In Shell script, $0, $?, $!, $$, $*, $#$@ And the meaning of the return value of linux command execution
(1) In shell script, $0, $?, $!, $$, $*, $#$@ And so on
1.1 brief description
ps: you can understand it by trying more in Linux environment
$$
The PID (ProcessID) of the Shell itself, that is, the current time the script is running Process ID No.)
$!
PID of the last background Process running in the Shell (PID of the last Process running in the background) Process ID No.)
$?
The end code (return value) of the last running command is the return value of the last instruction executed (showing the exit status of the last command. 0 indicates no error, and any other value indicates an error)
$-
Displays the current options used by the shell, with the same function as the set command
$*
List of all parameters. If "$*" is enclosed by "," and all parameters are output in the form of "$1 $2... $n", this option can have more than 9 parameters.
$@
List of all parameters. If "$@" is enclosed by ",", output all parameters in the form of "$1" $2 "..." $n ".
$@ is similar to $*, but can be used as an array
$#
Number of parameters added to Shell
$0
The file name of the Shell itself
$1～$n
The value of each parameter added to the Shell$ 1 is the first parameter, $2 is the second parameter.
1.2 script examples
Let's write a simple script and explain the meaning of each variable after execution
# touch variable
# vi variable
The script is as follows:
#!/bin/sh
echo "number:$#"
echo "scname:$0"
echo "first :$1"
echo "second:$2"
echo "argume:$@"
Save exit
Grant script execution permission
# chmod +x variable
Execute script
# ./variable aa bb
number:2
scname:./variable
first: aa
second:bb
argume:aa bb
Result analysis
By displaying the results, you can see:
$# is the number of parameters passed to the script
$0 is the name of the script itself
$1 is the first parameter passed to the shell script
$2 is the second parameter passed to the shell script
$@ is a list of all parameters passed to the script
(2) linux command execution return value description
Under Linux, whether you start a desktop program or run a command under the console, all programs will return a numerical value at the end. This value is called return value, or Error Number.
2.1. Under the console, there is a special environment variable $?, Save the return value of the previous program. We can try:
$ ls *.png
Diagram1.png eqn.png peazip.png
eqn4.png Mandelbrot.png x2.png
$ echo $?
0
First execute a command casually, such as some files like ls above, and then use echo $?, Print $? Value of ～
We find that the return value is 0. What does that mean?
As long as the return value is 0, it means that the program execution is successful ~
That is, if $? If the value of the variable is not 0, it indicates that an error occurred in the execution of the previous command.
2.2. We can try to ls a file that does not exist:
$ ls linuxgem
ls: cannot access linuxgem: there is no file or directory
$ echo $?
2
In this way, for some commands that print out a pile of unfamiliar English information, it is convenient to see whether it is successfully executed.
However, echo $? It's too tired. It doesn't matter. Can you put $? Put it in the prompt:
2.3. Put the return value of whether the last command was executed successfully in the prompt: export PS1="[$?]${PS1}"
tom@localhost ~ $ export PS1="[\$?]${PS1}"
[0]tom@localhost ~ $
export PS1="[$?]${PS1} "this command puts the value of $? At the front of the prompt, so that each time the command is executed, the value will be automatically updated, which is clear at a glance
2.3. Check the error prompt with perror
So if the returned value is not 0, how do we know that there is an error? Most programs will give a prompt when they make an error. If there is no prompt, you can check the error information with the program ， perror ， for example, if the return value is 2, we can run:
$ perror 2
OS error code 2: No such file or directory
This makes it clear that the file does not exist ~
2.4. Give the error code and its specific reason comparison table:
"OS error code 1: Operation not permitted" "OS error code 2: No such file or directory" "OS error code 3: No such process" "OS error code 4: Interrupted system call" "OS error code 5: Input/output error" "OS error code 6: No such device or address" "OS error code 7: Argument list too long" "OS error code 8: Exec format error" "OS error code 9: Bad file descriptor" "OS error code 10: No child processes" "OS error code 11: Resource temporarily unavailable" "OS error code 12: Cannot allocate memory" "OS error code 13: Permission denied" "OS error code 14: Bad address" "OS error code 15: Block device required" "OS error code 16: Device or resource busy" "OS error code 17: File exists" "OS error code 18: Invalid cross-device link" "OS error code 19: No such device" "OS error code 20: Not a directory" "OS error code 21: Is a directory" "OS error code 22: Invalid argument" "OS error code 23: Too many open files in system" "OS error code 24: Too many open files" "OS error code 25: Inappropriate ioctl for device" "OS error code 26: Text file busy" "OS error code 27: File too large" "OS error code 28: No space left on device" "OS error code 29: Illegal seek" "OS error code 30: Read-only file system" "OS error code 31: Too many links" "OS error code 32: Broken pipe" "OS error code 33: Numerical argument out of domain" "OS error code 34: Numerical result out of range" "OS error code 35: Resource deadlock avoided" "OS error code 36: File name