詳談python中subprocess shell=False與shell=True的區別

shell=True參數會讓subprocess.call接受字符串類型的變量作為命令,並調用shell去執行這個字符串,當shell=False是,subprocess.call隻接受數組變量作為命令,並將數組的第一個元素作為命令,剩下的全部作為該命令的參數。

舉個例子來說明

from subprocess import call  
import shlex  
cmd = "cat test.txt; rm test.txt"  
call(cmd, shell=True)

上述腳本中,shell=True的設置,最終效果是執行瞭兩個命令

cat test.txt 和 rm test.txt

把shell=True 改為False,

from subprocess import call  
import shlex  
cmd = "cat test.txt; rm test.txt"  
cmd = shlex(cmd)  
call(cmd, shell=False)

則調用call的時候,隻會執行cat的命令,且把 “test.txt;” “rm” “test.txt” 三個字符串當作cat的參數,所以並不是我們直觀看到的好像有兩個shell命令瞭。

也許你會說,shell=True 不是很好嗎,執行兩個命令就是我期望的呀。但其實,這種做法是不安全的,因為多個命令用分號隔開,萬一檢查不夠仔細,執行瞭危險的命令比如 rm -rf / 這種那後果會非常嚴重,而使用shell=False就可以避免這種風險。

總體來說

看實際需要而定,官方的推薦是盡量不要設置shell=True。

補充: python subprocess模塊的shell參數問題

昨天調試其他同學的代碼時,發現對於subprocess模塊所傳的args變量,與shell變量存在關聯,傳值不當會有各種問題。比較有趣,就記錄一下。

根據subprocess模塊的args定義如下:

args is required for all calls and should be a string, or a sequence of program arguments. Providing a sequence of arguments is generally preferred, as it allows the module to take care of any required escaping and quoting of arguments (e.g. to permit spaces in file names). If passing a single string, either shell must be True (see below) or else the string must simply name the program to be executed without specifying any arguments.

對於args,可傳string,也可傳list,但當傳string時,shell的值必須設為True。

當shell為True時

If shell is True, the specified command will be executed through the shell. This can be useful if you are using Python primarily for the enhanced control flow it offers over most system shells and still want convenient access to other shell features such as shell pipes, filename wildcards, environment variable expansion, and expansion of ~ to a user’s home directory.

就是調用瞭系統的 sh 來執行命令(args的string),這樣會導致一些猥瑣的安全問題,類似於SQL Injection攻擊:

from subprocess import call
filename = input("What file would you like to display?\n")
What file would you like to display?
non_existent; rm -rf / #
call("cat " + filename, shell=True) # Uh-oh. This will end badly...

所以,安心用shell=False吧,記得args傳list。

以上為個人經驗,希望能給大傢一個參考,也希望大傢多多支持WalkonNet。如有錯誤或未考慮完全的地方,望不吝賜教。