This question already has answers here:
What is the use of eval `opam config env` or eval $(opam env) and their difference?
(2 answers)
Closed 9 months ago.
I keep seeing
eval $(opam env)
e.g. when I try to install coq:
# brew install opam # for mac
# for ubuntu
conda install -c conda-forge opam
opam init
# if doing local env?
# eval $(opam env)
# - install coq
# local install
#opam switch create . 4.12.1
#eval $(opam env)
#opam repo add coq-released https://coq.inria.fr/opam/released
#opam install coq
# If you want a single global (wrt conda) coq installation (for say your laptop):
opam switch create 4.12.1
opam switch 4.12.1
opam repo add coq-released https://coq.inria.fr/opam/released
opam install coq
but I never know what it does (nor no one I talk to knows) and when I google it this comes up instead: What is the use of eval `opam config env`?
cross posted: https://discuss.ocaml.org/t/what-does-eval-opam-env-do-does-it-activate-a-opam-environment/9990
Clarification
Note that
eval `opam config env`
might be very similar. In the sense that I think the dashes '' are also command substitution as in $(...) but not sure. Which might make What is the use of eval `opam config env`? very related (though I don't know the difference for sure)
tldr: it activates your opam context according to your current switch -- similar to how a python virtual env is activated in python but for opam.
What eval $(opam env) does is command substitute $(opam env) i.e. run opam env in a subshell (that's what $( ) does) return the string it outputted and then give it to eval to evaluate as bash code. Command substitution is usually done to next the output of commands in bash. In python eval would interpret the input string to it as literal python code to evaluate, parse, run here similarly it would do the same but assume it's bash (is my guess).
What does opam env does? It returns the current bash env variables for the current swithc (i.e. siwth approximately euqal to opam environment). So therefore, doing:
eval $(opam env)
does this:
first runs in a subshell because thats what $(cmd) does. It's command substitution. Usually used to nest commands.
then it's output (so the output string of $(opam env)) is given to eval. opam env returns a string of env variables needed to "activate: the current opam env (similar to how you activate virtual envs in python).
finally eval evaluates the string it receives from the command substitution i.e. it parses the string as a bash command and runs it.
For compeleteness see the output of opam env:
(iit_synthesis) brandomiranda~ ❯ opam env
OPAM_SWITCH_PREFIX='/Users/brandomiranda/.opam/4.12.1'; export OPAM_SWITCH_PREFIX;
CAML_LD_LIBRARY_PATH='/Users/brandomiranda/.opam/4.12.1/lib/stublibs:/Users/brandomiranda/.opam/4.12.1/lib/ocaml/stublibs:/Users/brandomiranda/.opam/4.12.1/lib/ocaml'; export CAML_LD_LIBRARY_PATH;
OCAML_TOPLEVEL_PATH='/Users/brandomiranda/.opam/4.12.1/lib/toplevel'; export OCAML_TOPLEVEL_PATH;
PATH='/Users/brandomiranda/.opam/4.12.1/bin:/Users/brandomiranda/miniconda/envs/iit_synthesis/bin:/Users/brandomiranda/miniconda/condabin:/usr/local/bin:/Users/brandomiranda/.opam/4.12.1/bin:/opt/homebrew/bin:/usr/bin:/bin:/usr/sbin:/sbin'; export PATH;
Comment on backard ticks:
Please note : The back ticks shown around opam env after eval are essential. They change the order of application, which is very important. The back ticks tells the system to first evaluate opam env (which returns a string of commands) and then eval executes those commands in the string. Executing them doesn’t return anything, but it initializes the Opam environment behind the scenes.
ref: https://ocaml.org/docs/up-and-running
also see:
What does eval $(opam env) do, does it activate a opam environment?
What is the use of eval `opam config env` or eval $(opam env) and their difference?
https://ocaml.org/docs/up-and-running
https://discuss.ocaml.org/t/what-does-eval-opam-env-do-does-it-activate-a-opam-environment/9990
All details that lead to this answer:
From the man page for opam env:
This is most usefully used as eval $(opam env) to have further shell commands be evaluated in the proper opam context.
see: https://opam.ocaml.org/doc/man/opam-env.html
Slightly more details:
Finally, to get all the pieces together, what is the difference between command substitution (e.g. $( ) or ) and evaluating an expression (e.g. eval string or eval $(cmd) ) (in bash mainly). Ref: https://unix.stackexchange.com/a/23116/55620. My understanding is that $( ) runs a command in a subshells and outputs the string so that it's string is an input to another command e.g. eval. So to my understanding you can think of $(cmd) as replacing that expression and pasting the string of it back to to the command line for another commadn e.g. eval. While eval will take in the string and evaluate the expression in it instead. So what eval $(cmd) does is first evaluate $(cmd) replace it with the output string of running cmd in a subshell and then feeding it to eval -- where eval will pretend that input is code and evaluate it, parse and run it. So $(...) is usually done to nest command inside other commands.
