Evalute passed in variable in bash

Question

Is there a way to pass in a variable as an argument to a bash script and have it evaluated scoped by the bash script?

Given:

# cat /path/to/file/of/host/names
bob
tom
joe
etc...


# dofor
FILE=$1
shift
CMD=$*

while read host; do
    # recursively turns "ssh \$host hostname" from $@ into:
    # ssh bob hostname
    # ssh tom hostname
    # ssh joe hostname
    # etc...
    eval $CMD
done < $FILE

When:

# dofor /path/to/file/of/host/names "ssh \$host hostname"

Then: I'd receive the output from running ssh host hostname for each hostname listed in /path/to/file/of/host/names. e.g.:

bob.example.com
tom.example.com
joe.example.com
etc...

Answer 1

Try this:

#!/bin/bash

FILE=$1
shift
CMD=$*
cmds=""

source <({
cat $FILE | while read host; do
    echo $CMD | sed -e "s|\$host|$host|g"
done
})

Answer 2

You can use

#!/bin/bash
FILE=$1
exec 3<$FILE
while read -u 3 host; do
    $2 $host $3
done

Then if you have:

$ cat /tmp/hostnames
some.host1.com
another.host2.net
some.host3.org    
some.host4.com

and run dofor.sh /tmp/hostnames ssh ls, it will run in sequence:

ssh some.host1.com ls
ssh another.host2.net ls
ssh some.host3.org ls
ssh some.host4.com ls

EDIT1:

If you'd like to change ssh or ls into some longer commands (or parts of such), just use quotes:

dofor.sh /tmp/hostnames "ssh -p 23" "ls -lh /"

---

EDIT2:

With the following script, you'll be able to use $host variable in as many places in your command as you want:

#!/bin/bash
exec 3<$1
while read -u 3 host; do
    eval $2
done

(I made this one shorter - no useless introducing of $FILE variable.) The important part here is that you need to use single quotes around the command containing the $host variable:

 dofor.sh /tmp/hostnames 'echo "trying $host :"; ssh -p 23 myuser@$host "ls -lh /"'

But beware that it is dangerous to use eval (see l0b0's wall) because if the file /tmp/hostnames contained a command on some line, it would be executed. Better not use this as root!

Answer 3

This should work, but it's a really bad idea unless you completely control the contents of $FILE and it only contains very simple statements.

while read -r
do
    $REPLY
done < $FILE

For the SSH example, you could do something much simpler by just looping over the host names:

while read -r host
do
    echo -n "${host}:"
    ssh "$host" hostname
done 9< "$FILE"

That should return a list of my hostname:remote hostname entries.

If you have a file with actual commands, you have to make sure whitespace is handled properly. The only way to be able to handle any parameters (including for example echo commands where the parameters are multiple lines) would be to use a custom format. One relatively simple way would be to separate each command and argument with a ␀ character, and put an extra ␀ character between the last argument of a command and the next command.