保存子进程命令的错误消息

Question

使用子进程运行bash命令时,我可能会遇到命令无效的情况.在这种情况下,bash将返回错误消息.我们怎样才能抓住这条消息？我想将此消息保存到日志文件中.以下是一个示例,我尝试在不存在的目录中列出文件.

try:
    subprocess.check_call(["ls", "/home/non"]) 
    df = subprocess.Popen(["ls", "/home/non"], stdout=subprocess.PIPE)        
    output, err = df.communicate()
    # process outputs
except Exception as error:        
    print error
    sys.exit(1)

Bash会打印"ls:无法访问/ home/non:没有这样的文件或目录".我怎样才能收到此错误消息？except行捕获的错误明显不同,它说"Command'['ls','/ home/non']'返回非零退出状态2".

Answer 1

"ls:无法访问/ home/non:没有这样的文件或目录"是由ls命令生成的,而不是 bash这里.

如果要使用异常处理来处理不存在的文件,请使用subprocess.check_output():

#!/usr/bin/env python
from subprocess import check_output, STDOUT, CalledProcessError

try:
    output = check_output(['ls', 'nonexistent'], stderr=STDOUT)
except CalledProcessError as exc:
    print(exc.output)
else:
    assert 0

产量

ls: cannot access nonexistent: No such file or directory

Answer 2

您可以将stderr重定向到文件对象:

from subprocess import PIPE, CalledProcessError, check_call, Popen

with open("log.txt", "w") as f:
    try:
        check_call(["ls", "/home/non"], stderr=f)
        df = Popen(["ls", "/home/non"], stdout=PIPE)
        output, err = df.communicate()
    except CalledProcessError as e:
        print(e)
        exit(1)

输出到log.txt:

ls: cannot access /home/non: No such file or directory

如果你想要的信息除外:

try:
    check_call(["ls", "/home/non"])
    df = Popen(["ls", "/home/non"], stdout=PIPE)
    output, err = df.communicate()
except CalledProcessError as e:
    print(e.message)

对于python 2.6,e.message将无法正常工作.您可以使用check_output与python 2.6一起使用的类似版本的python 2.7 :

from subprocess import PIPE, CalledProcessError, Popen

def check_output(*args, **kwargs):
    process = Popen(stdout=PIPE, *args, **kwargs)
    out, err = process.communicate()
    ret = process.poll()
    if ret:
        cmd = kwargs.get("args")
        if cmd is None:
            cmd = args[0]
        error = CalledProcessError(ret, cmd)
        error.out = out
        error.message = err
        raise error
    return out

try:
    out = check_output(["ls", "/home"], stderr=PIPE)
    df = Popen(["ls", "/home/non"], stdout=PIPE)
    output, err = df.communicate()
except CalledProcessError as e:
    print(e.message)
else:
    print(out)