After I finish to install ORACLE, then there is the step that I have to register some environment values. One of them is PS1.
export PS1=$'\\n[$LOGNAME#\h:$ORACLE_SID]'
Kind of an explanation is "User OS prompt setting variable".
I can't understand when it is used and the variable is also quite weird.
Does anyone have an idea for it?
"have to" - says who? It is not a mandatory requirement.
5.1 Bourne Shell Variables
PS1 - The primary prompt string. The default value is ‘\s-\v\$ ’. See
Controlling the Prompt, for the complete list of escape sequences that
are expanded before PS1 is displayed.
6.9 Controlling the Prompt
\n - A newline.
\h - The hostname, up to the first '.'.
LOGNAME (login name) and ORACLE_SID are custom environment variables.
Related
EDIT: Original title said "bash prompt", but I use "zsh". I accepted #Artur R. Czechowski's answer because it was correct in a bash context and I was able to make mine work with his help. PROMPT is now ' ${ENV}${ENV:+ } %F{249}${PWD/#$HOME/~} %{$fg[green]%}'○' ', just adding the bit about ENV was all I needed.
ORIGINAL POST: As part of my job, I change environment variables frequently. So frequently that I feel I'm going to inevitably going to forget which one I'm in and do something I shouldn't. In case it's relevant to you answering my question, I change environment by running a shell script and my current bash prompt is PROMPT=' %F{249}${PWD/#$HOME/~} %{$fg[green]%}'○' '.
If I'm in a virtual environment, such as venv, then my prompt gets prefaced with (.venv). How do I get something similar with an environment variable such as DB_HOST or whatever variable I want? If DB_HOST changes, I want the bash prompt to change right away and persist.
PROMPT_COMMAND is your answer. Example:
myprompt() {
PS1="${DB_HOST}${DB_HOST:+ }\u#\h:\w\$ "
}
PROMPT_COMMAND=myprompt
It will always display current value of DB_HOST variable.
I recently discovered that the # sign indicates that you're root on the shell (at least in a bash shell), and the $ sign indicates that you're not.
What is the name of this sign and does it really have the meaning that I give to it?
Is it only a bash thing?
Example with default bash configuration on Ubuntu:
john#mycomputer /tmp $ echo "I'm a simple user"
mycomputer /tmp # echo "I'm the root user"
There does not appear to be a formal definition of the prompt character indicating your privileges.
This thorough answer to “What is the origin of the UNIX $ (dollar) prompt?” provides lots of historical background, but it does not have a name for this indicator either.
In the absence of an official term, I'd call this the “prompt privilege indicator” or the “prompt indicator symbol” or even just “prompt indicator.”
All shells have a prompt whose default string ends in an indicator symbol, which (by default) almost always indicates whether you are root. Most shells allow you to change the prompt and most shells have a way to specify the prompt indicator symbol.
Here's what you can find in the various shell docs and standards:
A segment of the bash man page:
PROMPTING
When executing interactively, bash displays the primary prompt PS1 when
it is ready to read a command, and the secondary prompt PS2 when it
needs more input to complete a command. Bash displays PS0 after it
reads a command but before executing it. Bash allows these prompt
strings to be customized by inserting a number of backslash-escaped
special characters that are decoded as follows:
…
\$ if the effective UID is 0, a #, otherwise a $
From the zsh man pages:
Shell state
%# A `#' if the shell is running with privileges, a `%' if not.
Equivalent to `%(!.#.%%)'. The definition of `privileged', for
these purposes, is that either the effective user ID is zero,
or, if POSIX.1e capabilities are supported, that at least one
capability is raised in either the Effective or Inheritable
capability vectors.
The POSIX standard doesn't even specify what the privileged prompt symbol should be:
PS1
Each time an interactive shell is ready to read a command, the value of this variable shall be subjected to parameter expansion and written to standard error. The default value shall be $ . For users who have specific additional implementation-defined privileges, the default may be another, implementation-defined value.
See also the dash man page (dash is a popular basic POSIX shell, often installed as /bin/sh):
PS1 The primary prompt string, which defaults to ``$ '', unless you
are the superuser, in which case it defaults to ``# ''.
POSIX and dash don't have anything dynamic in what you can set $PS1 to. The default prompt is set once and is replaced by any assignments to the PS1 variable, losing the ability to distinguish between root and unprivileged users with a single character. You'd have to write your own code to determine if you're privileged and use its output in your PS1 assignment.
It's called a prompt. These days, they can be configured in many different ways. This related question gives some background.
I happen to run some commands blindly, in order to get things done.
