4.2. Variable Assignment

=

the assignment operator (no space before and after)

Caution

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

   1 #!/bin/bash
   2 # Naked variables
   3 
   4 echo
   5 
   6 # When is a variable "naked", i.e., lacking the '$' in front?
   7 # When it is being assigned, rather than referenced.
   8 
   9 # Assignment
  10 a=879
  11 echo "The value of \"a\" is $a."
  12 
  13 # Assignment using 'let'
  14 let a=16+5
  15 echo "The value of \"a\" is now $a."
  16 
  17 echo
  18 
  19 # In a 'for' loop (really, a type of disguised assignment):
  20 echo -n "Values of \"a\" in the loop are: "
  21 for a in 7 8 9 11
  22 do
  23   echo -n "$a "
  24 done
  25 
  26 echo
  27 echo
  28 
  29 # In a 'read' statement (also a type of assignment):
  30 echo -n "Enter \"a\" "
  31 read a
  32 echo "The value of \"a\" is now $a."
  33 
  34 echo
  35 
  36 exit 0


Example 4-3. Variable Assignment, plain and fancy

   1 #!/bin/bash
   2 
   3 a=23              # Simple case
   4 echo $a
   5 b=$a
   6 echo $b
   7 
   8 # Now, getting a little bit fancier (command substitution).
   9 
  10 a=`echo Hello!`   # Assigns result of 'echo' command to 'a' ...
  11 echo $a
  12 #  Note that including an exclamation mark (!) within a
  13 #+ command substitution construct will not work from the command-line,
  14 #+ since this triggers the Bash "history mechanism."
  15 #  Inside a script, however, the history functions are disabled by default.
  16 
  17 a=`ls -l`         # Assigns result of 'ls -l' command to 'a'
  18 echo $a           # Unquoted, however, it removes tabs and newlines.
  19 echo
  20 echo "$a"         # The quoted variable preserves whitespace.
  21                   # (See the chapter on "Quoting.")
  22 
  23 exit 0

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

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