Bash vs POSIX Shell

Bash in an extension over the POSIX shell, so everything that works in the POSIX shell can also be used in bash.

Tests VS if then

The test command is present in most shells and it tests whether a boolean expression is true or false. You use it via [] expressions.

Bash and POSIX-compliant shells have also if constructs, and these are usually used together with test expressions.

The two following snippets for testing whether two variables are the same are therefore equivalent:

# using test on its own
[ "$a" -eq "$b" ] && echo 'a and b are equal'

# equivalent: using if construct
if [ "$a" -eq "$b" ]; then
  echo 'a and b are equal'
fi

If-then-else syntax

Note that the symbol for equality is a single equals operator:

#!/usr/bin/env sh

if [ -f myfile.txt ]; then
  cat myfile.txt
elif [ -f otherfile.txt ]; then
  cat otherfile.txt
elif [ "foo" = "bar" ]; then
  echo "this will never be run!"
else 
  echo "files not found!" 
fi

One-line if-then

TEMPLATE: if condition; then action; fi;

EXAMPLE: echo "foo" if a file exists

if [ -f file.txt ]; then echo "exists"; fi;

One-line if-then-else

TEMPLATE: if condition; then action1; else action2; fi;

EXAMPLE: echo "foo" if a file exists, echo "bar" if it doesn't

if [ -f file.txt ]; then echo "foo"; else echo "bar"; fi;

Quoting

You need to place a variable within double quotes whenever there's the risk that it might contain special characters such as $, \ and so on.

Test If Two values are Equal / Different

• to compare strings use = and !=

  foo='foo'
  
  if [ "$foo" = 'foo' ];then
    echo 'equal'
  elif [ "$foo" = 'bar' ];then
    echo 'something else'
  else
    echo 'not equal'
  fi
  

• for booleans, use = and != also

  b1=true
  b2=false
  
  # -a means AND and -o means OR
  
  if [ $b1 = true -a $b2 = true ];then
    echo 'they are not both true'
  elif [ $b1 = false -a $b2 = true ];then
    echo 'they are not both false'
  elif [ $b1 = true -o $b2 = false ];then
    echo 'at least one of them is true'
  else
    echo 'something else'
  fi 
  

• to compare numbers use -eq (equal) and -ne (not equal)

  # was last command a success?
  if [ "$?" -eq 0 ]; then
    echo 'great success!'
  else
    echo 'failure'
  fi 
  

References

• The Unix shell Humble if