I'm trying to write a shell script to automate a job for me. But i'm currently stuck.
Here's the problem :
I have a variable named var1 (a decreasing number from 25 to 0
and another variable named
var${var1} and this equals to some string.
then when i try to call var${var1} in anywhere in script via echo it fails.
I have tried $[var$var1], ${var$var} and many others but everytime it fails and gives the value of var1 or says operand expected error.
Thanks for your help
It's probably better if you use an array, but you can use indirection:
var25="some string"
var1=25
indirect_var="var$var1"
echo ${!indirect_var} # echoes "some string"
There's only one round of variable expansion, so you can't do it directly. You could use eval:
eval echo \${var$var1}
A better solution is to use an array:
i=5
var[$i]='foo'
echo ${var[$i]}
It sounds like you need bash variable indirection. Take a look at the link below.
http://mywiki.wooledge.org/BashFAQ/006
Related
I am looking for a way to add a string to a variable name in bash and evaluate this as a new variable. Example:
#!/bin/bash
file="filename"
declare ${file}_info="about a file"
declare ${file}_status="status of file"
declare ${file}_number="29083451"
echo ${${file}_info} # <-- This fails with error: "bad substitution"
Is there a way to do this?
I'm not actually implementing this in any production code anywhere. I just want to know if there is some way to create a variable name using a variable and a string. It seemed like an interesting problem.
Also note that I am not asking about bash indirection, which is the use of ${!x} to evaluate the contents of a variable as a variable.
You aren't asking about indirection, but that's what can help you:
info=$file\_info
echo ${!info}
I'm very new to bash scripting, and as I've been searching for information online I've found a lot of seemingly contradictory advice. The thing I'm most confused about is the $ in front of variable names. My main question is, when is and isn't it appropriate to use that syntax? Thanks!
Basically, it is used when referring to the variable, but not when defining it.
When you define a variable you do not use it:
value=233
You have to use them when you call the variable:
echo "$value"
There are some exceptions to this basic rule. For example in math expresions, as etarion comments.
one more question: if I declare an array my_array and iterate through
it with a counter i, would the call to that have to be $my_array[$i]?
See the example:
$ myarray=("one" "two" "three")
$ echo ${myarray[1]} #note that the first index is 0
two
To iterate through it, this code makes it:
for item in "${myarray[#]}"
do
echo $item
done
In our case:
$ for item in "${myarray[#]}"; do echo $item; done
one
two
three
I am no bash user that knows too much. But whenever you declare variable you would not use the $, and whenever you want to call upon that variable and use its value you would use the $ sign.
I would like to know if I can substitute a variable twice.
For example:
#global variable
TEST_SERV_EXT=""
#variables become from myconf.sh
TEST_SERV_EXT_FO='foo01'
TEST_SERV_EXT_BR='bar01'
I want dynamically construct those last two and assign them in TEST_SERV_EXT.
I tried something like this ${$TEST_SERV_COMP} but I'm getting "bad substitution" message.
I need something like php's feature "$$" or tcl's subst command.
Regards,
thandem
TEST_SERV_COMP=TEST_SERV_EXT_FO
TEST_SERV_EXT=${!TEST_SERV_COMP}
Look for indirect expansion in the bash manual.
I have some code that creates a variable of some name automatically and assigns some value to it. The code is something like the following:
myVariableName="zappo"
eval "${myVariableName}=zappo_value"
How would I access the value of this variable using the automatically generated name of the variable? So, I'm looking for some code a bit like the following (but working):
eval "echo ${${myVariableName}}"
(... which may be used in something such as myVariableValue="$(eval "echo ${${myVariableName}}")"...).
Thanks muchly for any assistance
If you think this approach is madness and want offer more general advice, the general idea I'm working on is having variables defined in functions in a library with such names as ${usage} and ${prerequisiteFunctions}. These variables that are defined within functions would be accessed by an interrogation function that can, for instance, ensure that prerequisites etc. are installed. So a loop within this interrogation function is something like this:
for currentFunction in ${functionList}; do
echo "function: ${currentFunction}"
${currentFunction} -interrogate # (This puts the function variables into memory.)
currentInterrogationVariables="${interrogationVariables}" # The variable interrogationVariables contains a list of all function variables available for interrogation.
for currentInterrogationVariable in ${currentInterrogationVariables}; do
echo "content of ${currentInterrogationVariable}:"
eval "echo ${${currentInterrogationVariable}}"
done
done
Thanks again for any ideas!
IIRC, indirection in bash is by !, so try ${!myVariableName}
Try:
echo ${!myVariableName}
It will echo the variable who's name is contained in $myVariableName
For example:
#!/bin/bash
VAR1="ONE"
VAR2="TWO"
VARx="VAR1"
echo ${VARx} # prints "VAR1"
echo ${!VARx} # prints "ONE"
I have multiple variables in a shell script; i was trying to save some code duplication and wanted to do something like following
# variables
FLAG=SIM
SIM_ICR_KEY_VAL="http://www.example.com/simi/icr"
REAL_ICR_KEY_VAL="http://www.example.com/real"
Based on the FLAG value i want to access the correct variable (without using IF's)
When i try this it echos the variable name & not the value itself.
echo $(echo ${FLAG}_ICR_KEY_VAL)
On further note; i need to use these substitutions inline in a sed statememt:
sed "s!${ISTR_KEY}=.*!${ISTR_KEY}=${SIM_ISTR_KEY_VAL}!" > tmp.file
... i am not sure its possible or not, please suggest
Reflection can be achieved with the infamous eval:
eval thisvar=\$${FLAG}_INC_KEY_VAL;
echo "We are using $thisvar"
Whenever you find yourself dynamically synthesizing a variable name, though, you are probably Doing It Wrong. You should consider alternatives like arrays:
ICR_KEY_VAL[0]="http://www.example.com/simi/icr"
ICR_KEY_VAL[1]="http://www.example.com/real"
SIM=0
echo ${ICR_KEY_VAL[$SIM]}
I don't know how to do it directly, but in bash you can do it indirectly:
FLAG=SIM
SIM_ICR_KEY_VAL="http://www.example.com/simi/icr"
REAL_ICR_KEY_VAL="http://www.example.com/real"
FLAG_ICR_KEY_VAL=${FLAG}_ICR_KEY_VAL
sed "s!${ISTR_KEY}=.*!${ISTR_KEY}=${!FLAG_ISTR_KEY_VAL}!" > tmp.file