I started to work with Jenkins recently, and then I had to use this export command to run the Jenkins WAR archive. What does the export command do in general, and why do we need to run this command, while running Jenkins (after the Jenkins home is set)?
export in sh and related shells (such as Bash), marks an environment variable to be exported to child-processes, so that the child inherits them.
export is defined in POSIX:
The shell shall give the export attribute to the variables corresponding to the specified names, which shall cause them to be in the environment of subsequently executed commands. If the name of a variable is followed by = word, then the value of that variable shall be set to word.
I guess you're coming from a Windows background. So I'll contrast them (I'm kind of new to Linux too). I found a user's reply to my comment, to be useful in figuring things out.
In Windows, a variable can be permanent or not. The term environment variable includes a variable set in the cmd shell with the SET command, as well as when the variable is set within the Windows GUI, thus set in the registry, and becoming viewable in new cmd windows.
E.g., the documentation for the set command in Windows "Displays, sets, or removes environment variables. Used without parameters, set displays the current environment settings."
In Linux, set does not display environment variables. It displays shell variables which it doesn't call/refer to as environment variables. Also, Linux doesn't use set to set variables (apart from positional parameters and shell options, which I explain as a note at the end), only to display them and even then only to display shell variables. Windows uses set for setting and displaying, e.g., set a=5, but Linux doesn't.
In Linux, I guess you could make a script that sets variables on bootup, e.g., /etc/profile or /etc/.bashrc, but otherwise, they're not permanent. They're stored in RAM.
There is a distinction in Linux between shell variables, and environment variables. In Linux, shell variables are only in the current shell, and environment variables, are in that shell and all child shells.
You can view shell variables with the set command (though note that, unlike Windows, variables are not set in Linux with the set command).
set -o posix; set (doing that set -o posix once first, helps not display too much unnecessary stuff). So set displays shell variables.
You can view environment variables with the env command.
Shell variables are set with, e.g., just a = 5.
Environment variables are set with export. Export also sets the shell variable.
Here you see shell variable zzz set with zzz = 5, and see it shows when running set, but it doesn't show as an environment variable.
Here we see yyy set with export, so it's an environment variable. And see it shows under both shell variables and environment variables:
$ zzz=5
$ set | grep zzz
zzz=5
$ env | grep zzz
$ export yyy=5
$ set | grep yyy
yyy=5
$ env | grep yyy
yyy=5
$
Other useful QnAs:
https://unix.stackexchange.com/questions/176001/how-can-i-list-all-shell-variables
https://askubuntu.com/questions/26318/environment-variable-vs-shell-variable-whats-the-difference
Note: One point which elaborates a bit and is somewhat corrective to what I've written, is that, in Linux bash, 'set' can be used to set "positional parameters" and "shell options/attributes", and technically both of those are variables, though the man pages might not describe them as such.
But still, as mentioned, set won't set shell variables or environment variables). If you do set asdf then it sets $1 to asdf, and if you do echo $1 you see asdf.
If you do set a=5 it won't set the variable a, equal to 5. It will set the positional parameter $1 equal to the string of "a=5". So if you ever saw set a=5 in Linux it's probably a mistake unless somebody actually wanted that string a=5, in $1.
The other thing that Linux's set can set, is shell options/attributes. If you do set -o you see a list of them. And you can do for example set -o verbose, off, to turn verbose on (by the way, the default happens to be off, but that makes no difference to this). Or you can do set +o verbose to turn verbose off. Windows has no such usage for its set command.
In simple terms, environment variables are set when you open a new shell session. At any time if you change any of the variable values, the shell has no way of picking that change. That means the changes you made become effective in new shell sessions.
The export command, on the other hand, provides the ability to update the current shell session about the change you made to the exported variable. You don't have to wait until new shell session to use the value of the variable you changed.
I have a .bashrc file with the following commands:
PS1="..."
export LC_ALL=...
What are these commands doing?
The first one sets the way your prompt looks (see my own post on pimping my prompt). In the case of the example you gave, it will cause your hostname, followed by a colon, followed by the current path to appear in the prompt. The second one sets the default language/localization settings to use the POSIX C configuration, rather than whatever the settings previously were. I strongly recommend changing the value from "C" to some variation of "utf-8" depending on your language, in order to support Unicode. Ex:
export LANG=en_US.utf-8
export LC_ALL=en_US.utf-8
PS1=.. Is setting the value of the prompt that is displayed
export LC_ALL is settiing an environment variable that will be available to programmes that bash executes. See http://pubs.opengroup.org/onlinepubs/7908799/xbd/envvar.html
The first one, provided it's exported or otherwise propagated to the user's shell process, sets the format of the command prompt, i.e. the little text at the beginning of the line that reads your input. Check out the "PROMPTING" section in man bash.
The second line exports the LC_ALL variable with value C, which sets your locale (to the standard "C" locale). If you experience internationalization-related problems, this may need to be changed.