The only puzzling thing to me is why $(cmd) without a cmd2 (eg eval) in the front interprets the strings of $(cmd) as bash commands to run instead of just evaluting e.g.
(iit_synthesis) brandomiranda~ ❯ $(echo v=1)
zsh: command not found: v=1
but
(iit_synthesis) brandomiranda~ ❯ v=1
(iit_synthesis) brandomiranda~ ❯ echo $v
1
does evaluate my string v=1 and then run it.
Thus what eval $(opam env) does is command substitute $(opam env) i.e. run opam env in a subshell return the string it outputted and then given to eval to evaluate as bash code. In python eval would interpret the input string to it as literal python code to evaluate, parse, run here similarly it would do the same but assume its bash (is my guess)
Long answer with all details:
toolchain = In software, a toolchain is a set of programming tools that is used to perform a complex software development task or to create a software product, which is typically another computer program or a set of related programs. https://www.google.com/search?q=toolchain&oq=toolchain&aqs=chrome..69i57j0i512l9.209j0j7&sourceid=chrome&ie=UTF-8
(iit_synthesis) brandomiranda~ ❯ opam env
OPAM_SWITCH_PREFIX='/Users/brandomiranda/.opam/4.12.1'; export OPAM_SWITCH_PREFIX;
CAML_LD_LIBRARY_PATH='/Users/brandomiranda/.opam/4.12.1/lib/stublibs:/Users/brandomiranda/.opam/4.12.1/lib/ocaml/stublibs:/Users/brandomiranda/.opam/4.12.1/lib/ocaml'; export CAML_LD_LIBRARY_PATH;
OCAML_TOPLEVEL_PATH='/Users/brandomiranda/.opam/4.12.1/lib/toplevel'; export OCAML_TOPLEVEL_PATH;
PATH='/Users/brandomiranda/.opam/4.12.1/bin:/Users/brandomiranda/miniconda/envs/iit_synthesis/bin:/Users/brandomiranda/miniconda/condabin:/usr/local/bin:/Users/brandomiranda/.opam/4.12.1/bin:/opt/homebrew/bin:/usr/bin:/bin:/usr/sbin:/sbin'; export PATH;
so opam env outputs a bunch of paths that are needed for the toolchain to work...perhaps here toolchain == opam? Based on What is the use of eval `opam config env` or eval $(opam env) and their difference? (question for eval opam config env ) I am inferring that what it means is that these are the bash env variables needed to "activate the ocaml/opam environment". Concluded that because opam env outputs a bunch of env variables.
So running opam env by itself only prints the output to the terminal the output of the command opam env.
$ usually refers that the identifier is a variable. e.g. $x is the variable x while x in the terminal is literally the string x (or command x if it exists).
$(command) or command is command substitution. So doing $(command) runs the output (e.g. the script that would have been printed to the terminal after running command) of command. I assume it's called command substitution because we substitute command with the output of it and try to run it. e.g. it says: In computing, command substitution is a facility that allows a command to be run and its output to be pasted back on the command line as arguments to another command in the wikipedia article so yes. https://en.wikipedia.org/wiki/Command_substitution
opam env = opam-env - Prints appropriate shell variable assignments to stdout. Returns the bindings for the environment variables set in the current switch, e.g. PATH, in a format intended to be evaluated by a shell. https://opam.ocaml.org/doc/man/opam-env.html note that same opam man page says This is most usefully used as eval $(opam env) to have further shell commands be evaluated in the proper opam context.
Finally, to get all the pieces together, what is the difference between command substitution (e.g. $( ) or ) and evaluating an expression (e.g. eval string or eval $(cmd) ) (in bash mainly). Ref: https://unix.stackexchange.com/a/23116/55620. My understanding is that $( ) runs a command in a subshells and outputs the string so that it's string is an input to another command e.g. eval. So to my understanding you can think of $(cmd) as replacing that expression and pasting the string of it back to to the command line for another commadn e.g. eval. While eval will take in the string and evaluate the expression in it instead. So what eval $(cmd) does is first evaluate $(cmd) replace it with the output string of running cmd in a subshell and then feeding it to eval -- where eval will pretend that input is code and evaluate it, parse and run it. So $(...) is usually done to nest command inside other commands.
