bash - Passing multiple sets of arguments to a command
2014-01
instances
contains several whitespace separated strings, as does snapshots
.
I want to run the command below, with each instance-snapshot pair.
ec2-attach-volume --instance $instances --device /dev/sdf $snapshots
For example, if instances
contains A B C
, and snapshots
contains 1 2 3
, I want the command to be called like so:
ec2-attach-volume -C cert.pem -K pk.pem --instance A --device /dev/sdf 1
ec2-attach-volume -C cert.pem -K pk.pem --instance B --device /dev/sdf 2
ec2-attach-volume -C cert.pem -K pk.pem --instance C --device /dev/sdf 3
I can do either one or the other with xargs -n 1
, but how do I do both?
Using the bash
shell, one possible method is:
instances=( `cat instances` )
snapshots=( `cat snapshots` )
for i in ${!instances[@]}; do
ec2-attach-volume -C cert.pem -K pk.pem \
--instance ${instances[i]} --snapshot ${snapshots[i]}
done
Here ${!instances[@]}
expands to all indexes of the instances
array (0, 1, 2, and so on). You can examine the array contents by doing declare -p instances snapshots
after the first two lines.
Here’s an approach that includes (some) error handling:
xargs –n 1 < instances > instances.1_per_line # “xargs –n 1” is equivalent to xargs –n 1 < snapshots > snapshots.1_per_line # “xargs –n 1 echo”. if [ $(wc –l < instances.1_per_line) != $(wc –l < snapshots.1_per_line) ] then echo "Different numbers of strings." >&2 else paste instances.1_per_line snapshots.1_per_line | while read inst snap do ec2-attach-volume -C cert.pem -K pk.pem --instance "$inst" \ --device /dev/sdf "$snap" done fi rm instances.1_per_line snapshots.1_per_line
This may work better than grawity’s answer for large files, as it doesn’t need to read the files into memory.
And this may work better in older versions of bash
.
(If $(command)
doesn’t work, try `command`
.)
On the other hand, my answer will fail
if any of the strings in the files contain quote characters.
To do it in parallel you can use GNU Parallel:
parallel --xapply ec2-attach-volume --instance {1} --device /dev/sdf {2} ::: `cat instances` ::: `cat snapshots`
Watch the intro videos to learn more: https://www.youtube.com/playlist?list=PL284C9FF2488BC6D1
In bash, you can use !*
to get all the arguments from the previous command. As an example, if you did cp /some/path /some/other/path
and then did mv !*
, the second command would be expanded to mv /some/path /some/other/path
.
Is there anything like this that can be used to access a specific argument from a command instead of all of them?
In !*
, !
is the history expansion prefix, and *
is the word designator that means all arguments. You can memorize the general syntax as bang-line-colon-column (!
line:
column). There are many possible shortcuts: the default line is the previous line, the default column specifier is “all”, and you can leave off the colon if the column specifier is non-numeric (but !3
would mean line 3). You can use !:0
to refer to the command name, !:1
, !:2
, etc, to refer to successive arguments, !:$
or !:*
for the last word, and more.
See also this post by Michael Mrozek at Unix Stack Exchange.
Personnally, I really dislike this “expansion with exclamation mark” feature which will even disturb if you try echo "Hello World!"
in interactive shells (so sourcing scripts that assume they will be run in non-interactive mode won't work at all).
So, I set set +o histexpand
and start recalling arguments with the following method:
Esc, 1, Esc, Ctrl-Y => Insert first argument of previous command.
note that the Esc-trick is because I don't have a meta key.