Category: Shell

Unix system use a lot of environment variables. Most of them are initialized when you log in and are maintained until you log out. This is a list of the most usual/usefull variables to know. Keep in mind, depending on the OS/distribution and your shell, some may or may not exist:

PATH – List directories the shell searches for commands
HOME – User's home directory
TERM – Indicate which terminal emulator is used
PS1 – Shell prompt in the Bourne shell and variants
MAIL – Path to user's mailbox
TEMP – Path to directory where processes can store temporary files
TZ – Timezone settings
PWD – Path of the current directory
HISTFILE – Name of the file in which command history is saved
HISTFILESIZE - Maximum number of lines contained in the history file
HOSTNAME - System's host name
LD_LIBRARY_PATH - List of directories where libraries should be searched for
USER - Current logged in user's name
DISPLAY - Network name of the X11 display to connect to, if available
SHELL - Indicate current shell path
TERMCAP – Display termcap database entry for the current term
OSTYPE – Type of operating system
MACHTYPE – The CPU architecture that the system is running on
EDITOR – The user's preferred text editor
PAGER –	The user's preferred text pager
MANPATH – List of directories to search for manual pages

You can also list all the environement variables on your current session, using the printenv command.

GNU screen/tmux

The best solution is simply to use a terminal multiplexer, like GNU screen or tmux. Simply launch a session, tape your command and then detach it (Ctrl-a d for screen). Easy and clean 😉

setsid

Debian contains a binary called setsid in the util-linux package. setsid can be use to start a process and detach it from the current shell (basically it create a new shell for the ‘orphaned’ process).

setsid doesn’t redirect the standard files descriptors (stdin, stdout and stderr) so you loose any process output except if you make a stdout+stderr redirection to a file:

setsid <command> > /tmp/output.txt &2>1

nohup

nohup as the name implies, makes your command ignore SIGHUP signal. Also by default nohup redirects the standard output and error to the file nohup.out, so the program won’t fail for writing to standard output when the shell is closed. Note that nohup doesn’t remove the process from the shell’s job control and also doesn’t put it in the background. Usage:

nohup <command> > /tmp/output.txt &

disown

Last option (and the more interesting) is the built-in bash command disown. disown removes the process from the shell’s job control, but still leaves it connected to the terminal. The results is that the shell won’t send it a SIGHUP when closed, but in the meantime you still get the output. The advantage is you can disown a already running program.

Simply suspend the program using Ctrl-z then use bg to put it in background. Then detach it:

disown %n

where n is the job number (use the command job to get it).

It’s often very useful to add customized message (with info or specific commands) on a VM. The easiest way to do that is to use the /etc/motd file. But what to do if you want to add a message just for one user ?

The first thing that everybody try is to add the message to the .bashrc.
That’s a bad habit:

zapan:~$ ssh me@zapan 'uptime'
You shouldn't see this message !
 17:16:45 up 4 days,  7:29,  4 users,  load average: 1.81, 1.25, 0.83

As you can see the content is displayed in case of a non-interactive non-login shell invoked via ssh. Instead add your message to the .bash_profile or .profile file.

Further Reading and sources

• http://hacktux.com/bash/bashrc/bash_profile

stat displays the detailed status of file using inode data. Theses info include the date the file was last modified (like ls), the time the file was last changed and the time that file was last accessed:

# stat /var/lib/dpkg/lock 
  File: `/var/lib/dpkg/lock'
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: 806h/2054d      Inode: 57030       Links: 1
Access: (0640/-rw-r-----)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2011-08-13 12:17:00.000000000 +0200
Modify: 2012-03-08 16:03:20.000000000 +0100
Change: 2012-03-08 16:03:20.000000000 +0100

stat can also be use to get information on a partition, using the -f option:

# stat -f /dev/sda1 
  File: "/dev/sda1"
    ID: 0        Namelen: 255     Type: tmpfs
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 2560       Free: 2514       Available: 2514
Inodes: Total: 6181154    Free: 6180411

And don’t forget the most useful option for scripting --format:

# stat --format "%A" /var/log/syslog
-rw-r-----

If you want to change the setting for the current terminal:

loadkeys fr

For changing this setting permanently:

dpkg-reconfigure console-data

For changing keymap during a X session:

setxkbmap -layout fr

To mount persistent SSH tunnel, nothing is better than autossh. But there is a small inconvenient, it doesn’t really like being started in a runlevel (go figure why). The solution is to use the /etc/rc.local file for this task:

autossh -i /foobar/.ssh/id_dsa -L 83:192.168.3.32:80 root@moya.farscape.net -oBatchMode=yes -N

Here we make a persistent tunnel with a forwarding of local machine port 83 to the moya.farscape.net machine port 80, using the user foobar key to initialize the connection. autossh will monitor the ssh tunnel and restart it as needed.

One annoying thing with text file is that files created on DOS/Windows machines have different line endings than files created on Unix/Linux. DOS uses carriage return and line feed \r\n as a line ending which Unix uses just line feed \n. So in the end you finish with a lot of supernumerary CTRL-M characters. To get rid of it you can:

Use sed

sed -e "s/^M//" filename > newfilename

Use vi

Inside vi [in ESC mode] type: :%s/^M//g
To enter ^M, type CTRL-V, then CTRL-M (hold down the CTRL key then press V and M in succession).

Use dos2unix

dos2unix filename