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Full Discussion: Loop through variables
Top Forums Shell Programming and Scripting Loop through variables Post 302839891 by durden_tyler on Saturday 3rd of August 2013 02:38:39 PM
Old 08-03-2013
Quote:
Originally Posted by biobill
...
Code:
Test: eval echo \$${src_file}
the_right_text
test: src_file_name=`eval echo \$${src_file}`
test: echo ${src_file_name}
29199{src_file}

so why when I manually do the eval it comes up with the right text, but when assigning the eval to a variable and then echo it it comes out wierd?
I am unable to reproduce your test in my system. This looks like the output of some shell script. I'll need to look at the script to figure out what's going on.

However, what's happening at the end is that the value of the variable "src_file_name" is the following two values appended to each other in the order shown:
(1) the value of $$
(2) the constant text "{src_file}"

Now, $$ is a built-in shell variable which holds the value of the PID, the process id. You can check the PID of each process in your shell by executing the following command and looking at the value in the 2nd column:

Code:
ps -ef

You can also check the value of your current shell PID:

Code:
echo $$

And view it and its descendent processes in the process list:

Code:
ps -ef | grep <the_number_you_saw_for_$$_above>

As for your problem, you are creating variable names within the loop, and you want to access their values for your file names. There is a construct "${!variable}" for that.

A simple test follows. I create and assign a variable and echo its value:

Code:
$ 
$ pointer_to_val=123456
$ echo $pointer_to_val
123456
$

Now, I assign the text "pointer_to_val" to another variable and echo it:

Code:
$ 
$ val=pointer_to_val
$ echo $val
pointer_to_val
$ 
$

So far, everything is as expected.

But in the last echo command above, let's say I want to see the value of pointer_to_val, i.e. I want to see the value 123456 that pointer_to_val points to.

To do that, I'd use ${!variable} construct:

Code:
$ 
$ echo ${val}
pointer_to_val
$ 
$ echo ${!val}
123456
$

This idea could be incorporated in your script as follows:

Code:
$ 
$ cat -n check_for_files_1.sh
     1    #!/usr/bin/ksh
     2    DATAPATH="/my/data/dir"
     3    src_file_1=file_one.txt
     4    src_file_2=file_two.sql
     5    src_file_3=file_three.py
     6    src_file_4=file_four.java
     7    src_file_5=file_five.c
     8    first=1
     9    last=5
    10    while [[ $first -le $last ]]
    11    do
    12      var=src_file_$first
    13      file=${!var}
    14      echo "${DATAPATH}/${file}"
    15      (( first += 1 ))
    16    done
$ 
$ 
$ . check_for_files_1.sh
/my/data/dir/file_one.txt
/my/data/dir/file_two.sql
/my/data/dir/file_three.py
/my/data/dir/file_four.java
/my/data/dir/file_five.c
$ 
$ 
$

And if the ${!variable} construct is not supported in your shell, you could use the old-fashioned eval construct:

Code:
$ 
$ cat -n check_for_files.sh
     1    #!/usr/bin/ksh
     2    DATAPATH="/my/data/dir"
     3    src_file_1=file_one.txt
     4    src_file_2=file_two.sql
     5    src_file_3=file_three.py
     6    src_file_4=file_four.java
     7    src_file_5=file_five.c
     8    first=1
     9    last=5
    10    while [[ $first -le $last ]]
    11    do
    12      var=src_file_${first}
    13      file=$(eval "echo \$$var")
    14      echo "${DATAPATH}/${file}"
    15      (( first += 1 ))
    16    done
$ 
$ 
$ . check_for_files.sh
/my/data/dir/file_one.txt
/my/data/dir/file_two.sql
/my/data/dir/file_three.py
/my/data/dir/file_four.java
/my/data/dir/file_five.c
$ 
$

 

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