Compound if statements with multiple expressions in Bash
Bash's [[ ]] and (( )) are more powerful and flexible than [ ].
[[ ]]is for strings and(( ))is for integer logic and arithmetic&&and||operators can be used inside[[ ]]and(( )), and()can be used for grouping- No need to quote variable expansions inside
[[ ]]or(( ))- Bash doesn't do word splitting or globbing in these contexts - Inside
(( )), there is no need for a$behind variable names to expand them [[ ]]and(( ))can span multiple lines, without the need for a line continuation with\
Using these, we can write clean, readable, and more reliable code.
Examples
Compound statements with integers
a=1 b=2 c=3
((a == 2 || (b == 2 && c == 3))) && echo yes # yields yes
Compound statements with strings
x=apple y=mango z=pear
[[ $x == orange || ($y == mango && $z == pear) ]] && echo yes # yields yes
[ equivalents for the above statements, using {}
[ "$a" -eq 2 ] || { [ "$b" -eq 2 ] && [ "$c" -eq 3 ]; }
[ "$x" == orange ] || { [ $y == mango ] && [ "$z" == pear ]; }
Related
- Is double square brackets [[ ]] preferable over single square brackets [ ] in Bash?
- How to use double or single brackets, parentheses, curly braces
- Comparing integers: arithmetic expression or conditional expression on Unix & Linux
- Test for non-zero length string in Bash: [ -n “$var” ] or [ “$var” ]
- Conditional Expressions - Bash Manual
For Bash, you can use the [[ ]] form rather than [ ], which allows && and || internally:
if [[ foo || bar || baz ]] ; then
...
fi
Otherwise, you can use the usual Boolean logic operators externally:
[ foo ] || [ bar ] || [ baz ]
...or use operators specific to the test command (though modern versions of the POSIX specification describe this XSI extension as deprecated -- see the APPLICATION USAGE section):
[ foo -o bar -o baz ]
...which is a differently written form of the following, which is similarly deprecated:
test foo -o bar -o baz