Tests & Conditionals

if [ -f /etc/passwd ]; then
  echo 'exists'
elif [ -d /tmp ]; then
  echo 'dir'
else
  echo 'neither'
fi

Test	True when
`-f file`	file exists (regular)
`-d dir`	directory exists
`-e path`	path exists (any type)
`-r/-w/-x`	readable/writable/executable
`-z str`	string is empty
`-n str`	string is non-empty
`n1 -eq n2`	numbers equal (`-ne -lt -le -gt -ge`)
`s1 = s2`	strings equal (`!=` not equal)

$ [ 5 -gt 3 ] && echo yes
yes

Exam trap: Use -eq/-lt/-gt for numbers, =/!= for strings. [ "$a" = "$b" ] - quote variables to survive empty values and spaces.