Bash Index : U - The 'U' Bash commands : description, flags and examples

• provided ufw is only a frontend to iptables (and unless you need some advanced features), there's no real gain using one or the other : stick to the one you're comfortable + efficient with
• if you already know and use iptables, ufw would only be a different tool to do the same thing (so if you're curious and have spare time, go for it)
• if you know none of these, ufw is a good starting point since it was designed to ease iptables firewall configuration

1. there is a "default maximum permissions" value :
   • for regular files : 666
   • for directories : 777
2. subtract the mask value (often defaults to 022) from this "default maximum permissions" value to get the default permissions :
   • for regular files : 666 - 022 = 644 (i.e. rw-r--r--)
   • for directories : 777 - 022 = 755 (i.e. rwxr-xr-x)

• for everybody, in :
  • /etc/profile
  • /etc/bashrc
• for me only, in :
  • ~/.bashrc

editor - manual mode
	link best version is /usr/bin/vim
	link currently points to /usr/bin/vim.basic			current setting
	link editor is /usr/bin/editor
	slave editor.1.gz is /usr/share/man/man1/editor.1.gz		symlinks /usr/share/man/man1/editor.1.gz -> /etc/alternatives/editor.1.gz -> /usr/share/man/man1/vim.1.gz
	slave editor.fr.1.gz is /usr/share/man/fr/man1/editor.1.gz
	
/bin/nano - priority 40
	slave editor.1.gz: /usr/share/man/man1/nano.1.gz
/usr/bin/emacs24 - priority 0
	slave editor.1.gz: /usr/share/man/man1/emacs.emacs24.1.gz
/usr/bin/vim - priority 50
/usr/bin/vim.basic - priority 30
	slave editor.1.gz: /usr/share/man/man1/vim.1.gz
	slave editor.fr.1.gz: /usr/share/man/fr/man1/vim.1.gz
	
/usr/bin/vim.tiny - priority 15
	slave editor.1.gz: /usr/share/man/man1/vim.1.gz
	slave editor.fr.1.gz: /usr/share/man/fr/man1/vim.1.gz
	

1. As root : update-alternatives --auto editor

   update-alternatives: using /usr/bin/vim to provide /usr/bin/editor (editor) in auto mode

2. update-alternatives --display editor

   editor - auto mode
   	link best version is /usr/bin/vim
   	link currently points to /usr/bin/vim
   	link editor is /usr/bin/editor
   	

There are 5 choices for the alternative editor (providing /usr/bin/editor).

  Selection    Path                Priority   Status
------------------------------------------------------------
* 0            /usr/bin/vim         50        auto mode
  1            /bin/nano            40        manual mode
  2            /usr/bin/emacs24     0         manual mode
  3            /usr/bin/vim         50        manual mode
  4            /usr/bin/vim.basic   30        manual mode
  5            /usr/bin/vim.tiny    15        manual mode

Press <enter> to keep the current choice[*], or type selection number: 1
update-alternatives: using /bin/nano to provide /usr/bin/editor (editor) in manual mode

1. As root : update-alternatives --set editor /usr/bin/vim.basic

   update-alternatives: using /usr/bin/vim.basic to provide /usr/bin/editor (editor) in manual mode

2. update-alternatives --display editor

   editor - manual mode
   	link best version is /usr/bin/vim
   	link currently points to /usr/bin/vim.basic
   	link editor is /usr/bin/editor
   	

• usermod -aG groupName kevin
  groupName is appended to the list of Kevin's secondary groups
• adduser kevin groupName
  details : man -P 'less -p "Add an existing user to an existing group"' adduser

• usermod -s /bin/bash stuart
• usermod -s /usr/sbin/nologin stuart : forbid login to Stuart

file /usr/sbin/{useradd,adduser}

/usr/sbin/useradd: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=493f6bd6c372a7a9a93e26de333ee303d3ed9e53, stripped
/usr/sbin/adduser: Perl script text executable

• useradd is a native binary compiled with the system
• adduser is a Perl script which uses useradd binary in back-end. adduser is more user friendly and interactive than its back-end useradd. There's no difference in features provided.

useradd -d /home/bob -g primaryGroup -m bob

useradd -d /home/bob -s /bin/bash -m bob

This way, the user account is created as disabled. (shown by the ! right after the login name in /etc/shadow) To enable it, unlock the account (blank password !!!) then login as this new user and set a password. (or, as root : passwd bob)

username='bob'; homeDir="/home/$username"; uid='7807'; gid='7800'; useradd --home "$homeDir" --uid "$uid" --gid "$gid" -m "$username"

useradd -G admins,ftp,www,developers stuart

Debian style :

Red Hat style :

1. adduser [other options] bob
2. passwd -l bob

• the ! prefixing the RegExp instructs Awk to apply on lines NOT matching the RegExp (lines not starting with total, here)
• here, uniq is somewhat equivalent to a SQL select distinct ... ;

echo -e "aaa\nbbb\naaa\nccc" | uniq -d

(nothing)

Nothing found because duplicates are not adjacent

echo -e "aaa\naaa\nbbb\nccc" | uniq -d

aaa

echo -e "aaa\naaa\nbbb\nccc" | sort | uniq -d

aaa

echo -e "aaa\nbbb\naaa\nccc" | uniq -u

aaa
bbb
aaa
ccc

echo -e "aaa\naaa\nbbb\nccc" | uniq -u

bbb
ccc

echo -e "aaa\naaa\nbbb\nccc" | sort -u

aaa
bbb
ccc

A hard limit can only be raised by root (any process can lower it). So it is useful for security: a non-root process cannot overstep a hard limit. But it's inconvenient in that a non-root process can't have a lower limit than its children.

A soft limit can be changed by the process at any time (to the extent permitted by the hard limit unless running as root of course). So it's convenient as long as processes cooperate, but no good for security.

1. (since limits can be set per user) : su - bob
2. soft limit : ulimit -Sn
3. hard limit : ulimit -Hn