The only puzzling thing to me is why $(cmd) without a cmd2 (eg eval) in the front interprets the strings of $(cmd) as bash commands to run instead of just evaluting e.g.
(iit_synthesis) brandomiranda~ ❯ $(echo v=1)
zsh: command not found: v=1
but
(iit_synthesis) brandomiranda~ ❯ v=1
(iit_synthesis) brandomiranda~ ❯ echo $v
1
does evaluate my string v=1 and then run it.
Thus what eval $(opam env) does is command substitute $(opam env) i.e. run opam env in a subshell return the string it outputted and then given to eval to evaluate as bash code. In python eval would interpret the input string to it as literal python code to evaluate, parse, run here similarly it would do the same but assume its bash (is my guess)
Eval vs $( ) details:
Playground 1
(iit_synthesis) brandomiranda~ ❯ clear
(iit_synthesis) brandomiranda~ ❯ $(echo v=1)
zsh: command not found: v=1
(iit_synthesis) brandomiranda~ ❯ v=1
(iit_synthesis) brandomiranda~ ❯ echo $v
1
Playground 2
(iit_synthesis) brandomiranda~ ❯ echo vv=2
vv=2
(iit_synthesis) brandomiranda~ ❯ $(echo vv=2)
zsh: command not found: vv=2
(iit_synthesis) brandomiranda~ ❯ eval $(echo vv=2)
(iit_synthesis) brandomiranda~ ❯ echo $vv
2
I'm trying to write a small script to change the current directory to my project directory:
#!/bin/bash
cd /home/tree/projects/java
I saved this file as proj, added execute permission with chmod, and copied it to /usr/bin. When I call it by:
proj, it does nothing. What am I doing wrong?
Shell scripts are run inside a subshell, and each subshell has its own concept of what the current directory is. The cd succeeds, but as soon as the subshell exits, you're back in the interactive shell and nothing ever changed there.
One way to get around this is to use an alias instead:
alias proj="cd /home/tree/projects/java"
You're doing nothing wrong! You've changed the directory, but only within the subshell that runs the script.
You can run the script in your current process with the "dot" command:
. proj
But I'd prefer Greg's suggestion to use an alias in this simple case.
The cd in your script technically worked as it changed the directory of the shell that ran the script, but that was a separate process forked from your interactive shell.
A Posix-compatible way to solve this problem is to define a shell procedure rather than a shell-invoked command script.
jhome () {
cd /home/tree/projects/java
}
You can just type this in or put it in one of the various shell startup files.
The cd is done within the script's shell. When the script ends, that shell exits, and then you are left in the directory you were. "Source" the script, don't run it. Instead of:
./myscript.sh
do
. ./myscript.sh
(Notice the dot and space before the script name.)
To make a bash script that will cd to a select directory :
Create the script file
#!/bin/sh
# file : /scripts/cdjava
#
cd /home/askgelal/projects/java
Then create an alias in your startup file.
#!/bin/sh
# file /scripts/mastercode.sh
#
alias cdjava='. /scripts/cdjava'
I created a startup file where I dump all my aliases and custom functions.
Then I source this file into my .bashrc to have it set on each boot.
For example, create a master aliases/functions file: /scripts/mastercode.sh
(Put the alias in this file.)
Then at the end of your .bashrc file:
source /scripts/mastercode.sh
Now its easy to cd to your java directory, just type cdjava and you are there.
You can use . to execute a script in the current shell environment:
. script_name
or alternatively, its more readable but shell specific alias source:
source script_name
This avoids the subshell, and allows any variables or builtins (including cd) to affect the current shell instead.
Jeremy Ruten's idea of using a symlink triggered a thought that hasn't crossed any other answer. Use:
CDPATH=:$HOME/projects
The leading colon is important; it means that if there is a directory 'dir' in the current directory, then 'cd dir' will change to that, rather than hopping off somewhere else. With the value set as shown, you can do:
cd java
and, if there is no sub-directory called java in the current directory, then it will take you directly to $HOME/projects/java - no aliases, no scripts, no dubious execs or dot commands.
