You can use the following command in a UNIX shell to concatenate a PEM-formatted certificate with \n (escaped new-line): 
awk 'NF {sub(/\r/, ""); printf "%s\\n",$0;}' certificate.pem | pbcopy
This copies the value into to your clipboard buffer so you can paste it where needed.

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When working with certificates or keys in rules or v2 requests, you will most likely require a string representation of the file. If you open a certificate file (cer, pem) with a text editor, you’ll see something like this: 
-----BEGIN CERTIFICATE-----
MIICzDCCAbQCCQDH8GvxPIeH+DANBgkqhkiG9w0BAQsFADAoMQswCQYDVQQGEwJh
cjEZMBcGA1UEAwwQaHR0cHM6Ly9uaWNvLmNvbTAeFw0xOTA0MDgxODA3NTVaFw0y
//
// more lines of base64-encoded information
//
nSWyabd+LiBGtLTMB+ZLbOIi3EioWPGw/nHOI8jzPrqhiCLuZCSQmiqrLQYNsc1W
-----END CERTIFICATE-----
The lines between the -----BEGIN CERTIFICATE----- header and -----END CERTIFICATE----- footer contain base64-encoded binary information. Public keys and private keys (.key files) will look similar, with just a different header/footer. For a string representation of a certificate/key file, you will need to concatenate everything in one line, with a \n (escaped new-line) sequence replacing the actual new lines in the file. So from the above sample you’d obtain something like this: 
"-----BEGIN CERTIFICATE-----
\nMIICzDCCAbQCCQDH8GvxPIeH+DANBgkqhkiG9w0BAQsFADAoMQswCQYDVQQGEwJh\n
[..all the other lines..]
-----END CERTIFICATE-----\n"

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