Overview of the Command: Comparing Processes Between Local and Remote Systems

The command provided:

diff (ps axo comm) (ssh ps axo comm)

is a creative use of process substitution in Bash, aimed at comparing the output of the ps axo comm command across two machines.

Breaking Down the Command

Let's break down the command into its parts:

ps axo comm: This part of the command displays a list of all running processes on the local machine, showing only their command names. ssh ps axo comm: This part connects to a remote machine via SSH and runs the process list command, listing the command names of processes running on that remote host. (): Known as process substitution, it allows the output of a command to be treated as a file. This creates a temporary file containing the output of the local ps axo comm and the remote ps axo comm commands. diff: This command is used to compare the contents of two files and show the differences.

Summary

The entire command:diff (ps axo comm) (ssh ps axo comm)

compares the command names of processes running on the local machine with those running on a remote machine specified by . The output of diff will highlight any differences based on the command names.

Explanation with a Typo Fix

Correcting the typo in the second part of the command, we get:

diff (ps axo comm) (ssh ps axo comm)

Breaking it down step-by-step:

ps axo comm: This command shows the command names of all processes running on the local system. ssh ps axo comm: This command runs the same ps axo comm on a remote system and sends the output back. (...): Process substitution creates a named pipe with a random name to temporarily store the output of each command. diff ... ...: The diff command compares the contents of the two temporary files, showing the differences.

The bash trick behind this involves:

Creating a named pipe by outputting the result of the command to a temporary file. Substituting the command with the temporary file name.

A simplified version of the above command is:

ps axo comm > /tmp/FILE1 ssh ps axo comm > /tmp/FILE2 diff /tmp/FILE1 /tmp/FILE2 rm /tmp/FILE1 /tmp/FILE2

Displaying Sorted Output

However, the default sorting by PID in the ps command might not be very informative for diff. A more useful approach is to sort the output by command name and display it side-by-side:

diff -y (ps axo comm | sort -k2) (ssh ps axo comm | sort -k2)

This command:

Sorts the output of ps axo comm and ssh ps axo comm by the second column (command name). Then compares the side-by-side output using the -y option of diff, which displays the differences in a 2-column format.

By displaying the differences in a side-by-side format, you can more easily see which processes are running locally and not on the remote host or vice versa.

Conclusion

In summary, using diff with process substitution allows you to compare the process lists of a local and a remote Linux machine. This is a useful technique for system administrators to check for discrepancies between two systems.

Note: Ensure SSH access and proper permissions are set up to avoid any security issues.