My $HOME is /Users/jleffler; my $CDPATH is:
:/Users/jleffler:/Users/jleffler/mail:/Users/jleffler/src:/Users/jleffler/src/perl:/Users/jleffler/src/sqltools:/Users/jleffler/lib:/Users/jleffler/doc:/Users/jleffler/work
Use exec bash at the end
A bash script operates on its current environment or on that of its
children, but never on its parent environment.
However, this question often gets asked because one wants to be left at a (new) bash prompt in a certain directory after execution of a bash script from within another directory.
If this is the case, simply execute a child bash instance at the end of the script:
#!/usr/bin/env bash
cd /home/tree/projects/java
echo -e '\nHit [Ctrl]+[D] to exit this child shell.'
exec bash
To return to the previous, parental bash instance, use Ctrl+D.
Update
At least with newer versions of bash, the exec on the last line is no longer required. Furthermore, the script could be made to work with whatever preferred shell by using the $SHELL environment variable. This then gives:
#!/usr/bin/env bash
cd desired/directory
echo -e '\nHit [Ctrl]+[D] to exit this child shell.'
$SHELL
I got my code to work by using. <your file name>
./<your file name> dose not work because it doesn't change your directory in the terminal it just changes the directory specific to that script.
Here is my program
#!/bin/bash
echo "Taking you to eclipse's workspace."
cd /Developer/Java/workspace
Here is my terminal
nova:~ Kael$
nova:~ Kael$ . workspace.sh
Taking you to eclipe's workspace.
nova:workspace Kael$
simply run:
cd /home/xxx/yyy && command_you_want
When you fire a shell script, it runs a new instance of that shell (/bin/bash). Thus, your script just fires up a shell, changes the directory and exits. Put another way, cd (and other such commands) within a shell script do not affect nor have access to the shell from which they were launched.
You can do following:
#!/bin/bash
cd /your/project/directory
# start another shell and replacing the current
exec /bin/bash
EDIT: This could be 'dotted' as well, to prevent creation of subsequent shells.
Example:
. ./previous_script (with or without the first line)
On my particular case i needed too many times to change for the same directory.
So on my .bashrc (I use ubuntu) i've added the
1 -
$ nano ~./bashrc
function switchp
{
cd /home/tree/projects/$1
}
2-
$ source ~/.bashrc
3 -
$ switchp java
Directly it will do: cd /home/tree/projects/java
Hope that helps!
It only changes the directory for the script itself, while your current directory stays the same.
You might want to use a symbolic link instead. It allows you to make a "shortcut" to a file or directory, so you'd only have to type something like cd my-project.
You can combine Adam & Greg's alias and dot approaches to make something that can be more dynamic—
alias project=". project"
Now running the project alias will execute the project script in the current shell as opposed to the subshell.
You can combine an alias and a script,
alias proj="cd \`/usr/bin/proj !*\`"
provided that the script echos the destination path. Note that those are backticks surrounding the script name.
For example, your script could be
#!/bin/bash
echo /home/askgelal/projects/java/$1
The advantage with this technique is that the script could take any number of command line parameters and emit different destinations calculated by possibly complex logic.
to navigate directories quicky, there's $CDPATH, cdargs, and ways to generate aliases automatically
http://jackndempsey.blogspot.com/2008/07/cdargs.html
http://muness.blogspot.com/2008/06/lazy-bash-cd-aliaes.html
https://web.archive.org/web/1/http://articles.techrepublic%2ecom%2ecom/5100-10878_11-5827311.html
In your ~/.bash_profile file. add the next function
move_me() {
cd ~/path/to/dest
}
Restart terminal and you can type
move_me
and you will be moved to the destination folder.
You can use the operator && :
cd myDirectory && ls
While sourcing the script you want to run is one solution, you should be aware that this script then can directly modify the environment of your current shell. Also it is not possible to pass arguments anymore.
Another way to do, is to implement your script as a function in bash.
function cdbm() {
cd whereever_you_want_to_go
echo "Arguments to the functions were $1, $2, ..."
}
This technique is used by autojump: http://github.com/joelthelion/autojump/wiki to provide you with learning shell directory bookmarks.
You can create a function like below in your .bash_profile and it will work smoothly.
The following function takes an optional parameter which is a project.
For example, you can just run
cdproj
or
cdproj project_name
Here is the function definition.
cdproj(){
dir=/Users/yourname/projects
if [ "$1" ]; then
cd "${dir}/${1}"
else
cd "${dir}"
fi
}
Dont forget to source your .bash_profile
This should do what you want. Change to the directory of interest (from within the script), and then spawn a new bash shell.
