Ubuntu – What does >&2 mean in a shell script

bashcommand lineredirect

I am doing my homework and I have a question about one line. I don't understand what this >&2 means:

if test -d $2 
  then echo "$2: Is directory" >&2 ; exit 21
fi

Best Answer

It is simply displaying the message "/blah/blah/: Is directory" to stderr. Also known as Standard Error which is denoted by &2.

Without the &2 messages are displayed on stdout. Also known as Standard Output which is denoted by &1.

More details on displaying messages to &>2 can be found here:

In your command posted, both messages for stdout and stderr will appear on your terminal screen. However some applications will separate the stderr messages and perform special processing.

Most people don't bother redirecting echo error messages to >&2 but it is technically the correct way of doing things.

For more reading on stdin, stdout and stderr from user or system administrator perspective see:

Confused about stdin, stdout and stderr?

For a programmers perspective of stdin, stdout, stderr which are &0, &1 and &2 respectively see:

Linux Programmer's Manual

Related Solutions

Ubuntu – What does “%U” mean when calling a command

It's a parameter for the Exec key in .desktop files (defined in the Desktop Entry Specification) that describes how arguments to the program (from the file manager/program launcher, e.g. multiple selected files) should be handled:

%u A single URL. Local files may either be passed as file: URLs or as file path.

%U A list of URLs. Each URL is passed as a separate argument to the executable program. Local files may either be passed as file: URLs or as file path.

So, e.g., /usr/share/applications/gedit.desktop contains:

Exec=gedit %U

which means that arguments to gedit will be treated as a list of URLs (or local files).

For a complete list of possible parameters, see The Exec key.

Ubuntu – What does “[[ $- != i ]] && return” mean

$- 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.

Best Answer

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Ubuntu – What does “%U” mean when calling a command

Ubuntu – What does “[[ $- != *i* ]] && return” mean

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Ubuntu – What does “[[ $- != i ]] && return” mean