Shell – Define variables with a variable name

shell-scriptvariable

What's wrong with this script?
I'm trying to define A1=1, B1=1, C1=1

LIST="A B C"
for x in $LIST
do
    "$x"1=1
done

and the result is:

./x.: line 7: A1=1: command not found
./x.: line 7: B1=1: command not found
./x.: line 7: C1=1: command not found

Best Answer

A variable assignment has the form of a variable name, followed by the equal sign, followed by the (optional) value.

This is a valid assignment:

ABC=123

"$x"1=1 is not a valid assignment, because "$x"1 is not a variable name. It may be evaluated to a variable name, but it isn't. The shell, in fact, believes it is a command.

One way for doing what you want to achieve is this:

eval "$x"1=1

Another way in bash (but not in other shells) is:

declare "$x"1=1

Or also (again bash-only):

let "$x"1=1

(There is no much difference in your case.)

But, as Jakuje noted in the comments, you probably want to go with arrays, if your shell has them (ksh, bash or zsh).

For completeness:

eval executes arbitrary commands. So, if on the right side of the equal sign you have a variable that expands to some command, that command will be executed. The following code:
```
x=a
y='$(echo hello)'
eval "$x=$y"
```
is equivalent to a=hello.
declare is a bash builtin to assign variables and won't execute any command. The following code:
```
x=a
y='$(echo hello)'
declare "$x=$y"
```
is equivalent to a='$(echo hello)'.
let is similar to declare, in that it doesn't execute commands. But contrary to declare, let may be used for arithmetic operations:
```
let a="1 + 2"
```
is equivalent to a=3.

Related Solutions

Bash – if env variable exists then exit out of script

Regarding testing whether a "variable is 0", one can use:

if [ "$MYVAR" -eq 0 ] ; then exit ; fi

Or, if you prefer:

if [[ $MYVAR -eq 0 ]] ; then exit ; fi

The expression [[ -n "$MYVAR" ]] tests for a non-empty string value of MYVAR. The string "0" is non-empty: it has one character, the same as the string "1".

exit will cause the script to exit. return, by contrast, is used in functions or sourced scripts when you want to quit the function or sourced script but continue with execution of the calling script. Running exit in a function would cause both the function and its calling script to exit.

Bash Arrays – Using Variables in Array Names

Some ideas:

A "parameter expansion" of a variable value (the ${...} part):
```
echo "arr$COUNTER[0] = ${arr$COUNTER[0]}"
```
will not work. You may get around by using eval (but I do not recommend it):
```
eval echo "arr$COUNTER[0] = \${arr$COUNTER[0]}"
```
That line could be written as this:
```
i="arr$COUNTER[0]"; echo "$i = ${!i}"
```
That is called indirection (the !) in Bash.
A similar issue happens with this line:
```
declare -a "arr$COUNTER=($field)"
```
Which should be split into two lines, and eval used:
```
declare -a "arr$COUNTER"
eval arr$COUNTER\=$ \$field $
```
Again, I do not recommend using eval (in this case).
As you are reading the whole file into the memory of the shell, we may as well use a simpler method to get all lines into an array:
```
readarray -t lines <"VALUES_FILE.txt"
```
That should be faster than calling awk for each line.

An script with all the above could be:

#!/bin/bash
valfile="VALUES_FILE.txt"

readarray -t lines <"$valfile"             ### read all lines in.

line_count="${#lines[@]}"
echo "Total number of lines $line_count"

for ((l=0;l<$line_count;l++)); do
    echo "Counter value is $l"             ### In which line are we?
    echo "Field = ${lines[l]}"             ### kepth only to help understanding.
    k="arr$l"                              ### Build the variable arr$COUNTER
    IFS=" " read -ra $k <<<"${lines[l]}"   ### Split into an array a line.
    eval max=\${#$k[@]}                    ### How many elements arr$COUNTER has?
    #echo "field $field and k=$k max=$max" ### Un-quote to "see" inside.
    for ((j=0;j<$max;j++)); do             ### for each element in the line.
        i="$k[$j]"; echo "$i = ${!i}"      ### echo it's value.
    done
done
echo "The End"
echo

However, still, AWK may be faster, if we could execute what you need in AWK.

A similar processing could be done in awk. Assuming the 6 values will be used as an IP (4 of them) and the other two are a number and an epoch time.

Just a very simple sample of an AWK script:

#!/bin/sh
valfile="VALUES_FILE.txt"
awk '
NF==6 { printf ( "IP: %s.%s.%s.%s\t",$1,$2,$3,$4)
        printf ( "number: %s\t",$5+2)
        printf ( "epoch: %s\t",$6)
        printf ( "\n" )
    }
' "$valfile"

Just make a new question with the details.

Best Answer

Related Solutions

Bash – if env variable exists then exit out of script

Bash Arrays – Using Variables in Array Names

Related Question