This is a BASH script tip for improving multi-variable assignments from a single input line.

Example

On OS X, /usr/bin/sw_vers -productVersion will output the current computer’s OS version:

$ /usr/bin/sw_vers -productVersion
10.11.4

When assign the components from the version string into separate variables I often see this:

major=$( /usr/bin/sw_vers -productVersion | awk -F'.' '{print $1}' )
minor=$( /usr/bin/sw_vers -productVersion | awk -F'.' '{print $2}' )
patch=$( /usr/bin/sw_vers -productVersion | awk -F'.' '{print $3}' )

This works fine, but calls sw_vers and awk three times to parse the same string into variables.

We could remove the repeated calls to sw_vers by assigning it’s output to a variable and then use that variable as input to the awk commands:

version=$( /usr/bin/sw_vers -productVersion )
major=$( awk -F'.' '{print $1}' <<< "${version}" )
minor=$( awk -F'.' '{print $2}' <<< "${version}" )
patch=$( awk -F'.' '{print $3}' <<< "${version}" )

Better, but still executes awk three times in order to parse the string and assign the variables.

read

BASH has a builtin command named read designed for this situation.

This is what the same variable assignment looks like when using read:

IFS='.' read -r major minor patch < <( /usr/bin/sw_vers -productVersion )

Command explained:

I start by defining the separator I want to use to split the input string:

IFS='.'

Then, I call the read command followed by the variable names I want to assign each value to:

read -r major minor patch

I use process substitution to pass the output from sw_vers to the input for the read command:

< <( /usr/bin/sw_vers -productVersion )

When executed, read will use the separator to split the input line and assign each value one by one to each of the variable names defined.

Matching number of variables and values

If I define fewer variable names than there are available values, read will assign one value to each variable until there are no more variables to assign. After that it will just add the remainder of the line to the last variable (including separators).

Consider this from the above example, but here I’ve defined fewer variable names than values:

IFS='.' read -r major minor < <( /usr/bin/sw_vers -productVersion )

# Now the variables hold the following values:
(major=10)
(minor=11.4)

If I define more variable names than there are values, read will just fill each variable until there are no more values to assign and then assign empty values to the remaining variables:

IFS='.' read -r major minor patch beta < <( /usr/bin/sw_vers -productVersion )

# Now the variables hold the following values:
(major=10)
(minor=11)
(patch=4)
(beta=)

Only assign a subset of values from the line

If you don’t want want the entire line assigned to variables you should parse the line before passing it to read. Then you can control what variables and values will be associated:

read -r minor patch < <( /usr/bin/sw_vers -productVersion | awk -F'.' '{print $2"\ "$3}' )

# Now the variables hold the following values:
(minor=11)
(patch=4)

man page

Because read is a BASH builtin command it doesn’t have it’s own man page.

You can use this command to jump to the read section in the bash man page:

man bash | less -p "^       read "

Here are more links to Apple’s bash man page (search for “read [” to find the correct section.):

• Open in Terminal.app
• Open in Browser
  
  

Final Notes

By saving computer resources (removing unnecessary subshells and pipes), we’ve reduced the execution time of the variable assignment in our example by 55% (real time).

This is the time each example took:

• sw_vers and awk each called three times:

  real	0m0.031s  
  user	0m0.016s  
   sys	0m0.016s
  

• sw_vers output to variable and awk called three times:

  real	0m0.022s
  user	0m0.009s
   sys	0m0.011s
  

• read:

  real	0m0.014s
  user	0m0.002s
   sys	0m0.003s