How do I actually write "\n\r" to an environment variable?

Question

I have tried command substition, but it replaced '\n' with space

$ export GREENIE="$(python -c 'print "\n\r"*4')"
$ echo $GREENIE |hexdump
0000000 200d 200d 200d 0a0d                    
0000008
$

All I need is like this:

$ echo $GREENIE |hexdump
00000000  0a0d 0a0d 0a0d 0a0d
00000008
$

How can I do that?

http://shellcheck.net/ is your friend for catching bugs of this kind (unquoted expansions). You'd still have an extra newline from the `echo` after catching what it found for you, but that would have been a smaller set of problems. — Charles Duffy, Dec 24 '16 at 22:29
As an aside, by the way -- all-caps variable names are actually in space reserved for variables with meaning to the operating system and shell (which is why the shell itself uses `PS1`, `PATH`, etc). Use lower-case names for your own variables to prevent overwriting a name with meaning to the system by mistake; see http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html (discussing environment variables -- but shell variables share the same namespace: setting a shell variable with the name of an existing environment variable will overwrite the latter). — Charles Duffy, Dec 24 '16 at 22:32
btw -- `GREENIE=$'\n\r\n\r\n\r\n\r'` will assign the literal string in question to your variable in bash (just a regular shell variable, not an environment variable, until you `export` it) — Charles Duffy, Dec 24 '16 at 22:34

Mark Reed · Accepted Answer · 2017-01-03T02:58:43.083

Your Python one-liner works just fine as-is; the problem is your test. No matter how you set the variable, you won't see its value properly in your hexdump unless you quote its expansion, like this:

$ printf '%s' "$GREENIE"  | hexdump
0000000 0a 0d 0a 0d 0a 0d 0a 0d
0000008

(I used printf instead of echo to avoid confusion, since echo adds an extra newline on the end, which shows up in the output of hexdump even though it's not part of the value of $GREENIE. I could also have used echo -n since this is a bash-specific question, but I try to avoid that since it's not part of POSIX.)

Without the quotes, the shell does word-splitting on the newlines, so you get nothing but the carriage returns back from printf:

$ printf '%s' $GREENIE | hexdump
0000000 0d 0d 0d 0d
0000004

If you use echo instead (with bash's -n option to avoid the confusing extra newline), it just joins those carriage-returns back together with the default separator (space):

$ echo -n $GREENIE  | hexdump
0000000 0d 20 0d 20 0d 20 0d
0000007

You could also use shell built-ins to generate the string, although it doesn't have a convenient string-multiplier operator. You can use a dumb for loop:

GREENIE=$(for ((i=0;i<4;++i)); do printf '\n\r'; done)

But it's short enough that you can just type it out:

GREENIE=$(printf '\n\r\n\r\n\r\n\r')

If we're being bash-specific, you could use ANSI strings and skip the printf:

GREENIE=$'\n\r\n\r\n\r\n\r'

or combine them and get cute with brace-expansion:

GREENIE=$(printf '%s' $'\n\r'{,,,})

Either way, if you are just using the variable inside a shell script, you don't need to export it to the environment; that's only for making it accessible to other programs:

$ GREENIE=hello
$ echo $GREENIE
hello
$ python -c 'import os; print os.getenv("GREENIE")'
None
$ export GREENIE
$ python -c 'import os; print os.getenv("GREENIE")'
hello

You can also export a variable into the environment of a particular program without setting it in the shell at all, by placing the assignment on the command line where you run the program:

$ unset GREENIE
$ echo $GREENIE

$ GREENIE=hello python -c 'import os; print os.getenv("GREENIE")'
hello
$ echo $GREENIE

$

And if you're not exporting it to the environment, the usual convention is to give it a lowercase name:

$ greenie=hello
$ echo $greenie
hello

How do I actually write "\n\r" to an environment variable?

1 Answers1