Ubuntu – What does 2>/dev/null mean

bashcommand lineredirectstdout

I would like a brief explanation of the following command line:

grep -i 'abc' content 2>/dev/null

The > operator redirects the output usually to a file but it can be to a device. You can also use >> to append.

If you don't specify a number then the standard output stream is assumed, but you can also redirect errors:

> file redirects stdout to file
1> file redirects stdout to file

2> file redirects stderr to file

&> file redirects stdout and stderr to file
> file 2>&1 redirects stdout and stderr to file

/dev/null is the null device it takes any input you want and throws it away. It can be used to suppress any output.

Note that > file 2>&1 is an older syntax which still works, &> file is neater, but would not have worked on older systems.

It's sometimes useful to allow parameter expansions side-effects to occur.

For example, setting a default value

read -p "Enter your name: " name
: ${name:=John Doe}  # if the user entered an empty string
echo "$name"

$- means 'current flags'.
echo $- returns "himBH". Those are all defaults.
so ... [[ $- != *i* ]] && return actually does what it says above in a comment: it checks if the interactive flag is set. The [[ and ]] make it a boolean so it ends up in a "true" or "false". "false && return" makes it go on "true && return" makes it execute the return.

The default flags explained in more detail:

h is for "hash all": this tells bash to remember the locations of commands it has found through querying your PATH.

i is for "interactive": entering input & getting back output.

m is for "monitor": this enables job control

B is for "brace expand". This allows you to use brace expansion

H is for "history expand". This is what enables you to rerun a command from your history by prefacing its number with an exclamation point

By the way. I have ...

# If not running interactively, don't do anything
case $- in
    *i*) ;;
      *) return;;
esac

Basically does the same but easier to read I assume.