Switched On Tech Design

Category: Terminal

  • How to easily find the full path of a command in Ubuntu

     

    If you’re writing scripts or making cron jobs you will need to know the full path of the commands you’re using; rather than just being able to use “ls” you would have to use “/bin/ls” instead. You could use the find command here but there’s a quicker and more elegant way: which. Use it like so:

     

    which ls

     

    will return:

     

    /bin/ls

     

    Not everything will be in /bin, e.g.:

     

    which timeout

     

    will likely return:

     

    /usr/bin/timeout

     

    Simple but it will make your life quite a bit easier when writing scripts, particularly with new commands or command which you don’t use often.

  • Using rsync with a non-standard SSH port

     

    There are many reasons you may be using an SSH port other than 22; perhaps you changed it as a security measure, or perhaps you have multiple machines behind your firewall which you are port forwarding to and thus have to use other ports in addition to 22. Rsync is an extremely powerful file synchronization tool which by default uses SSH to connect your source and destination, thus if you have changed your SSH port you will need to tell rsync. This can be easily done with the e switch like so (using 2222 as the new SSH port as an example):

     

    rsync -e “ssh -p 2222” /path/to/source [email protected]:/path/to/destination

     

    As a practical example using the other options -avzP (our typical selection) your command might look like:

     

    rsync -avzP -e “ssh -p 2222” /home/user/myfile [email protected]:/home/user/

     

     

  • How to exclude results from grep

     

    Sometimes you may wish to further filter grep output, such as in the following situation:

     

    # zfs get all | grep compressratio

    backup01         compressratio         1.23x                  –
    backup01         refcompressratio      1.00x                  –
    backup01/data    compressratio         1.50x                  –
    backup01/data    refcompressratio      1.50x                  –
    backup01/photos  compressratio         1.05x                  –
    backup01/photos  refcompressratio      1.05x                  –

    Here we only really want to see the compressratio results, not the refcompressratio. We can pipe the grep output to another grep command, this time inverting the results with the -v switch.

     

    # zfs get all | grep compressratio | grep -v refcompressratio

    backup01         compressratio         1.21x                  –
    backup01/data    compressratio         1.50x                  –
    backup01/photos  compressratio         1.05x                  –

    This excludes any line containing refcompressratio, making the results easier to read. This is particularly helpful when you have a large number of results from grep.