Bash – Variable Substitution with Exclamation Mark

bashvariable substitution

I have the following lines in my .cfg bash script file

DDF_SOURCE="siebel_DATA_DATE_FORMAT"
DATA_DATE_FORMAT=${!DDF_SOURCE}

how is ${!DDF_SOURCE} evaluated? It would be !siebel_DATA_DATE_FORMAT, which doesn't make sense to me.

Best Answer

That is an indirect expansion, documented in man bash section EXPANSION, subsection Parameter Expansion:

If the first character of parameter is an exclamation point (!), a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect expansion.

bash-4.2$ DDF_SOURCE="siebel_DATA_DATE_FORMAT"

bash-4.2$ siebel_DATA_DATE_FORMAT='Hello Indirect Redirection'

bash-4.2$ DATA_DATE_FORMAT=${!DDF_SOURCE} # siebel_DATA_DATE_FORMAT must get value before this line

bash-4.2$ echo $DATA_DATE_FORMAT
Hello Indirect Redirection

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bash – Variable Not Expanding Inside Another Variable

You can use eval, but the next maintainer will curse your name. Have you tried just appending the variable when using the parsed data, or expanding the variable when creating the data?

If the file is created that way, can't you just use some [redacted] techniques to convince the originator to change their wicked ways?

If change is literally not possible, then you must have control over which variables are possible. Otherwise your script is vulnerable to all sorts of injection attacks, such as inputs like 3|sample1.txt|$(rm --arr --eff /)/operation/. Since you obviously have that under control, you can do some literal replacements of variables with their values on a case by case basis:

IFS='/' read -a pathnames <<< "$fpath"
for pathname in "${pathnames[@]}"
do
    if [ "${pathname::1}" = '$' ]
    then
        variable_name="${pathname:1}"
        printf '%s' "${!variable_name}"
    else
        printf '%s' "$pathname"
    fi
done

With some additional boilerplate to add slashes between pathnames.

Bash variable substitution in a JSON string

JSON=\''{"hostname": "localhost", "outdir": "'"$OUTDIR"'", "port": 20400, "size": 100000}'\'

That is get out of the single quotes for the expansion of $OUTDIR. We did put that expansion inside double-quotes for good measure even though for a scalar variable assignment it's not strictly necessary.

When you're passing the $JSON variable to echo, quotes are necessary though to disable the split+glob operator. It's also best to avoid echo for arbitrary data:

printf '%s\n' "$JSON"

Best Answer

Related Solutions

bash – Variable Not Expanding Inside Another Variable

Bash variable substitution in a JSON string

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