is as follows:

2.3 Use popen ()

In the example of executing shell commands, we used the popen () function. You may have found it carefully. You can also use popen () to get the result of the command.
The function of this function is to create a pipeline, fork a process, and then execute the shell, and the output of the shell can be obtained by reading the file. This method is recommended to avoid creating temporary files without being limited by the number of output characters.
popen () uses the FIFO pipeline to execute external programs. It determines the input / output direction of the command by whether the type is r or w, and r and w are relative to the pipeline of the command. r means that the command is read from the pipe, w means that the command is output to its stdout through the pipe, and popen () returns the file stream pointer of the FIFO pipe. pclose () is used to close this pointer after use.
Sample code is as follows:

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