ssh breaks out of while-loop in bash

Question

I use this bash-code to upload files to a remote server, for normal files this works fine:

for i in `find devel/ -newer $UPLOAD_FILE`
do
    echo "Upload:" $i
    if [ -d $i ]
    then
        echo "Creating directory" $i
        ssh $USER@$SERVER "cd ${REMOTE_PATH}; mkdir -p $i"
        continue
    fi
    if scp -Cp $i $USER@$SERVER:$REMOTE_PATH/$i
    then
        echo "$i OK"
    else
        echo "$i NOK"
        rm ${UPLOAD_FILE}_tmp
    fi
done

The only problem is that for files with a space in the name, the for-loop fails, so I replaced the first line like this:

find devel/ -newer $UPLOAD_FILE | while read i
do
    echo "Upload:" $i
    if [ -d $i ]
    then
        echo "Creating directory" $i
        ssh $USER@$SERVER "cd ${REMOTE_PATH}; mkdir -p $i"
        continue
    fi
    if scp -Cp $i $USER@$SERVER:$REMOTE_PATH/$i
    then
        echo "$i OK"
    else
        echo "$i NOK"
        rm ${UPLOAD_FILE}_tmp
    fi
done

For some strange reason, the ssh-command breaks out of the while-loop, therefore the first missing directory is created fine, but all subsequent missing files/directories are ignored.

I guess this has something to do with ssh writing something to stdout which confuses the "read" command. Commenting out the ssh-command makes the loop work as it should.

Does anybody know why this happens and how one can prevent ssh from breaking the while-loop?

As an aside -- all-uppercase variable names are reserved by convention for environment variables and builtins; they shouldn't be used for variables local to a script. — Charles Duffy, Mar 03 '15 at 17:15
`while read` will break badly when your filenames contain literal backslashes. Safer to use `read -r`. Similarly, `read` will strip trailing whitespace from names; to avoid that, you need to clear `IFS`. — Charles Duffy, Mar 03 '15 at 17:16
Also, `find | while read` emits a newline-delimited stream, but legitimate filenames on UNIX are allowed to contain newlines. Think about what happens if someone does a `mkdir -p devel/$'\n'/etc` and then writes to `devel/$'\n'/etc/passwd`; you'd better hope at this point that your script doesn't have permission to write to `/etc/passwd` on the remote machine. — Charles Duffy, Mar 03 '15 at 17:18
Also, `[ -d $i ]` will misbehave if `$i` contains spaces or a wildcard expression, or is empty. Either use `[[ -d $i ]]` (if your shell is bash or another ksh derivative) or `[ -d "$i" ]`, with the quotes (for POSIX compatibility). — Charles Duffy, Mar 03 '15 at 17:20
I just now found this problem, and just wanted to note, that the error condition only occurs when `ssh` successfully makes a connection with the remote machine. It does not happen when the connection is refused. — Pysis, Aug 09 '17 at 19:23

choroba · Accepted Answer · 2016-03-08T09:45:23.393

297

The problem is that ssh reads from standard input, therefore it eats all your remaining lines. You can just connect its standard input to nowhere:

ssh $USER@$SERVER "cd ${REMOTE_PATH}; mkdir -p $i" < /dev/null

You can also use ssh -n instead of the redirection.

edited Mar 08 '16 at 09:45

answered Feb 22 '12 at 10:34

choroba

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You can also use ssh -n to accomplish the same thing. – jordanm Feb 22 '12 at 19:34
3

This is fairly buggy, by the way. What if `$1` is a filename with spaces? You end up creating more than one directory. Similarly for `$REMOTE_PATH`. `printf %q` could be used to quote variables to be safely passed through the extra shell that ssh invokes. – Charles Duffy Mar 03 '15 at 17:14
Thank you. :D It took me some debugging before I noticed that the issue was executing commands with ssh and not another part of script. – wojciii Sep 01 '16 at 09:18
Could you elaborate what do you mean by ssh reads from stdin? I tried "ssh $USER@$SERVER "some commands"; echo "blabla"; and it works fine, so I don't understand what exactly it means? – ethan Sep 26 '17 at 12:38
@ethan: What do you mean by "works fine"? What did you expect? The echo is run after ssh is closed, so ssh can't read from it. – choroba Sep 26 '17 at 12:51

Charles Duffy · Answer 2 · 2015-03-03T17:24:25.130

Another approach is to loop over a FD other than stdin:

while IFS= read -u 3 -r -d '' filename; do
  if [[ -d $filename ]]; then
    printf -v cmd_str 'cd %q; mkdir -p %q' "$REMOTE_PATH" "$filename"
    ssh "$USER@$SERVER" "$cmd_str"
  else
    printf -v remote_path_str '%q@%q:%q/%q' "$USER" "$SERVER" "$REMOTE_PATH" "$filename"
    scp -Cp "$filename" "$remote_path_str"
  fi
done 3< <(find devel/ -newer "$UPLOAD_FILE" -print0)

The -u 3 and 3< operators are critical here, using FD 3 rather than the default FD 0 (stdin).

The approach given here -- using -print0, a cleared IFS value, and the like -- is also less buggy than the original code and the existing answer, which can't handle interesting filenames correctly. (Glenn Jackman's answer is close, but even that can't deal with filenames with newlines or filenames with trailing whitespace).

The use of printf %q is critical to generate commands which can't be used to attack the remote machine. Consider what would happen with a file named devel/$(rm -rf /)/hello with code which didn't have this paranoia.

ssh breaks out of while-loop in bash

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