Can't install any packages: "uname: command not found"

I'm a sysadmin at a university helping researchers get set up with rstudio-server on a new Ubuntu 18.04 computational server. It seems that I can install packages in RStudio just fine, but other users are seeing error messages like this one:

/usr/lib/R/bin/R: line 8: uname: command not found
/usr/lib/R/bin/R: line 143: exec: sh: not found
Warning in install.packages :
  error in running command
Warning in install.packages :
  installation of package ‘Rcpp’ had non-zero exit status

I've double checked that which uname returns /bin/uname for each of their accounts, and that both /bin and /usr/bin are in $PATH for each of them. I can't figure out for the life of me what might be causing uname to not be found in rstudio, but installing packages in R using terminal/SSH works fine for all of them, and I have no problems with my account in RStudio.

Thanks in advance for pointing me in the right direction!