Variable Assignment in Shell Scripting

=

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: 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: 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)

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