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Updated SC2087 (markdown)

koalaman
2015-08-09 09:05:27 -07:00
parent eb057bbea3
commit 70315713b7
1 changed files with 15 additions and 3 deletions
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18
SC2087.md

@@ -14,11 +14,23 @@
### Rationale:
When the end token of a here document is not quoted, parameter expansions and command substitutions will be expanded. This means that the hostname printed will be that of the client, and not of the server.
When the end token of a here document is unquoted, parameter expansion and command substitution will happen on in contents of the here doc.
In other words, before sending the commands to the server, the client replaces `$HOSTNAME` with localhost, thereby sending `echo "Logged in on localhost"` to the server.
This means that before sending the commands to the server, the client replaces `$HOSTNAME` with localhost, thereby sending `echo "Logged in on localhost"` to the server. This has the effect of printing the client's hostname instead of the server's.
By quoting the here token, local expansion will not take place, so the server sees `echo "Logged in on $HOSTNAME"` which is expanded and printed with the server's hostname, which is usually the intention.
Scripts with any kind of variable use are especially problematic because all references will be expanded before the script run. For example,
ssh host << EOF
x="$(uname -a)"
echo "$x"
EOF
will never print anything, neither client nor server details, since before evaluation, it will be expanded to:
x="Linux localhost ... x86_64 GNU/Linux"
echo ""
By quoting the here token, local expansion will not take place, so the server sees `echo "Logged in on $HOSTNAME"` which is expanded and printed with the server's hostname, which is usually the intention.
### Exceptions: