1 Linux:Comparing Remote with Local
2 =================================
3 :author: Aaron Ball
4 :email: nullspoon@iohq.net
5
6
7 == {doctitle}
8
9 Today I ran into quite the conundrum. I've been needing to compare the contents
10 of a file on a remote server with the contents of a file on my local server
11 over ssh. My original solution was to copy the file from one server to the
12 other over sftp and compare their contents with diff.
13
14 However, when you need to compare more than just the contents of a file, things
15 can get really messy. Take for instance another situation where I needed to
16 compare the contents of a directory. First, I needed to output the contents of
17 a directory on BOTH servers into two files I had to create. Then I had to sftp
18 into one server, copy it over to the other server, and run diff to compare
19 their contents. Talk about complicated, yeah?
20
21 All this copying and sftp-ing around frustrated me to the point where I wanted
22 to find another solution. Sadly, my solution is a bit complicated for someone
23 who doesn't know the linux command line super well, but at least it works and
24 it works fast. I'll go slowly because if I don't, I won't get it myself either.
25
26 ----
27 ssh username@ServerIP 'ls -1 /tmp' | diff <(ls -1 /tmp)
28 ----
29
30 Here's our example. The end result of this example is to get a comparison of
31 the contents of a remote /tmp directory and the contents of our local /tmp
32 directory.
33
34 First things first, we have to run a command remotely to get the contents of
35 said remote directory. To do this, we run simply
36
37 ----
38 ssh username@ServerIP 'ls -1 /tmp'
39 ----
40
41 That gets a list of the files and folders in the /tmp directory. Specifically,
42 the '-1' switch gives us one file or folder per line.
43
44 Next up we pipe that into the diff command.
45
46 For those of you who may not know about this functionality, piping basically
47 takes the output of one command, and feeds it to another. In this case, we are
48 taking the listed contents of a remote directory and feeding it to the diff
49 command. Now, we do this by using the following.
50
51 ----
52 ... | diff ...
53 ----
54
55 Basically, the diff command works by finding the difference between the first
56 thing it is given and the second thing it is given. Generally speaking, diff
57 works like the following.
58
59 ----
60 diff <file1> <file2>
61 ----
62
63 In this case we are saying diff which means to substitute what was piped in
64 with the -.
65
66 Up to this point, we have the contents of our remote directory and we have run
67 the diff command. All we need now is to give it the second input to compare our
68 first to. This brings us to our final step getting the contents of a local
69 directory.
70
71 This is about one of the most common linux command line functions performed.
72 However, due to the fact that we want to compare the contents of the directory
73 with the contents of another directory, things get a bit more complicated
74 sadly. Do accomplish this, we need to run a nested command.
75
76 Ordinarily running ls -1 /tmp after a diff command would result in an error
77 rather than giving us what we want. To substitute a command for a file and so
78 compare the command's output, we need to encase it in <(). Our final piece of
79 the command should look like this.
80
81 ----
82 <(ls -1 /tmp)
83 ----
84
85 This completes our command. If you try to run the entire thing, you should be
86 asked for your password to the remote server. Upon entering your password, the
87 command should run as expected, comparing the files and folders in the two
88 directories.
89
90 The final command again looks like this...
91
92 ----
93 ssh username@ServerIP 'ls -1 /tmp' | diff <(ls -1 /tmp)
94 ----
95
96 If you want to get really tricky, you can compare the contents of a
97 remote file and the contents of a local file. We'll take httpd.conf for
98 instance.
99
100 ----
101 ssh username@ServerIP 'cat /etc/httpd/conf/httpd.conf' | diff <(cat /etc/httpd/conf/httpd.conf)
102 ----
103
104 Hopefully that description wasn't too confusing. It's a complicated command to
105 run (probably the worst I have ever used actually), but with some practice, it
106 should become pretty easy if you understand how it works.
107
108 Let me know if I didn't describe anything well enough and I will do my best to
109 help out and update the post so it is more user friendly.
110
111 Thanks for reading!
112
113
114 Category:Linux
115 Category:SSH
116
117
118 // vim: set syntax=asciidoc:
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