if statement in .bashrc not working correctly

Question

I set up .bashrc to show a long prompt if the terminal is at least 80 characters wide. Otherwise it shows a degraded, smaller prompt.

I use an if statement to achieve this:

dp_smaller_than=80
if [ $COLUMNS -lt $dp_smaller_than ]
then
    PROMPT_COMMAND="degr_prompt"
else
    PROMPT_COMMAND="full_prompt"
fi

It only works correctly if I put a sleep 0 or other command before this statement, otherwise it shows the following error:

[: -lt: unary operator expected

Note: I have the degr_prompt and full_prompt functions defined in the beginning of the file.

Solution 1

COLUMNS variable is set by bash upon receipt of a SIGWINCH. (see Shell Variables section in man bash). It is not set in your script because this is not an environment variable. See yourself:

$ export COLUMNS=$COLUMNS
$ ./your-script.sh

Now it works. You need to retrieve value of COLUMNS in your script. You can use tput for example:

$ tput cols
186

Solution 2

COLUMNS is not set somehow in .bashrc.

Just get the columns via tput cols like this:

dp_smaller_than=80
if [ $(tput cols) -lt $dp_smaller_than ]
then
    PROMPT_COMMAND="degr_prompt"
else
    PROMPT_COMMAND="full_prompt"
fi

Comments

[polym]

can you provide the statement, which sets COLUMNS?

[psimon]

@polym, it is set automatically.

[psimon]

You're right. Strange, $COLUMNS only get set if a command gets executed. I tested it with echo $COLUMNS.

[polym]

+1 because of SIGWINCH info.

[phemmer]

@psimon man bash: checkwinsize - If set, bash checks the window size after each command and, if necessary, updates the values of LINES and COLUMNS.. So if you've got checkwinsize set, you just need to execute a command (a simple : would suffice).

[psimon]

@Patrick it seems that not all commands work for this. sleep, echo, printf work, but alias, shopt, [ or setting variables don't (I have these before the if statement in .bashrc). Is it in connection with interactivity?