#!/bin/bash
# saved as mov_dir.sh
cd ~/mt/v3/rt_linux-rt-tools/
bash
If you run this, it will take you to the directory of interest and when you exit it it will bring you back to the original place.
root#intel-corei7-64:~# ./mov_dir.sh
root#intel-corei7-64:~/mt/v3/rt_linux-rt-tools# exit
root#intel-corei7-64:~#
This will even take you to back to your original directory when you exit (CTRL+d)
I did the following:
create a file called case
paste the following in the file:
#!/bin/sh
cd /home/"$1"
save it and then:
chmod +x case
I also created an alias in my .bashrc:
alias disk='cd /home/; . case'
now when I type:
case 12345
essentially I am typing:
cd /home/12345
You can type any folder after 'case':
case 12
case 15
case 17
which is like typing:
cd /home/12
cd /home/15
cd /home/17
respectively
In my case the path is much longer - these guys summed it up with the ~ info earlier.
As explained on the other answers, you have changed the directory, but only within the sub-shell that runs the script. this does not impact the parent shell.
One solution is to use bash functions instead of a bash script (sh); by placing your bash script code into a function. That makes the function available as a command and then, this will be executed without a child process and thus any cd command will impact the caller shell.
Bash functions :
One feature of the bash profile is to store custom functions that can be run in the terminal or in bash scripts the same way you run application/commands this also could be used as a shortcut for long commands.
To make your function efficient system widely you will need to copy your function at the end of several files
/home/user/.bashrc
/home/user/.bash_profile
/root/.bashrc
/root/.bash_profile
You can sudo kwrite /home/user/.bashrc /home/user/.bash_profile /root/.bashrc /root/.bash_profile to edit/create those files quickly
Howto :
Copy your bash script code inside a new function at the end of your bash's profile file and restart your terminal, you can then run cdd or whatever the function you wrote.
Script Example
Making shortcut to cd .. with cdd
cdd() {
cd ..
}
ls shortcut
ll() {
ls -l -h
}
ls shortcut
lll() {
ls -l -h -a
}
If you are using fish as your shell, the best solution is to create a function. As an example, given the original question, you could copy the 4 lines below and paste them into your fish command line:
function proj
cd /home/tree/projects/java
end
funcsave proj
This will create the function and save it for use later. If your project changes, just repeat the process using the new path.
If you prefer, you can manually add the function file by doing the following:
nano ~/.config/fish/functions/proj.fish
and enter the text:
function proj
cd /home/tree/projects/java
end
and finally press ctrl+x to exit and y followed by return to save your changes.
(NOTE: the first method of using funcsave creates the proj.fish file for you).
You need no script, only set the correct option and create an environment variable.
shopt -s cdable_vars
in your ~/.bashrc allows to cd to the content of environment variables.
Create such an environment variable:
export myjava="/home/tree/projects/java"
and you can use:
cd myjava
Other alternatives.
Note the discussion How do I set the working directory of the parent process?
It contains some hackish answers, e.g.
https://stackoverflow.com/a/2375174/755804 (changing the parent process directory via gdb, don't do this) and https://stackoverflow.com/a/51985735/755804 (the command tailcd that injects cd dirname to the input stream of the parent process; well, ideally it should be a part of bash rather than a hack)
It is an old question, but I am really surprised I don't see this trick here
Instead of using cd you can use
export PWD=the/path/you/want
No need to create subshells or use aliases.
Note that it is your responsibility to make sure the/path/you/want exists.
I have to work in tcsh, and I know this is not an elegant solution, but for example, if I had to change folders to a path where one word is different, the whole thing can be done in the alias
a alias_name 'set a = `pwd`; set b = `echo $a | replace "Trees" "Tests"` ; cd $b'
If the path is always fixed, the just
a alias_name2 'cd path/you/always/need'
should work
In the line above, the new folder path is set
This combines the answer by Serge with an unrelated answer by David. It changes the directory, and then instead of forcing a bash shell, it launches the user's default shell. It however requires both getent and /etc/passwd to detect the default shell.
#!/usr/bin/env bash
cd desired/directory
USER_SHELL=$(getent passwd <USER> | cut -d : -f 7)
$USER_SHELL
Of course this still has the same deficiency of creating a nested shell.