performing regex on shell script variable?

ocicat · 07-16-2007, 12:21 PM

I'm simply not familiar with the intricacies of C shell scripting.

How do I change the contents of a variable through a regular expression? Assuming I want to simplify an absolute pathname by using tilde notation, how would this be done within a C shell script?

In Perl, this would be done similarly to:

Code:

my $s = `pwd`;
$s =~ s/^\Q$ENV{HOME}/~/;

It appears that tools like grep or sed all work on files, so I'm confused given that the value I want to translate is already contained in a variable.

Thanks for any insight which can be shared.

Tinkster · 07-16-2007, 12:58 PM

They quite happily work on anything you throw at them.

Code:

tmp=`echo $VARIABLE|sed 's/do_your/magic/'`

Use grep analogously :}

Cheers,
Tink

makyo · 07-17-2007, 01:26 PM

Hi.

The form of the assignment statement will vary from Bourne shell relatives (e.g. bash) to cshell (e.g. tcsh), but as Tinkster has written the key concept is to use echo. For example:

Code:

#!/bin/tcsh

# @(#) s1       Demonstrate set, setenv in tcsh.

echo
echo "$version" | sed 's/options.*//'
echo " Using shell $SHELL"

set script = "hello"

set SCRIPT="world"

setenv VAR " This will be an environment variable."

echo
echo " script is ${script}, SCRIPT is ${SCRIPT}"
echo " ( evaluating $script is same as ${script} )"
echo " The environment variable is :${VAR}:"
env | grep VAR

echo
echo " Argument 0 is $0"
set program = `basename $0`
echo " The result of basename on argument 0 is $program"
echo " All arguments from argv are $argv"

exit 0

producing:

Code:

% ./s1 one two three

tcsh 6.13.00 (Astron) 2004-05-19 (i386-intel-linux)
 Using shell /bin/tcsh

 script is hello, SCRIPT is world
 ( evaluating hello is same as hello )
 The environment variable is : This will be an environment variable.:
VAR= This will be an environment variable.

 Argument 0 is ./s1
 The result of basename on argument 0 is s1
 All arguments from argv are one two three

See man tcsh for details ... cheers, makyo

makyo · 07-17-2007, 02:10 PM

Hi.

As a follow-up, the later Bourne family shells can do some processing directly on variables:

Quote:

${parameter/pattern/string}
${parameter//pattern/string}
The pattern is expanded to produce a pattern just as in pathname
expansion. Parameter is expanded and the longest match of pat-
tern against its value is replaced with string. In the first
form, only the first match is replaced. The second form causes
all matches of pattern to be replaced with string. If pattern
begins with #, it must match at the beginning of the expanded
value of parameter. If pattern begins with %, it must match at
the end of the expanded value of parameter. If string is null,
matches of pattern are deleted and the / following pattern may
be omitted. If parameter is @ or *, the substitution operation
is applied to each positional parameter in turn, and the expan-
sion is the resultant list. If parameter is an array variable
subscripted with @ or *, the substitution operation is applied
to each member of the array in turn, and the expansion is the
resultant list.

-- man bash on patterns

For example:

Code:

#!/bin/sh

# @(#) s1       Demonstrate patterns and variables.

set -o nounset
echo
echo "GNU bash $BASH_VERSION" >&2
echo

t1="hello bad world"

echo " The value of t1 is :$t1:"
echo " Result of t1 processed :${t1/?bad?/ GOOD }:"

exit 0

producing:

Code:

% ./s1

GNU bash 2.05b.0(1)-release

 The value of t1 is :hello bad world:
 Result of t1 processed :hello GOOD world:

This is but one of the many reasons that most people prefer to write shell scripts in the Bourne shell family ... cheers, makyo