too long" "OS error code 37: No locks available" "OS error code 38: Function not implemented" "OS error code 39: Directory not empty" "OS error code 40: Too many levels of symbolic links" "OS error code 42: No message of desired type" "OS error code 43: Identifier removed" "OS error code 44: Channel number out of range" "OS error code 45: Level 2 not synchronized" "OS error code 46: Level 3 halted" "OS error code 47: Level 3 reset" "OS error code 48: Link number out of range" "OS error code 49: Protocol driver not attached" "OS error code 50: No CSI structure available" "OS error code 51: Level 2 halted" "OS error code 52: Invalid exchange" "OS error code 53: Invalid request descriptor" "OS error code 54: Exchange full" "OS error code 55: No anode" "OS error code 56: Invalid request code" "OS error code 57: Invalid slot" "OS error code 59: Bad font file format" "OS error code 60: Device not a stream" "OS error code 61: No data available" "OS error code 62: Timer expired" "OS error code 63: Out of streams resources" "OS error code 64: Machine is not on the network" "OS error code 65: Package not installed" "OS error code 66: Object is remote" "OS error code 67: Link has been severed" "OS error code 68: Advertise error" "OS error code 69: Srmount error" "OS error code 70: Communication error on send" "OS error code 71: Protocol error" "OS error code 72: Multihop attempted" "OS error code 73: RFS specific error" "OS error code 74: Bad message" "OS error code 75: Value too large for defined data type" "OS error code 76: Name not unique on network" "OS error code 77: File descriptor in bad state" "OS error code 78: Remote address changed" "OS error code 79: Can not access a needed shared library" "OS error code 80: Accessing a corrupted shared library" "OS error code 81: .lib section in a.out corrupted" "OS error code 82: Attempting to link in too many shared libraries" "OS error code 83: Cannot exec a shared library directly" "OS error code 84: Invalid or incomplete multibyte or wide character" "OS error code 85: Interrupted system call should be restarted" "OS error code 86: Streams pipe error" "OS error code 87: Too many users" "OS error code 88: Socket operation on non-socket" "OS error code 89: Destination address required" "OS error code 90: Message too long" "OS error code 91: Protocol wrong type for socket" "OS error code 92: Protocol not available" "OS error code 93: Protocol not supported" "OS error code 94: Socket type not supported" "OS error code 95: Operation not supported" "OS error code 96: Protocol family not supported" "OS error code 97: Address family not supported by protocol" "OS error code 98: Address already in use" "OS error code 99: Cannot assign requested address" "OS error code 100: Network is down" "OS error code 101: Network is unreachable" "OS error code 102: Network dropped connection on reset" "OS error code 103: Software caused connection abort" "OS error code 104: Connection reset by peer" "OS error code 105: No buffer space available" "OS error code 106: Transport endpoint is already connected" "OS error code 107: Transport endpoint is not connected" "OS error code 108: Cannot send after transport endpoint shutdown" "OS error code 109: Too many references: cannot splice" "OS error code 110: Connection timed out" "OS error code 111: Connection refused" "OS error code 112: Host is down" "OS error code 113: No route to host" "OS error code 114: Operation already in progress" "OS error code 115: Operation now in progress" "OS error code 116: Stale NFS file handle" "OS error code 117: Structure needs cleaning" "OS error code 118: Not a XENIX named type file" "OS error code 119: No XENIX semaphores available" "OS error code 120: Is a named type file" "OS error code 121: Remote I/O error" "OS error code 122: Disk quota exceeded" "OS error code 123: No medium found" "OS error code 124: Wrong medium type" "OS error code 125: Operation canceled" "OS error code 126: Required key not available" "OS error code 127: Key has expired" "OS error code 128: Key has been revoked" "OS error code 129: Key was rejected by service" "OS error code 130: Owner died" "OS error code 131: State not recoverable" "MySQL error code 132: Old database file" "MySQL error code 133: No record read before update" "MySQL error code 134: Record was already deleted (or record file crashed)" "MySQL error code 135: No more room in record file" "MySQL error code 136: No more room in index file" "MySQL error code 137: No more records (read after end of file)" "MySQL error code 138: Unsupported extension used for table" "MySQL error code 139: Too big row" "MySQL error code 140: Wrong create options" "MySQL error code 141: Duplicate unique key or constraint on write or update" "MySQL error code 142: Unknown character set used" "MySQL error code 143: Conflicting table definitions in sub-tables of MERGE table" "MySQL error code 144: Table is crashed and last repair failed" "MySQL error code 145: Table was marked as crashed and should be repaired" "MySQL error code 146: Lock timed out; Retry transaction" "MySQL error code 147: Lock table is full; Restart program with a larger locktable" "MySQL error code 148: Updates are not allowed under a read only transactions" "MySQL error code 149: Lock deadlock; Retry transaction" "MySQL error code 150: Foreign key constraint is incorrectly formed" "MySQL error code 151: Cannot add a child row" "MySQL error code 152: Cannot delete a parent row"
reference resources: https://blog.csdn.net/u012359618/article/details/51442966