Unix - eval command example

Have you ever have the need to remotely execute a long list of command to a list of unix machines to perform tasks like gathering their information? Well, if you did then you will mostly came across an issue where you ran out of quoting variables to quote the long list of command that you need to execute.

For example using "" (double quote) to execute a command uptime remotely.

root@myserver # ssh root@server1 "uptime"

If you need to execute uptime along with for example df and search for string "nfs" using awk you can use the below command.

root@myserver # ssh root@server1 "uptime;df |awk '/nfs/'"

So far the above two example are simple and doable without much issue as we have used double quote (") and single quote (') to wrap our command. What if you need to execute more command and ran of out quotes? Well one easy solution is to ssh multiple times to same machine to execute different sets of command. To some that might look troublesome and inefficient. Another more efficient way is to use eval command.

Eval Command. Eval is a built in linux or unix command. The eval command is used to execute the arguments as a shell command on unix or linux system.

How to use it? First save the commands that you need to execute in a file. You can use vi editor. Below is the content of the file which contains the needed commands to be executed on a unix machine.

root@myserver # cat commands.txt hostname;uptime;sar -u 3 3;df |awk '/nfs/';echo "hostname"

Next create a simple script to do what we need where in this case is to ssh remotely to a unix machine to executed the needed command contains in file commands.txt. Below is what the content of the script would look like.

root@myserver # cat script.sh commands=`cat commands.txt` ssh root@server1 eval $commands

Remember to set the script to executable with chmod and it is ready to be used.