4.2. Variable Assignment


the assignment operator (no space before and after)


Do not confuse this with = and -eq, which test, rather than assign!

Note that = can be either an assignment or a test operator, depending on context.

Example 4-2. Plain Variable Assignment

#!/bin/bash # Naked variables echo # When is a variable "naked", i.e., lacking the '$' in front? # When it is being assigned, rather than referenced. # Assignment a=879 echo "The value of \"a\" is $a." # Assignment using 'let' let a=16+5 echo "The value of \"a\" is now $a." echo # In a 'for' loop (really, a type of disguised assignment): echo -n "Values of \"a\" in the loop are: " for a in 7 8 9 11 do echo -n "$a " done echo echo # In a 'read' statement (also a type of assignment): echo -n "Enter \"a\" " read a echo "The value of \"a\" is now $a." echo exit 0

Example 4-3. Variable Assignment, plain and fancy

#!/bin/bash a=23 # Simple case echo $a b=$a echo $b # Now, getting a little bit fancier (command substitution). a=`echo Hello!` # Assigns result of 'echo' command to 'a' echo $a # Note that including an exclamation mark (!) within a #+ command substitution construct #+ will not work from the command line, #+ since this triggers the Bash "history mechanism." # Inside a script, however, the history functions are disabled. a=`ls -l` # Assigns result of 'ls -l' command to 'a' echo $a # Unquoted, however, removes tabs and newlines. echo echo "$a" # The quoted variable preserves whitespace. # (See the chapter on "Quoting.") exit 0

Variable assignment using the $(...) mechanism (a newer method than backquotes). This is actually a form of command substitution.

# From /etc/rc.d/rc.local R=$(cat /etc/redhat-release) arch=$(uname -m)