$0 $1 $# $@ $* $? Detailed explanation of various symbols of $$

An overview,

There are two types of characters in the shell: normal characters and metacharacters.

1. Common characters

There is no special meaning in the Shell other than its literal meaning, that is, plain plain text;

2. Metacharacters

Is a reserved character of the Shell that has special meaning in the Shell.

Today I’ll focus on the various uses of the shell character $.

Escape the $character

The character $is often used in Linux shell scripts. Here are some common uses of $

Example 1

Create script peng.sh as follows:

#/bin/bash
total=$[ The $1 * $2 + $3 ]
echo "The $1 * $2 + $3 = $total"
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Run as follows:

./peng.sh 4 5 6
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The results are as follows:

Example 2

When entering a string into a shell script, use Spaces as delimiters, or double or single quotation marks if the string already has Spaces, as in

#/bin/bash
echo "The $1 is pretty nice!"
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Run as follows:

. / peng. Sh a LinuxCopy the code

The results are as follows:

Example 3

#! /bin/sh
echo "Parameter number:$#"
echo "Script name:$0"
echo "Parameter 1:The $1"
echo 2 "parameters:$2"
echo "List of all parameters:$@"
echo "pid:$$"
if [ The $1= 100]then
 echo "Command exit status: $?" 
 exit 0 The exit status is 0
else
 echo "Command exit status: $?"
 exit 1 Parameter error, exit state 1
fi
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Execution Result:

Example 4

Use the for loop for parameter traversal

Example:

#! /bin/bash
number=65             # define an exit value
index=1               Define a counter
if [ -z "The $1" ];then   If no parameter is entered, return the script usage and exit, exit value 65
   echo "Usage:$0+ parameters"
   exit $number
fi
echo "listing args with \$*:"    # Enter on screen, iterate through the parameters in $*
for arg in $*                                          
do
   echo "arg: $index = $arg"                 
   let index+=1
done
echo
index=1                         Reset the counter to 1
echo "listing args with \"\$@\":"    # loop through the arguments in "$@"
for arg in "$@"
do
   echo "arg: $index = $arg"
   let index+=1
done
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Example 5

Tip 1: The difference between the traversal parameters in “∗” and *” and ∗” and *

Example:

#! /bin/bash
number=11
if [ $# -eq 0 ];then
   echo "Usage: $0+ parameters"
   exit $number
fi
for i in $*               # iterates over the arguments in $*. Each argument is independent, and $# is iterated over
do
   echo $i
done
echo
for i in "$*"            # Iterate over arguments in "$*", where "$*" is expanded to a single string containing all positional arguments, traversed only once
do
   echo $i
done
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**$? * * that

Under Linux, whether you start a desktop program or run a command from the console, all programs return a numeric value at the end of the program. This value is called the return value, or Error Number.

1. In the console, there is a special environment variable $? Save the return value of the previous program, we can try:

Do a random command, like ls above, and then echo? To print? To print? To print? The value of the ~

We find that the return value is 0. What does that mean? As long as the return value is 0, the program executed successfully. That is, if $? A variable with a value other than 0 indicates an error in the execution of the previous command.

2. We can try to ls a file that does not exist:

This makes it easy to see if a command that prints a bunch of unfamiliar Information in English is successfully executed. But echo every time? I’m too tired. It’s okay. Can you? I’m too tired. It’s okay. Can you? I’m too tired. It’s okay. Can you? Put it in the prompt:

3. Place the return value of the last command execution success in the prompt:export PS1="[\$?] ${PS1}"

root@ubuntu:/home/peng/zhh# export PS1="[\$?] ${PS1}"
[0]root@ubuntu:/home/peng/zhh# ls
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export PS1=”[\$?] ${PS1}” Is placed at the top of the prompt, so that the value is automatically updated each time the command is executed

4. Use Perror to view error messages

So if the value returned is not zero, how do we know what went wrong? If no error message is displayed, use the perror program to check for error messages. For example, if the return value is 2, run the following command:

$ perror 2
OS error code   2:  No such file or directory
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The file does not exist

This command requires the installation of apt-get install perror

5. Give a comparison table of error codes and their specific causes:

"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"
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