While taking a look at this awesome thread I noticed that some examples use
PS1="Blah Blah Blah"
and some use
PROMPT_COMMAND="Blah Blah Blah"
(and some use both) when setting the prompt in a Bash shell. What is the difference between the two? A Stack Overflow search and even a bit of broader Google searching aren't getting me results, so even a link to the right place to look for the answer would be appreciated.
PROMPT_COMMAND can contain ordinary Bash statements whereas the PS1 variable can also contain the special characters, such as '\h' for hostname, in the variable.
For example, here is my Bash prompt that uses both PROMPT_COMMAND and PS1. The Bash code in PROMPT_COMMAND works out what Git branch you might be in and displays that at the prompt, along with the exit status of the last run process, hostname and basename of the pwd.
The variable RET stores the return value of the last executed program. This is convenient to see if there was an error and the error code of the last program I ran in the terminal. Note the outer ' surrounding the entire PROMPT_COMMAND expression. It includes PS1 so that this variable is reevaluated each time the PROMPT_COMMAND variable is evaluated.
PROMPT_COMMAND='RET=$?;\
BRANCH="";\
ERRMSG="";\
if [[ $RET != 0 ]]; then\
ERRMSG=" $RET";\
fi;\
if git branch &>/dev/null; then\
BRANCH=$(git branch 2>/dev/null | grep \* | cut -d " " -f 2);\
fi;
PS1="$GREEN\u#\h $BLUE\W $CYAN$BRANCH$RED$ERRMSG \$ $LIGHT_GRAY";'
Example output looks like this in a non-Git directory:
sashan#dhcp-au-122 Documents $ false
sashan#dhcp-au-122 Documents 1 $
And in a Git directory you see the branch name:
sashan#dhcp-au-122 rework mybranch $
Update
After reading the comments and Bob's answer, I think that writing it as he describes is better. It's more maintainable than what I originally wrote above, where the PS1 variable is set inside the PROMPT_COMMAND, which itself is a super complicated string that is evaluated at runtime by Bash.
It works, but it's more complicated than it needs to be. To be fair, I wrote that PROMPT_COMMAND for myself about 10 years ago and it worked and didn't think too much about it.
For those curious as to how I've amended my things, I've basically put the code for the PROMPT_COMMAND in a separate file (as Bob described) and then echo the string that I intend to be PS1:
GREEN="\[\033[0;32m\]"
CYAN="\[\033[0;36m\]"
RED="\[\033[0;31m\]"
PURPLE="\[\033[0;35m\]"
BROWN="\[\033[0;33m\]"
LIGHT_GRAY="\[\033[0;37m\]"
LIGHT_BLUE="\[\033[1;34m\]"
LIGHT_GREEN="\[\033[1;32m\]"
LIGHT_CYAN="\[\033[1;36m\]"
LIGHT_RED="\[\033[1;31m\]"
LIGHT_PURPLE="\[\033[1;35m\]"
YELLOW="\[\033[1;33m\]"
WHITE="\[\033[1;37m\]"
RESTORE="\[\033[0m\]" #0m restores to the terminal's default colour
if [ -z $SCHROOT_CHROOT_NAME ]; then
SCHROOT_CHROOT_NAME=" "
fi
BRANCH=""
ERRMSG=""
RET=$1
if [[ $RET != 0 ]]; then
ERRMSG=" $RET"
fi
if which git &>/dev/null; then
BRANCH=$(git branch 2>/dev/null | grep \* | cut -d " " -f 2)
else
BRANCH="(git not installed)"
fi
echo "${GREEN}\u#\h${SCHROOT_CHROOT_NAME}${BLUE}\w \
${CYAN}${BRANCH}${RED}${ERRMSG} \$ $RESTORE"
And in my .bashrc file:
function prompt_command {
RET=$?
export PS1=$(~/.bash_prompt_command $RET)
}
PROMPT_DIRTRIM=3
export PROMPT_COMMAND=prompt_command
From the GNU Bash documentation page (Bash Reference Manual):
PROMPT_COMMAND
If set, the value is interpreted as a command to execute before
the printing of each primary prompt ($PS1).
I never used it, but I could have used this back when I only had sh.
The difference is that PS1 is the actual prompt string used, and PROMPT_COMMAND is a command that is executed just before the prompt. If you want the simplest, most flexible way of building a prompt, try this:
Put this in your .bashrc file:
function prompt_command {
export PS1=$(~/bin/bash_prompt)
}
export PROMPT_COMMAND=prompt_command
Then write a script (Bash, Perl, or Ruby: your choice), and place it in file ~/bin/bash_prompt.
