Exporting environment variable

Question

I want to export an environment variable whose value contains a variable but I don't want that variable (which is within the value) to be evaluated during the export process. i.e.

export STY=for i in {0..3}; do echo $i; done

so my requirement is that when I echo STY, the output that I need should be

echo $STY
for i in {0..3}; do echo $i; done 

But $i is evaluated during export. I need to preserve it as it is.

Answer 1

If you want what you said you wanted:

export STY="for i in {0..3}; do echo \$i; done"

If you want what you probably actually meant:

function STY() {
    for i in {0..3}; do echo $i; done
}
export -f STY

Answer 2

Short: use eval "$hello".

(This answer originally said: use eval $hello but another poster pointed out that eval "$hello" is safer.)

As shown below:

bash-2.05a$ export hello='for i in 0 1 2 3; do echo $i; done'

Simply saying $hello at the beginning of a line does not work - because for loops are processed before $variable expansion (phase oriented languages, I hate 'em):

bash-2.05a$ $hello
bash: for: command not found

But eval'ing works:

bash-2.05a$ eval $hello
0
1
2
3
bash-2.05a$ 

What you were (or might have been) doing wrong:

bash-2.05a$ export hello='for i in {0..4}; do echo $i; done'
bash-2.05a$ $hello
bash: for: command not found
bash-2.05a$ eval $hello
{0..4}
bash-2.05a$ for i in {0..4}; do echo $i; done
{0..4}
bash-2.05a$ 

I.e. at least in the version of bash I am using, {0..4} doesn't do what you thought it should.

http://www.cyberciti.biz/faq/bash-for-loop/ says that the {0..4} feature came in bash 3.0+.

Perhaps you, like me (at woprk, not home), are using an obsolete version of bash?

Answer 3

The reason why $hello doesn't run as you expected:

• Expansion is performed on the command line after it has been split into tokens.
• for...do...done is a Compound Command which is determined in tokenlize stage.
• for is not a command.

So, you cannot do this:

$ "for" i in {0..4}; do echo $i; done
sh.exe": syntax error near unexpected token `do'

When bash split the command-line into tokens, It see "for" not for, so token do is unexpected.

---

In your case:

$ hello='for i in {0..4}; do echo $i; done'
$ $hello
sh.exe": for: command not found

Bash finds one token $hello, then variable-expansion&&word-spliting take place in order.
Actually, bash parses the result as a Simple Command:

• command(for) takes rest as args(i,in,{0..4}, ;, do, ...)
• for, do, ;, done, etc lost their special meanings.

read more for detail

Answer 4

The problem has two sides:

• storing complex expressions in variable
• executing stored expression

To store expression, escape sequences and special quoting might be needed (that seems to be right in your example). Execution can be performed in two ways:

• function call as in Borealid's answer: data=`STY()` - standard output of function is stored into variable
• expression evaluation: data=eval $STY for variable definition given in your example