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fouric
Asked: 2013-11-08 22:03:00 +0800 CST

What does "${debian_chroot:+($debian_chroot)}" do in my terminal prompt?

  • 772

In my terminal prompt definition in my .bashrc file, among other things, I have this snippet of code:

${debian_chroot:+($debian_chroot)}

What does this do, and do I need it?

bash

4 Answers

  1. Best Answer

    The important part to answer this question is this snippet from /etc/bash.bashrc:

    if [ -z "$debian_chroot" ] && [ -r /etc/debian_chroot ]; then
    debian_chroot=$(cat /etc/debian_chroot)
fi

    It means if the variable $debian_chroot is empty and the file /etc/debian_chroot exists and is readable the variable is set to the content of the file.

    Now what is this for? The file /etc/debian_chroot is when you have a chrooted debian system inside another debian system (ubuntu is based on debian). So this is for a better overview. To distinguish whether you are in the chroot or not.

    When you have a chroot of another system for example in /srv/nfs4/netboot/ you can set a name for this chroot in /srv/nfs4/netboot/etc/debian_chroot (in my case it's a nfs4 pxe netboot drive):

    user@host:~# echo "netboot" >/srv/nfs4/netboot/etc/debian_chroot

    And then when you chroot inside:

    chroot /srv/nfs4/netboot/

    Your prompt looks like this:

    (netboot)user@host:~#
    • 116

  2. Generally, ${var:+value} means:

    if $var is defined and not null; then use 'value'; else do nothing

    The debian_chroot variable is defined in /etc/bash.bashrc file. It takes the content of /etc/debian_chroot file if this file exists and is readable. By default this file doesn't exists.

    For more details, see:

    Now, to understand better what exactly it is happening there, do the following in terminal:

    radu@Radu:~$ PS1='${var:+($var)}\u@\h:\w\$ '
radu@Radu:~$ var="test"
                  ----
                   |
  ------------------
  |
  V
(test)radu@Radu:~$ var=""
radu@Radu:~$ var="and so on"
(and so on)radu@Radu:~$
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  3. If the environment variable $debian_chroot exists and is not empty ${debian_chroot:+($debian_chroot)} is replaced by ($debian_chroot) (that is the value of $debian_chroot with parens around it).

    $debian_chroot is set in /etc/bash.bashrc to the contents of /etc/debian_chroot if that file exists (it doesn't by default) and $debian_chroot doesn't have a value yet.

    ${debian_chroot:+($debian_chroot)} is usually used to define your Bash prompt, for example

    PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '

    As the name suggests you can use this variable to indicate which chroot you are in by placing etc/debian_chroot into your chroot root folders.

    If you don't know what a chroot is chances are you don't need that ;-) But you still may abuse it to include some other information into your Bash prompt

    By default it doesn't do anything.

    • 21

  4. If you never need to use debian_chroot then it's a handy place to put the time the command prompt was displayed by using:

    export PROMPT_COMMAND='debian_chroot=$(date +%r)'

    Type this in your terminal and watch your command prompt change with the time:

    rick@alien:~$ export PROMPT_COMMAND='debian_chroot=$(date +%r)'

(09:14:59 PM)rick@alien:~$

    After the time is set once, to get a running clock which updates every second use:

    while sleep 1;do tput sc;tput cup $(($(tput lines)-1)) 1;printf `date +%r`;tput rc;done &
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