Execute a command on multiple hosts, but only print command if it is successful?

Question

Here's what I want to do.

I want to check over 100 hosts and see if a file exists on that host. If the file does exist, then I want to print the hostname and the output of the command.

In this example example, assume that I have three hosts: host1.example.org host2.example.org host3.example.org . The file /etc/foobar exists on host2.example.org, but not on host1.example.org or host3.example.org .

1. I want to run ls -l /etc/foobar on each host in the list.
2. If this file exists on that host, then print the hostname and the output of the command.
3. If the file does not exist on that host, then don't print anything. I don't want the extra noise.

HOSTLIST="host1.example.org host2.example.org host3.example.org"
for HOST in $HOSTLIST
do
    echo "### $HOST"
    ssh $HOST "ls -ld /etc/foobar"
done

Ideal output would be:

### host2.example.org
drwx------ 3 root root 4096 Apr 10 16:57 /etc/foobar

But the actual output is:

### host1.example.org
### host2.example.org
drwx------ 3 root root 4096 Apr 10 16:57 /etc/foobar
### host3.example.org

I don't want the lines for host1.example.org or host3.example.org to print.

I am experimenting braces to contain the output spit out by echo and ssh, but I can't figure out the magic syntax to do what I want. I am sure that I have done this in the past without control characters,

HOSTLIST="host1.example.org host2.example.org host3.example.org"
for HOST in $HOSTLIST
do
    # If 'ls' shows nothing, don't print $HOST or output of command
    # This doesn't work
    { echo "### $HOST" && ssh $HOST "ls -ld /etc/foobar" ; } 2>/dev/null
done

Solution 1

in this issue i recommend use pssh. Thx pssh you could very easy run command on many remote servers at once.

put host into (i.e hosts_file)- each server in 1 line like:
host1.tld
host2.tld

Usage:

pssh -h hosts_file "COMMAND"

in you example it will be

pssh -h hosts_file "ls -l /etc/foobar"

Solution 2

This worked for me:

for HOST in $HOSTLIST; do
  ssh $HOST '[ -f /etc/passwd ] && echo $(hostname) has file'
done

Solution 3

Check out clusterssh to easily execute a command on may systems. Be extremely careful with executing commands on so many systems at once, it you make a unfortunate typo all servers will be affected.

The command you want to execute would go something like (refer to bash manual page for details):

[ -e some_existing_file ] && { hostname; ls -l; }

Solution 4

set -- host1.example.org host2.example.org
for host; do
        ssh "$host" sh -c '[ -e /etc/foobar ] && { printf %s\\n "$1"; ls -ld /etc/foobar; }' _ "$host"
done

View more solutions

Comments

[jw013]

Avoid using all UPPERCASE variable names. Such names like PAGER, HOSTNAME, PATH, etc. are often reserved or may be reserved in the future by the shell for special purposes and clobbering them may have unintended side effects.

[Mark Norgren]

@jw013 : Interesting. I thought that was just a convention. Do you have an authoritative source which recommends that I avoid all uppercase names, since it's not mentioned in the BASH manual

[jw013]

Not really, just the folk wisdom of #bash and the fact that all the variables that are special to the shell have uppercase names.

[jw013]

Here's a source from the Wooledge Wiki.

[rogerdpack]

See serverfault.com/q/2533/27813