以下內容轉自
https://blog.csdn.net/fengqingting2/article/details/41940149
https://blog.csdn.net/windone0109/article/details/8895875
https://blog.csdn.net/zgl07/article/details/43196823
今天遇到要在os.system中連續執行多條語句的情況，語句如下：
cmd1="cd /d %s" % os.path.dirname(file_path)
cmd2="python setup.py install"
如果分為兩次執行，即:os.system(cmd1) os.system(cmd2)，在執行第二條語句的時候會提示找不到setup.py文件，也就是沒有進入到相應的路徑，查閱了類似的帖子，有說明如下：
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你需要了解os.system的工作原理：
Execute the command (a string) in a subshell. This is implemented by calling the Standard C function system(), and has the same limitations...
關於子進程的創建需要明確兩點：
- 父進程的環境變量(environment variables)會默認傳遞到子進程中(工作目錄
PWD就是環境變量之一) - 使用
system函數，子進程無法影響父進程中的環境變量
根據這兩點，執行os.system(path)引發的工作目錄變更作用范圍僅限於第一個被創建的子進程，故而當前的工作目錄沒有變更，到執行os.system(upd)就會報錯。關於這點，可以用REPL驗證如下：
>>> import os
>>> os.getcwd()
'/Users/user1'
>>> os.system('cd /etc')
0
>>> os.getcwd()
'/Users/user1'
要達到預期的效果，如果仍使用os.system，有兩種方法。第一種方法是確保工作目錄的變更和svn都在子進程中進行，可以使用復合語句（如os.system('cd path-to-repo && svn ci')）或多個語句（如os.system('cd path-to-repo; svn ci')）。第二種方法則是先在父進程中切換工作目錄(os.chdir('path-to-repo'))，再利用1.中提到的原理，執行子進程即可(os.system('svn ci'))。
最后采用cmd="cd /d %s && python setup.py install" % os.path.dirname(file_path)的形式解決了問題，即采用&&連接多個命令
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兩者的區別是：
os.system(cmd)的返回值只會有0(成功),1,2
os.popen(cmd)會吧執行的cmd的輸出作為值返回。
python調用Shell腳本，有兩種方法：os.system(cmd)或os.popen(cmd),前者返回值是腳本的退出狀態碼，后者的返回值是腳本執行過程中的輸出內容。實際使用時視需求情況而選擇。
現假定有一個shell腳本test.sh：
#!/bin/bash
1. echo "hello world!"
2. exit 3
os.system(cmd):
該方法在調用完shell腳本后，返回一個16位的二進制數，低位為殺死所調用腳本的信號號碼，高位為腳本的退出狀態碼，即腳本中“exit 1”的代碼執行后，os.system函數返回值的高位數則是1，如果低位數是0的情況下，則函數的返回值是0×100,換算為10進制得到256。
如果我們需要獲得os.system的正確返回值，那使用位移運算可以還原返回值：
1. >>> n = os.system(test.sh)
2. >>> n >> 8
3. >>> 3
os.popen(cmd):
這種調用方式是通過管道的方式來實現，函數返回一個file-like的對象，里面的內容是腳本輸出的內容（可簡單理解為echo輸出的內容）。使用os.popen調用test.sh的情況：
python調用Shell腳本，有兩種方法：os.system(cmd)或os.popen(cmd),前者返回值是腳本的退出狀態碼，后者的返回值是腳本執行過程中的輸出內容。實際使用時視需求情況而選擇。
明顯地，像調用”ls”這樣的shell命令，應該使用popen的方法來獲得內容
以前就疑惑popen和system有什么不同，今天實際的應用，才讓我恍然大悟
os.popen（）可以實現一個“管道”，從這個命令獲取的值可以繼續被調用。而os.system不同，它只是調用，調用完后自身退出，可能返回個0吧
比如，我想得到ntpd的進程id，就要這么做：
os.popen('ps -C ntpd | grep -v CMD |awk '{ print $1 }').readlines()[0]
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問題：/bin/xxx.py是一個返回碼為1的程序。當python 程序使用os.system(”./bin/xxx.py”) 這樣調用的時候， 成功運行后os.system 的返回值出現了問題，變成了256 ，也就是0×100。而不是正常應該返回的1。
解決：查閱了文檔發現os.system()的返回為：
On Unix, the return value is the exit status of the process encoded in the format specified for wait().
而os.wait()的返回為：
a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number
is zero);
os.system的返回值並不是執行程序的返回結果。而是一個16位的數，它的高位才是返回碼。也就是說os.system()返回256即 0×0100，返回碼應該是其高位0×01即1。所以要獲取程序運行退出的值（比如C的main函數中的return 0），需要處理一下。
ret = os.system('./a.out')
ret >>= 8
這樣才能獲取到正確的返回值。另外還要注意：python獲取到的值是無符號整數，所以返回負值的時候，打印出來是很大的正值。比如返回-1，python 會獲取到255，-2則254，以此類推。所以最好就判斷是否為0就可以了，實在要判斷自己寫的c程序返回值，建議返回0,1,2,3等值，出錯返回 -1。
另外，我遇到一次明明處理好了返回值，c程序調試信息提示也該返回值0了，結果python獲取到的是 -1，而且無論c程序返回多少，python都獲取-1。后來排查c程序的問題，發現原來是因為我這個python程序本身是由另一個C程序調用的，而調 用它的那個C程序中將SIGCLD信號忽略了（這表明python是根據子進程退出時產生的信號來獲取返回值的），我將那個C程序的SIGCLD綁定到函 數，即使那個函數什么也不做，python也能獲取到正確的返回值了。
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linux命令執行后無論成功與否都有一個返回值：
如果為 0，則表示命令執行成功，其它值則表示錯誤，具體的錯誤碼含義如下：
"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"