• jim3692@discuss.online
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    3 months ago

    In bash, when you redirect the output of a command to /dev/null, like cat /etc/passwd >/dev/null, you are silencing the output.

    There are cases that this is useful, for example when checking if an application is installed:

    node -v >/dev/null && echo "Node.js is installed"

    This line tries to get the version of Node.js, but it silences the output. That’s because we don’t care about the version. We only care about whether the execution was successful, which implies the existence of Node.js in the system.

    • Korthrun@lemmy.sdf.org
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      3 months ago

      Dear linux newbies of the fediverse:

      Please do not run cat for the sole purpose of copying a single files content to STDOUT

      Your system almost certainly has a pager on it (e.g. ‘less’, ‘more’, ‘most’). Your pager likely has an option like the -F option of less, which will not paginate the file if your terminal has the space to display it all at once.

      You do not need to involve cat to get a files contents into a variable. Any POSIX compliant shell will support MYVAR=$(</tmp/myfile)

      You do not need to involve cat to iterate over the lines of a file. You can do things like:

      while read myline
      do
          printf "found '%s'\n" "$myline"
      done </tmp/myfile
      

      If you want to concatenate multiple files, but do not care if they all exist, you might use /dev/null to suppress the “no such file” error from cat as such cat file1 file2 file3 2>/dev/null. Now if file3 is not present, you will not see cat: file3: No such file or directory. 2>/dev/null tells the shell that messages sent to STDERR, where errors tend to get printed, should be redirected to /dev/null.


      Please do not invoke a command only to see if it is available in the directories listed your PATH environment variable

      As an aside this is not the same as seeing if it’s installed.

      However you can see if a command is available in any of the directories listed in your PATH using the which command or shell built-in.

      You might want to do something like:

      #!/bin/bash
      
      which node &> /dev/null
      HAS_NODE="$?"
      
      # ... MORE CODE HERE ...
      
      if [[ $HAS_NODE ]]
      then
          # something you only do if node is present
          :
      else
          # do something else or print a friendly error
          :
      fi
      

      This way you don’t see the output of the “which” command when you run the script, but you do get it’s exit code. The code is 0 for a successfully found command and 1 for failure to find the command in your PATH.

      • qqq@lemmy.world
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        3 months ago

        Alternatively, use your shell however you want. And which isn’t POSIX so I wouldn’t use that in a shell script you intend to share.

        • Korthrun@lemmy.sdf.org
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          3 months ago

          Once upon a time I would have been more particular about the “which issue”. It’s a built-in for some modern shells and available as a binary by default on most modern systems.

          You are correct though, if you want to write a 100% POSIX compliant shell script you’re better off using command, type or actually looping over the contents of $PATH and checking for the presence of your desires binary.

          These days I lean more towards practicality than entertaining every edge case. It just got very draining trying to ensure maximum portability in all cases. Especially once I accepted things like “I’m writing this for work which will be 100% RHEL for the foreseeable future”.

          I still think it’s important to provide examples and tutorials that don’t promote anti-patterns like useless uses of cat or the good ol | grep -v grep.

      • Gronk@aussie.zone
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        3 months ago

        Huh TIL thank you, suppose I should make the leap to learn bash properly instead of clinging onto my perl scripts

          • Gronk@aussie.zone
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            3 months ago

            I absolutely love perl, I’ve fallen out of professional development but I would take a job to maintain a legacy perl codebase in a heartbeat.