I've read that while using a cron you define variables like always:
var = <value>
But you can't use variable values on < value > such as:
PATH=$PATH
So how could I introduce the PATH inside PATH plus HOME/FOLDER for instance? Normally I would do...
PATH=$HOME/FOLDER:$PATH
But if what I've read is correct, that isn't available...right?
my crontab(5) page agrees with you:
The value string is not parsed for environmental substitutions or replacement of variables, thus lines like
PATH = $HOME/bin:$PATH
will not work as you might expect.
However, if you're specifically interested in $HOME, you can use this:
An alternative for setting up the commands path is using the fact that many shells will treat the tilde(~) as substitution of $HOME, so if you use bash for your tasks you can use this:
SHELL=/bin/bash
PATH=~/bin:/usr/bin/:/bin
Related
I'm trying to get going with some more advanced Ansible playbooks and have hit a wall. I'm trying to get Ansible to do what this /bin/bash 'for' loop does;
for i in $(</filename.txt);do '/some/command options=1 user=usera server=$i';done
filesnames.txt contains 50-100 hostnames.
I can't use jinja templates as the command has to be run, not just the config file updated.
Any help would be greatly appreciated.
Thanks in advance,
Jeremy
you can use jinja templates, but differently
your specific code is not doing something that is most advisable
for multi-line code you should use shell module.
example of multi-code piece of call:
- name: run multiline stuff
shell: |
for x in "${envvar}"; do
echo "${x}"
done
args:
executable: /bin/bash
note I'm explicitly setting executable, which will ensure bash-isms would work.
I just used envvar as an example, of arbitrary environment variable available.
if you need to pass specific env variables, you should use environment clause of the call to shell module, refer to: http://docs.ansible.com/ansible/playbooks_environment.html
For simple variables you can just use their value in shell: echo "myvar: {{myvar}}"
If you wish to use an ansible list/tuple variable inside bash code, you can make it bash variable first. e.g. if you have a list of stuff in mylist, you can expand it and assign into a bash array, and then iterate over it. the shell code of the call to shell would be:
mylist_for_bash=({{mylist|join(" ")}})
for myitem in "${mylist_for_bash[#]}"; do
echo "my current item: ${myitem}"
done
Another approach would be to pass it as string env variable, and convert it into an array later in the code.
NOTE:
of course all this works correctly only with SPACELESS values
I've never had to pass array with space containing items
I'm reading this:
You can delete a variable with the command unset varname. Normally this is not useful, since all variables that don't exist are assumed to be null, i.e., equal to empty string "". But if you use the option nounset which causes the shell to indicate an error when it encounters an undefined variable, then you may be interested in unset.
My first question is: I cannot see why the use of unset be not useful; if I want to put my variable to null I can use it (or set variable="" or variable=). On the other hand, if I have a variable that doesn't exist, I don't know why I should have to use it..
My second question is: Why may I be interested in unset in that case?
There is a relevant difference between unset and empty variables.
When you can't tell in front which variables will be used, you can process the output of set (examples: https://stackoverflow.com/a/43419722/3220113 and https://stackoverflow.com/a/28104421/3220113 ).
You might have a situaton where you have sourced a read-only config file, but you do not want all lines set in your environment. In that case you might want to unset the settings you do not need.
When you write some utility that uses some variables, you do not want to leave garbage in the environment. Next to using local variables using unset is another possibility.
I think I have found the answer to my question.
1) If you need to remove the definition and the content of a variable you can use unset command. However, unless you turn on the nounset set option, Korn Shell will allow using variables which don't exist, and it will default the content of such a variable as an empty string. That's why you normally don't use unset: because you normally leave the nounset option off and test variables via conditional logic. Hence in these cases, i.e. the inhibition of the use of a variable, it is not useful. (Obviously, it remains useful for deleting variables - as noted by #Walter A, i.e. "" is not unset, the complete removal of the variable.)
2) That said, it follows that if you use the nounset, unset command makes sense. Indeed, if you unset a variable, the shell will disallow using it.
I want to define variable in shell script as:
value1 = 40 (this can be number or character)
and want to use as in a text like:
$value1_position.xyz (I basically want 40_position.xyz)
How do I do this?
this should do:
${value1}_position.xyz
beware that the variable should be declared with this syntax
value1=40
notice the absence of spaces around the =
To define a variable, simply make sure there are no spaces between the variable name and value
value1=40
To use that variable in bash substitution, creating what you want, use the $ replacement symbol like so:
${value1}_position.xyz
To append that to your text file
echo "${value1}_position.xyz" >> file.txt
I noticed that in shell script when we declare a variable, the preceding dollar sign is not needed, although when we want to access this variable later we should add a dollar sign in front of this variable name.
just like:
#!/bin/sh
VAR_1=Hello
VAR_2=Unix
echo "$VAR_1 $VAR_2"
This is different from other languages, like Perl we will always have the preceding dollar sign with the variable name, I just want to know any good reason for shell script to do it in this way, or it's just a convention...?
Shell is a different language than Perl is a different language than C++ is a different language than Python. You can add "with different rules" to each of the languages.
In shell an identifier like VAR_1 names a variable, the dollar sign is used to invoke expansion. $var is replaced with var's content; ${var:-foo} is replaced with var's content if it is set and with the word foo if the variable isn't set. Expansion works on non-variables as well, e.g. you can chain expansion like ${${var##*/}%.*} should leave only a file base name if var contains a file name with full path and extension.
In Perl the sigil in front of the variable tells Perl how to interpret the identifier: $var is a scalar, #var an array, %var a hash etc.
In Ruby the sigil in front of the varible tells Ruby its scope: var is a local variable, $var is a global one, #var is an instance variable of an object and ##var is a class variable.
In C++ we don't have sigils in front of variable names.
Etc.
In the shell, the $ sign is not part of the variable name. It just tells the shell to replace the following word with the contents of the variable with the same name, i.e. $foo means "insert the contents of the variable foo here".
This is not used when assigning to the variable because there you explicitly don't want to insert the old contents; you want to use the variable itself (in some ways this is similar to dereferencing pointers).
It's basically a syntactical convention.
DOS/.bat file syntax works the same way.
1) to create a variable, no metacharacter.
2) to "dereference" the contents of the variable, use the metacharacter.
DOS:
set VAR=123
echo %VAR%
My problem lies with my confusion with shell variables.
To my understanding, variables allow me to store a value (String in this case) and to call it later in my code. So if I wanted to have a variable that holds the path to some set of scripts, I could ideally just store it like this:
SPTH = '/home/Foo/Documents/Programs/ShellScripts/Butler'
//Later on in the script//
cd $SPTH
./script1
What I'm trying to do, with probably the wrong syntax, is to set the path to variable SPTH.
Then I use cd with argument $SPTH.
Ideally this would allow me to run the file there without typing in the path. However it doesn't work. The $SPTH is ignored and the result is as if cd was used alone.
So what am I doing wrong? And what would be a way to do this?
Don't use spaces...
(Incorrect)
SPTH = '/home/Foo/Documents/Programs/ShellScripts/Butler'
(Correct)
SPTH='/home/Foo/Documents/Programs/ShellScripts/Butler'
To add to the above correct answer :-
For my case in shell, this code worked (working on sqoop)
ROOT_PATH="path/to/the/folder"
--options-file $ROOT_PATH/query.txt