Linux – the difference between renice and chrt commands in Linux

In your login shell's profile files you can set up some things that you will use during your session, and which only need to be done once. Some ideas:

• create a temporary file containing the IP address you connected from, later on you can include it in some scripts setting firewall rules.
• run ssh-agent, ask for your SSH keys, and store the SSH agent environment variables in a file.
• if that is a limited machine, and your co-workers want to be aware of each others logins, write(1) messages informing them of your login.

In a non-login shell's files (.bashrc) you should configure your shell:

• Fancy prompt
• set aliases
• set history options
• define custom shell functions
• export environment variables, (maybe PAGER, EDITOR if system-wide settings suck)
• load ssh-agent variables saved in .bash_profile

Usually, you would include .bashrc from .bash_profile with the following. Then login shell gets all your customizations, and .bash_profile file does not have to duplicate things that are already in .bashrc.

[[ -f ~/.bashrc ]] && . ~/.bashrc

A hard link traditionally shares the same file system structures (inode in unixspeak), while a soft-link is a pathname redirect.

• Hardlinks must be on the same filesystem, softlinks can cross filesystems.
• Hardlinked files stay linked even if you move either of them (unless you move one to another file system triggering the copy-and-delete mechanism). Softlinked files break if you move the target (original), and sometimes when you move the link (Did you use an absolute or relative path? Is it still valid?).
• Hardlinked files are co-equal, while the original is special in softlinks, and deleting the original deletes the data. The data does not go away until all hardlinks are deleted.
• Softlinks can point at any target, but most OS/filesystems disallow hardlinking directories to prevent cycles in the filesystem graph (with the exception of the . and .. entries in unix directories which are hard links).
• Softlinks can require special support from filesystem walking tools. Read up on readlink (2).

(Some details brought back to mind by mat1t. Thanks.)