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Recent content - 1 new post
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1. Bash Parameter Expansion
Published Book page by john
[ http://kwlug.org/node/770 ]

We've seen environment variables, these are really just an arbitrary
group of variables that have special meaning to shells or commands. We
can use variables for our own personal use too. Typically variables are
used in shell scripts or in complex shell commands and can be
substituted into a command line by prefixing the variable name with a
dollar sign ($.) This is also called Parameter Expansion.
There are interesting ways to assign variables as well as substituting
them. You've seen the typical assignment and substitution:

filename="firecracker coleslaw recipe.txt"
echo "$filename"

Beyond simple substitution we can use syntax that will modify the
variables value for the substitution and optionally modify the variable
at the same time. Additionally it's possible to cause a shell command or
script to fail if a variable is not set.
For my first example I'll be using the for command we will rename files
en mass by changing the file extension from .TXT to .txt.
First I'll show you a simple loop that uses a file glob to select files
and uses the for command to assign each file name to a variable one at a
time, looping for each file name, then we'll print it just to show it
works:

for filename in *.TXT
do
    echo "$filename"
done

If all goes well, and you have .TXT files in the current directory,
you'll see a list of file names. Now we will want to use the mv command
to rename the files, but do to so we need to give mv the destination
name, a filename with a .txt extension. This is Linux so there are
several ways to do this. We could use sed to do a text replace, but it's
far easier to use a special Bash substitution syntax, that does the
replace for us. We will use the ${varname/%.TXT$/.txt}. This command
does a pattern match on the contents of varname and, because of the /%
it matches trailing text so if the filename ends with .TXT it is
replaced by .txt.
Here is the combined result:

for filename in *.TXT
do
    mv "$filename" "${filename/%.TXT/.txt}"
done

The above substitution command changes the substitution, not the
variable contents so if we were to look at $filename after the mv
command it would still show the .TXT extension.



There are many types of pattern substitution, some allowing replacement
text and others simple removing matched text. Check the man page for a
full listing of substitution commands. 

The pattern is the same as shell glob pattern matching and you can use
variables as part of the pattern or the replacement text. Here is a more
complex example:


#!/bin/bash

old="$1"
new="$2"
# Now do the replace
for filename in *$old
do
    mv "$filename" "${filename/%$old/$new}"
done

For the above commands to work it should be copied into a file (let's
say rename.ext,) then the file should be run using bash rename.ext .from
.to. The variables $1 and $2 are set to command line arguments 1 and 2.
So to change .TXT to .txt use bash rename.ext .TXT .txt. If you omit the
first line (#!/bin/bash) you can make a shell function from this too.
There are many types of special substitution commands that change how
the variable is substituted and others that change the variable itself.
Consider these two similar substitutions, they substitute the same
value, but they differ in that one sets var and the other doesn't. These
particular substitutions take place only when the variable is empty.

# Set var to nothing, this expands to a value, but doesn't set var.
var=""
echo ${var:-not set}
echo "the value of var=$var"

# try again using different expansion, this sets the value of var too.
var=""
echo ${var:=not set}
echo "the value of var=$var"

The above examples are very useful in setting variables to default
values inside scripts if they are not set var="${var:-default}".
Or perhaps you want the length of the variables contents

var="hello world"
echo ${#var}

Or perhaps a sub string of a variable

# Print out the from letter 6 the following 5 chars.
var="hello world"
echo ${var:6:5}

# Print from letter 0 the next 5 chars
echo ${var:0:5}

There are many more variable substitution syntaxes to use. Some care if
the variable is set, others just that it is empty (or unset)
Of the pattern matching syntaxes, some work on the beginning of a
string, some on the end. Some match the most text, others the least.
Some will substitute all occurrences of a pattern while others only
substitute the first occurrence. I won't list them all here, check the
man page (man bash and use the / command to search for Parameter
Expansion.


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