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Unix Quick Reference

This is just a location where I store various commands I found handy for Unix.

Shell (mainly BASH)

Count all files in directory tree(s)

I was actually using this to count files in a maildir type directory. I needed to know how many total e-mails each user had, then I wanted to know how many they stored in their Inbox.

At a different domain, I needed to know only specific users. They all had account names of the form 'mca-something' so, since 'mca' is pretty uncommon, I just grep'd that (could have used egrep '^mca' even better, I guess).

Note: this is really not accurate as most IMAP servers store several configuration and control files in the directory, but since that is 2-5 per directory, and I had users storing tens of thousands of e-mails in the Inbox, I didn't break it down any further. You can always look a the Maildir and see some kind of pattern to send to egrep if you want more accuracy.

# count all files in all subdirectories
for dir in `ls`; do echo -n " $dir " ; find $dir -type f | wc -l ; done
# count all files in all specific subdirectories identified by a pattern (mca)
for dir in `ls | grep mca`; do echo -n " $dir " ; find $dir -type f | wc -l ; done
# find inn a subdirectory, ie the Inbox
for dir in `ls`; do echo -n " $dir " ; find $dir/Maildir/cur -type f | wc -l ; done


Create new key, no passphrase

Create a new rsa key with no passphrase. Useful when you want two machines to talk to each other using automated processes, though it is very insecure if the primary storage is ever disabled.

  • -C parameter allows you to define a comment (default is user@hostname)
  • -t defines the type of key to create (rsa, dsa, etc…)
  • -b is the number of bits to use in the key. Some keys (dsa) have a fixed size. Larger number of bits is harder to crack, but uses more resources.
  • -f define the file to create for the private key. The public key will be the same, with .pub added
ssh-keygen -t rsa -b 4096 -f id_rsa -C 'new comment for key' -N ''

Create missing host keys

Create new host keys (run by root only). When a Unix system is initially set up, several ssh keys are created to identify the system. The following command allows you to change those. The one command will generate all key pairs which are not already created, with an empty passphrase for the private keys.

# enter root password to become root
ssh-keygen -A
exit # return to unprivileged user

Upgrade existing private key storage

Upgrade existing rsa private key to newer storage format. This only affects the encryption on the private key. It does not alter the key at all, so it still works as you are used to

ssh-keygen -p -o -f ~/.ssh/id_rsa

Change passphrase and/or comment on existing private key

  • -p tells it to change the passphrase
  • -c tells it to change the comment
  • -f tells which file contains the information
ssh-keygen -p -c -f ~/.ssh/id_rsa

Using multiple key pairs

You can have multiple key pairs for a single user, by simply generating them with different file names, then passing the -i (identity) flag on the command line. WARNING: if you mess up the -f parameter, you can end up overwriting your default, which is stored as id_rsa (or something similar), so back up your stuff first. The following example assumes rsa.

# make a copy in case we mess up
cp ~/.ssh/id_rsa ~/.ssh/id_rsa.original
cp ~/.ssh/ ~/.ssh/
# generate two new keys for two separate applications
ssh-keygen -t rsa -b 4096 -f id_rsa.server1 -C 'key for server1' -N 'passphrase for this key'
ssh-keygen -t rsa -b 4096 -f id_rsa.server2 -C 'key for server2' -N 'passphrase for this key'

Copy to server1:~/.ssh/authorized_keys, and to server2:~/.ssh/authorized_keys

To go to a machine named server, which uses the default, simply execute

ssh server

however, to go to server1, using its separate key pair

ssh -i "~/.ssh/id_rsa.server1" server1

and do something similar for server2

See config file section for a way to make it easier

using the config file

There are two files which, by default, allow you to make life easier on yourself when using ssh. ~/.ssh/config is local, and /etc/ssh/ssh_config is global. The global file location may be different for other operating systems, but I haven't run into that yet.

We'll concentrate on the local config file. Basically, this is a standard text file, with restrictive permissions (0600). The file contains a stanza which begins with the keyword Host (case insensitive), followed by multiple line which set parameters for ssh when called. Each line is a keyword, as space, and a value.

Host myshortname
# use this for myother server
Host myother
   IdentityFile ~/.ssh/realname2_rsa
   User remoteusername
   Port 43214

The example lists two entries. Note that whitespace is ignored, so indentation is done in the second one to make it easier to read by humans.

The first stanza simply creates an alias (myshortname) for a connection. Issuing the command

ssh myshortname

uses the default identity file (~/.ssh/id_rsa), username (your current user name) and port (22) to connect to

The second does an override of several parameters. The following two commands are equivilent:

ssh myother
# is the same as
ssh -p 43214 -i "~/.ssh/realname2_rsa"

There are pages and pages of options by running the man sh_config command, where you can include other files, set X11 forwarding, basically everything.

NOTE: I especially like this since I always get ssh -p and scp -P mixed up, and programs which use ssh (rsync, etc…) will use this file.


quickreference/unix.txt · Last modified: 2018/08/26 23:11 by rodolico