Creating directories with variable names in bash

Question

I am trying to create directories whose names are variables defined in an array. When I run my code below I do not understand the errors

#!/bin/bash
#
Output_Base_Dirs=(/home/user/CORDEX/OUTPUT/historical /home/user/CORDEX/OUTPUT/rcp45)
Input_Base_Dirs=(/home/user/CORDEX/INPUT/historical /home/user/CORDEX/INPUT/rcp45)
Input_Data_Dirs=(CLMcom-CCLM4-8-17_v1/CNRM-CERFACS-CNRM-CM5_r1i1p1/day/native)

Var_Dirs=(precip tmin tmax)
Vars=(pr tasmin tasmax)

# Create directories called by the variable name, if not there
###########################################################
for i in "${Output_Base_Dirs[@]}"
do
   echo $Var_Dirs[i]
   #  if [ ! -d ${Var_Dirs[i]} ]; then
   echo $Var_Dirs[i]
   echo "Directory doesn't exist. Creating now"
   mkdir "$Output_Base_Dirs[i]/$Var_Dirs[i]"
   echo "File created"
#  else
#   echo "Directory exists"
#  fi
done
 exit 0

When I run the above code with the if construct in place I get an error at the line of the if, as follows

precip[i]
./merge_files.sh: line 55: /home/user/CORDEX/OUTPUT/historical: syntax error: operand expected (error token is "/home/user/CORDEX/OUTPUT/historical")

When I run the code with the if construct commented out, I get the following precip[i]

Directory doesn't exist. Creating now
mkdir: cannot create directory ‘/home/user/CORDEX/OUTPUT/historical[i]/precip[i]’: No such file or directory
File created

precip[i]
Directory doesn't exist. Creating now
mkdir: cannot create directory ‘/home/user/CORDEX/OUTPUT/historical[i]/precip[i]’: No such file or directory
File created

Why does

echo $Var_Dirs[i]

print "precip[i]"?

I will appreciate to understand what is the problem and how to do it properly.

Forgot to mention that this was run on cygwin, but ubuntu 18.04 gives the same errors. — Zilore Mumba
– Zilore Mumba, Commented Sep 13, 2019 at 9:45
Oh boy, you're using values in Output_Base_Dirs as indexes for Var_Dirs — oguz ismail
– oguz ismail, Commented Sep 16, 2019 at 5:00
There is this thing called "GNU Bash Manual", you should check it out, it's really interesting! — rindeal
– rindeal, Commented Sep 18, 2019 at 23:15
I think I close this with the following comments: 1) Sorry I have had no internet connection for some days; 2) It is now my understanding that shell scriting has limited programming capabilities. I have done the same work with a lot of ease in perl. My perl script was not taking a unix command, which is why I tried bash; 3) I did not use values in $Output_Base_Dirs as indices. The index used was [i] (check the loop). The system itself was taking values from $Output_Base_Dirs as indices. The same result was ogt on cygwin and in linux ubuntu; 4) Thank you all who looked at this. — Zilore Mumba
– Zilore Mumba, Commented Sep 22, 2019 at 18:30

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

Quoting from Bash Reference Manual § 6.7 Arrays;

Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid conflicts with pathname expansion.

...

Referencing an array variable without a subscript is equivalent to referencing the array with a subscript of 0.

That means,

$Var_Dirs[i]

is the same as

${Var_Dirs[0]}[i]

, thus history[i], precip[i], etc. in the output.

The correct syntax is:

${Var_Dirs[i]}

But,

fixing it won't make your script work. Because "${Output_Base_Dirs[@]}" expands to the values in array Output_Base_Dirs, and you're using them as indexes.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Sep 13, 2019 at 9:34

oguz ismail

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5 Comments

Zilore Mumba Over a year ago

Thanks @ismail. I have tried "${Var_Dirs[i]}", ${Var_Dirs[i]}, $Var_Dirs[i] and "$Var_Dirs[i]", I still get the same error in every case tried.

Zilore Mumba Over a year ago

The same combinations have been tried, quoted with curly brackets, without, unquoted with without. Why is it printing precip[i]? I am concluding there is no solution to my problem, maybe it is my platform which is not ok. thanks for your time.

Zilore Mumba Over a year ago

$Output_Base_Dirs[i] was also tried with all combinations as with Var_Dirs[i] in my earlier comment, same result. I am wondering why echo ${Var_Dirs[i]} prints precip[i] instead of evaluating at i=0.

Zilore Mumba Over a year ago

"which bash" gives "/usr/bin/bash"

Zilore Mumba Over a year ago

@ismail thanks for suggesting starting from a simple construction. Indeed the code you provided works and prints "foo". I went a step further and introduced a for loop as follows: foo=(foo1 foo2); for i in "${foo[@]}" ; do; printf "%s\t%s\n" "$i", "${foo[i]}"; done. My system is strange, the output I get is: line 1 - foo1, foo1. line2- foo2, foo1. This is on cygwin. I will send you what I get on linux ubuntu in due course. Thanks.

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