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.

My dumbass can only come up with three:

1. You are already root (ok, fine)
2. You have made /dev/ writable by non-privileged users
3. Your non-privileged user already owns the symlink /dev/nul. Which "ok, fine", but also the point of command would have to be to functionally do nothing other than print out the error ln: failed to create symbolic link '/dev/nul': File exists

I would love to understand the use case behind #2. I am also curious to see even 7 more cases, let alone your figurative million.

In regards to #3 even if the behaviour of ln was to replace a symlink if it already existed, it'll probably have to unlink() the existing symlink, which I'm pretty sure is gonna get you a permission denied error on any /dev filesystem with sane permissions.

Oopsie, let's correct that and pretend it didn't happen.