Yrurtj

I am trying to learn how to use getopts so that I can have scripts with parsed input (although I think getopts could be better). I am trying to just write a simple script to return partition usage percentages. The problem is that one of my bash functions does not seem to like that I reference $1 as an variable within the function. The reason I reference $1 is because the get_percent function can be passed a mount point as an optional argument to display instead of all of the mount points.

The script

#!/usr/bin/bash



set -e

set -u

set -o pipefail



get_percent(){

    if [ -n "$1" ] 

    then

        df -h $1 | tail -n +2 | awk '{ print $1,"t",$5 }'

    else

        df -h | tail -n +2 | awk '{ print $1,"t",$5 }'

    fi

}



usage(){

    echo "script usage: $(basename $0) [-h] [-p] [-m mount_point]" >&2

}



# If the user doesn't supply any arguments, we run the script as normal

if [ $# -eq 0 ];

then

    get_percent

    exit 0

fi

# ...

The Output

$ bash thing.sh

thing.sh: line 8: $1: unbound variable



$ bash -x thing.sh

+ set -e

+ set -u

+ set -o pipefail

+ '[' 0 -eq 0 ']'

+ get_percent

thing.sh: line 8: $1: unbound variable

set -u will abort exactly as you describe if you reference a variable which has not been set. You are invoking your script with no arguments, so get_percent is being invoked with no arguments, causing $1 to be unset.

Either check for this before invoking your function, or use default expansions (${1-default} will expand to default if not already set to something else).

This is the effect of set -u.

You could check $# inside the function and avoid referencing $1 if it is not set.

Since this is bash you can sidestep the check for $1 being set and just use "$@" (when double-quoted, this parameter disappears completely if it has no values, which avoids it being caught by set -u):

get_percent() {

    df -h "$@" | awk 'NR>1 { printf "%st%sn", $1, $5 }'

}

I've also tweaked the rest of the line slightly so that you don't get {space}{tab}{space} between the two values you output but insead you get just a {tab}. If you really want the two invisible spaces then change the awk to use printf "%s t %sn", $1, $5.

Your key problem is here:

if [ -n "$1" ] 

By wrapping $1 in quotes, you're asking bash to perform string interpolation before it evaluates the -n operator.

Instead, what you want is more like this:

if [ -n $1 ]

This allow bash to directly inspect the variable, without it being packed up in another expression first.

Since you're already using bash, and have tossed compatibility with sh out the window, you should probably take this one step further:

if [[ -n $1 ]]

This is because [ is probably an actual binary on your system, and if bash finds [ in your PATH, it will use that [, not its builtin implementation. You won't have this problem with [[, so you know it's bash's implementation that gets used, and you know that it's the same version of bash that's running the rest of your script. (This might seem like a nonissue, but there are scenarios where multiple versions of bash might exist on the same system. Most commonly with Chef Habitat, but I'm sure it also happens in various cobbled sandbox environments, too.)

Finally, for stylistic reasons, I'd suggest

if [[ -n ${1} ]]

as the preferred form. The reason for the {...} is because it makes it easier to reference arguments beyond ${9}. ${10} works, $10 doesn't.