Department of Engineering

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Variables

The shell maintains a list of variables, some used by the shell to determine its operation (for instance the PATH variable which determines where the shell will look for programs), some created by the user.

A shell variable can be given a type (just as variables in Java or Fortran can), but this isn't necessary. Typing

      pix=/usr/lib/X11/bitmaps

(note: no spaces around the `=' sign) creates a variable `pix' whose value can be accessed by preceding the name with a `$'. `cd $pix' will take you to a directory of icons. If you type

      pix=8

then pix will be treated as a number if the context is appropriate. The let command built into the shell performs integer arithmetic operations. It understands all the C operators except for $++$ and $--$. The typeset command can be used to force a variable to be an integer, and can also be used to select the base used to display the integer, so

pix=8
let "pix=pix<<2"
echo $pix
typeset -i2 pix
echo $pix

will print 32 then 2#100000 onscreen. The let command (note that $ signs aren't necessary before the operands) uses C's << operator to shift bits 2 to the left and the typeset command makes pix into a base 2 integer.

typeset can do much else too. Look up the shell man page for a full list. Here are just a few examples

# You can control the how many columns a number occupies and whether 
# the number is right or left justified. The following
# typeset command right-justifies j in a field of width 7. 
j=1
typeset -R7 j
echo "($j)"

# Variables can be made read-only. This j=2 assignment causes an error.
typeset -r j
j=2

# String variables can be made upper or lower case.
string=FOO
echo $string
typeset -l string
echo $string