The script can use any information it likes to construct a prompt. This is much simpler, IMO, because you don't have to learn the somewhat baroque substitution language that was developed just for the PS1 variable.
You might think that you could do the same by simply setting PROMPT_COMMAND directly to ~/bin/bash_prompt, and setting PS1 to the empty string.
This at first appears to work, but you soon discover that the readline code expects PS1 to be set to the actual prompt, and when you scroll backwards in history, things get messed up as a result.
This workaround causes PS1 to always reflect the latest prompt (since the function sets the actual PS1 variable used by the invoking instance of the shell), and this makes readline and command history work fine.
From man bash:
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing each primary prompt.
PS1
The value of this parameter is expanded (see PROMPTING below) and used as the primary prompt string. The default value is ''\s-\v\$ ''.
If you simply want to set the prompt string, using PS1 alone is enough:
PS1='user \u on host \h$ '
If you want to do something else just before printing the prompt, use PROMPT_COMMAND. For example, if you want to sync cached writes to disk, you can write:
PROMPT_COMMAND='sync'
Yeah, so to try to really nail this down:
PROMPT_COMMAND is a handy Bash convenience variable/function, but there is, strictly speaking, nothing that cannot also be done using PS1 alone, correct?
I mean, if one wants to set another variable with scope outside the prompt: depending on the shell, that variable would probably need to be declared first outside $PS1 or (worst case) one might have to get fancy with something waiting on a FIFO prior to calling $PS1 (and armed again at the end of $PS1); the \u \h might cause some trouble, particularly if you're using some fancy regex; but otherwise: one can accomplish anything PROMPT_COMMAND can by using command substitution within $PS1 (and, maybe in corner cases, explicit subshells)?
Right?
The difference is that
if you output an incomplete line from PROMPT_COMMAND, it will screw your Bash prompt
PS1 substitutes \H and friends
PROMPT_COMMAND runs its contents, and PS1 uses its contents as the prompt.
PS1 does variable expansion and command substitution at each prompt. There isn't any need to use PROMPT_COMMAND to assign a value to PS1 or to run arbitrary code. You can easily do export PS1='$(uuidgen) $RANDOM' once in file .bash_profile. Just use single quotes.
I spent so much time on this I just wanted to share what worked for me. I looked at a lot of the SO posts about PROMPT_COMMAND and PS1 and tried many combinations of single quotes, double quotes, calling functions... I could not get the prompt to update each time without printing control characters or the literal expanded but not processed prompt string, or without just setting PS1 in PROMPT_COMMAND as we are advised not to do. My problem was setting variables (colors) that contained control characters; these had to be hard-coded after the variable name in PS1. PROMPT_COMMAND is set to a function that sets variables and they are used (escaped) in a double-quoted PS1 string. This is for a powerline-style prompt that changes colors with each command.
icon1=#unicode powerline char like
#these: https://github.com/ryanoasis/powerline-extra-symbols#glyphs
icon2=#same
#array of ANSI colors. 2 for rgb mode then the rgb values
#then 'm' without '\]' control character. these are from
#the solarized theme https://ethanschoonover.com/solarized/
declare -a colors=(
"2;220;50;47m"
"2;203;75;22m"
"2;181;137;0m"
"2;133;153;0m"
"2;42;161;152m"
"2;38;139;210m"
"2;108;113;196m"
"2;211;54;130m"
"2;0;43;54m"
"2;7;54;66m"
"2;88;110;117m"
"2;101;123;131m"
"2;131;148;150m"
"2;147;161;161m"
)
#outside of vars set in PROMPT_COMMAND it's ok to have control chars
LEN=${#colors[#]}
BG="\[\e[48;"#set bg color
FG="\[\e[38;"#set fg color
TRANSP="1m\]"#transparency
BASE2="2;238;232;213m\]"#fg (text) color
myfunc(){
RAND=$(($RANDOM % $LEN))
COLOR1=${colors[$RAND]}
COLOR2=${colors[($RAND + 1) % $LEN]}
COLOR3=${colors[($RAND + 2) % $LEN]}
}
PROMPT_COMMAND=myfunc
#note double quotes and escaped COLOR vars followed by hard-coded '\]' control chars
PS1="$BG$TRANSP$FG\$COLOR1\]$icon1$BG\$COLOR1\]$FG$TRANSP$BG\$COLOR1\]$FG$BASE2
[username hard-coded in unicode] $BG\$COLOR2\]$FG\$COLOR1\]$icon2$BG\$COLOR2\]$FG$BASE2
\w $BG\$COLOR3\]$FG\$COLOR2\]$icon2$BG\$COLOR3\]$FG$BASE2 [more unicode]
\[\e[0m\]$FG\$COLOR3\]$icon2\[\e[0m\] "
That ought to get you going!