Bash for loop to execute commands on remote servers

Question

So I have a list of servers with n length. I need to open a connection and edit a file and close it.

Here's what I currently have:

#!/bin/bash

server_list=(a b c d)

for i in "${server[@]}"; do ssh "${server[@]}"; cd /etc; cp file file.bak; perl -pi -i 's/find/replace/g' file; exit; done

The only issue I have is that I can't exit the ssh connection and move on to the next in the array. I have used the -n, -t and -T options to no avail.

Thanks.

Would it be possible for you to NOT have a backup on your remote box, perform your global replace locally the just do scp file "${server[@]}/etc/" — NastyDiaper
– NastyDiaper, Commented Feb 3, 2017 at 21:45
The important thing to understand here is that ssh "${server[@]}" is a distinct command from cd /etc, and as your code is currently written, cd /etc won't be run until ssh "${server[@]}" exits. — Charles Duffy
– Charles Duffy, Commented Feb 3, 2017 at 21:48

codeforester · Accepted Answer · 2017-02-03 21:59:36Z

1

Your current code isn't sending any commands to the ssh sessions. Use a heredoc to pass your commands into ssh:

    #!/bin/bash

    server_list=(a b c d)
    for i in "${server_list[@]}"; do
      #
      # As per Charles' suggestion - "bash -s" makes sure the commands
      # would run with Bash rather than the default shell on the remote
      # server.
      #
      # I have left your commands exactly as in your question.
      # They can be written as a single command as per @chepner's recommendation 
      ssh "$i" bash -s << "EOF"
        cd /etc
        cp file file.bak
        perl -pi -i 's/find/replace/g' file
        exit
EOF
    done

edited Feb 3, 2017 at 21:59

answered Feb 3, 2017 at 21:43

codeforester

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17 Comments

Charles Duffy Over a year ago

Do you really mean to have everything but the EOF (shebang included) four-spaces off the margin? I tried to edit, but think you had something going on concurrently, so we ended up stomping on each other there.

chepner Over a year ago

@CharlesDuffy That might have been me; I was trying to remove the -n option from ssh that would have prevented the heredoc from being read.

chepner Over a year ago

The entire remote script could be reduced to perl -pi.bak 's/find/replace/g' /etc/file, rending the heredoc unnecessary.

Charles Duffy Over a year ago

BTW, I'd suggest ssh "$i" "bash -s", so you aren't relying on the user's configured default interpreter. I mean, for all we know they could be using something completely noncompliant like fish.

chepner Over a year ago

@CharlesDuffy That's the kind of detail I think one can ignore unless the OP brings it up.

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