Resuming interrupted s3 download with awscli

Use s3cmd it has a --continue function built in. Example:

# Start a download
> s3cmd get s3://yourbucket/yourfile ./
download: 's3://yourbucket/yourfile' -> './yourfile' [1 of 1]
    123456789 of 987654321     12.5% in 235s   0.5 MB/s

[ctrl-c] interrupt

# Pick up where you left off
> s3cmd --continue get s3://yourbucket/yourfile ./

Note that S3 cmd is not multithreaded where awscli is multithreaded, e.g. awscli is faster. A currently maintained fork of s3cmd, called s4cmd appears to provide the multi-threaded capabilities while maintaining the usability features of s3cmd:

https://github.com/bloomreach/s4cmd

There is a "hidden" command in the awscli tool which allows lower level access to S3: s3api.† It is less user friendly (no s3:// URLs and no progress bar) but it does support the range specifier on get-object:

   --range  (string) Downloads the specified range bytes of an object. For
   more   information   about   the   HTTP    range    header,    go    to
   http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35.

Here's how to continue the download:

$ size=$(stat -f%z myfile) # assumes OS X. Change for your OS
$ aws s3api get-object \
            --bucket mybucket \
            --key myfile \
            --range "bytes=$size-" \
            /dev/fd/3 3>>myfile

You can use pv for a rudimentary progress bar:

$ aws s3api get-object \
            --bucket mybucket \
            --key myfile \
            --range "bytes=$size-" \
            /dev/fd/3 3>&1 >&2 | pv >> myfile

(The reason for this unnamed pipe rigmarole is that s3api writes a debug message to stdout at the end of the operation, polluting your file. This solution rebinds stdout to stderr and frees up the pipe for regular file contents through an alias. The version without pv could technically write to stderr (/dev/fd/2 and 2>), but if an error occurs s3api writes to stderr, which would then get appended to your file. Thus, it is safer to use a dedicated pipe there, as well.)

† In git speak, s3 is porcelain, and s3api is plumbing.