By Mendel Cooper
This can be the whole reference at the Borne back shell (bash). quantity 1Volume 2 includes all appendices.
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This makes $d an integer. d = 2334 2334 + 1 = d = 2335 # What about null variables? e="" echo "e = $e" # e = let "e += 1" # Arithmetic operations allowed on a null variable? echo "e = $e" # e = 1 echo # Null variable transformed into an integer. # What about undeclared variables? echo "f = $f" # f = let "f += 1" # Arithmetic operations allowed? echo "f = $f" # f = 1 echo # Undeclared variable transformed into an integer. # Variables in Bash are essentially untyped. exit 0 Untyped variables are both a blessing and a curse.
Example 7-3. /bin/bash # Arithmetic tests. # The (( ... )) construct evaluates and tests numerical expressions. # Exit status opposite from [ ... ] construct! Chapter 7. k. " # Division result < 1. # Rounded off to 0. " # Illegal division by 0. # 1 # What effect does the "2>/dev/null" have? # What would happen if it were removed? # Try removing it, then rerunning the script. # ======================================= # # (( ... )) also useful in an if-then test. var1=5 var2=4 if (( var1 > var2 )) then #^ ^ Note: Not $var1, $var2.
There exists a dedicated command called [ (left bracket special character). It is a synonym for test, and a builtin for efficiency reasons. This command considers its arguments as comparison expressions or file tests and returns an exit status corresponding to the result of the comparison (0 for true, 1 for false). 02, Bash introduced the [[ ... ]] extended test command, which performs comparisons in a manner more familiar to programmers from other languages. Note that [[ is a keyword, not a command.