grep a shell script variable contents

kushalkoolwal · 02-02-2009, 08:50 PM

Hi,

I have a bash shell script which has a variable $DEVICE. Now I would like to use "grep" to do some pattern matching on the contents of variable $DEVICE.

For example, suppose variable DEVICE has "ABC XYZ" stored in it. Now I would like to grep "ABC" on $DEVICE.

I am not ale to figure out how to do it. It is easy to do a grep on file:
grep "ABC" <file-name> but how do you do on a variable?

Thanks

weibullguy · 02-02-2009, 09:11 PM

One way would be...

Code:

echo $DEVICE | grep -o ABC

Someone else will have another that is probably better.

ErV · 02-02-2009, 11:36 PM

Quote:

Originally Posted by kushalkoolwal

Hi,

I have a bash shell script which has a variable $DEVICE. Now I would like to use "grep" to do some pattern matching on the contents of variable $DEVICE.

For example, suppose variable DEVICE has "ABC XYZ" stored in it. Now I would like to grep "ABC" on $DEVICE.

I am not ale to figure out how to do it. It is easy to do a grep on file:
grep "ABC" <file-name> but how do you do on a variable?

Thanks

You can't get value from $DEVICE using grep. You can extract contents of assignment operation using something like

Code:

grep "^\$DEVICE[ \t]*=[ \t]*.*$" file.txt| sed 's/^\$DEVICE[ \t]*=[ \t]\(.*\.)$/\1/g'

(grep finds line with assignment, sed cuts off left part of assignment. Probably can be made 2x times shorter)
But you can't extract value from script without running it, because if there are operations that change value of $DEVICE later, grep/sed won't handle it, and you will get incorrect result.

kushalkoolwal · 02-02-2009, 11:38 PM

Quote:

Originally Posted by weibullguy

One way would be...

Code:

echo $DEVICE | grep -o ABC

Someone else will have another that is probably better.

Thanks that just worked for me! Exactly what I was looking for...

Dr_P_Ross · 02-03-2009, 04:35 AM

Why not use the pattern-matching capabilities of parameter expansion in bash?
Call this little script "tryme":

Quote:

#!/bin/bash

vartest=${1/*ABC*}

if [ "x$vartest" == "x" ];
then
echo vartest matches
else
echo vartest does not match
fi

Then, assuming you have made the script executable:

Quote:

% ./tryme BBBABC
vartest matches
% ./tryme "BBB ABC"
vartest matches
% ./tryme "BBB ABBC"
vartest does not match
% ./tryme "..ABCXY"
vartest matches
% ./tryme "..A BCXY"
vartest does not match
%

archtoad6 · 02-03-2009, 07:47 AM

OP,
It looks like you've got your answer, but I'm curious: What problem prompted your Q?

Using grep -o, Post #2
weibullguy,
Nice piece of code. grep -o is one of my favorites. (Who was the 6th Marx brother? . . . Greppo)

Using grep & sed

Quote:

Originally Posted by ErV

You can extract contents of assignment operation using something like

Code:

grep "^\$DEVICE[ \t]*=[ \t]*.*$" file.txt| sed 's/^\$DEVICE[ \t]*=[ \t]\(.*\.)$/\1/g'

I afraid the above misses the point of the OP.
This code won't work because bash doesn't use '$' as part of the variable name in assignment statements.
Code:
```
grep "^DEVICE[ \t]*=[ \t]*.*$" file.txt| sed 's/^DEVICE[ \t]*=[ \t]\(.*\.)$/\1/g'
```

The '[ \t]*' surrounding the '=' is not only unnecessary, but wrong:

Code:

$ DDD = xxx
bash: DDD: command not found
$ DDD= xxx
bash: xxx: command not found
$ DDD =xxx
bash: DDD: command not found
$ DDD=xxx

There must be no spaces in a bash variable assignment.

Code:

grep "^DEVICE=.*$" file.txt| sed 's/^DEVICE=\(.*\.)$/\1/g'

There is no need to pipe grep to sed -- sed has its own line selection mechanism.
Code:
```
sed '/^DEVICE=/s/^DEVICE=\(.*\.)$/\1/g' file.txt
```
The 2nd '.' in '$.*\.)' is unnecessary & misplaced.
Code:
```
sed '/^DEVICE=/s/^DEVICE=\(.*$$/\1/g' file.txt
```
Escaping '(' & ')' can be avoided by using the "-r" option.
Code:
```
sed -r '/^DEVICE=/s/^DEVICE=(.*)$/\1/g' file.txt
```

Commas are better delineators.

Code:

sed -r '/^DEVICE=/s,^DEVICE=(.*)$,\1,g' file.txt

The 'g' in 's...g' is only necessary for multiple instaces in a given line, the '^' & '$' framing the search pattern preclude this possibility.
Code:
```
sed -r '/^DEVICE=/s,^DEVICE=(.*)$,\1,' file.txt
```
If your intent is to print only the assigned values, you will need to include the "-n" & ",p" options:
Code:
```
sed -nr '/^DEVICE=/s,^DEVICE=(.*)$,\1,p' file.txt
```
Finally, there is no need for the '\1' mechanism (& w/ it the "-r" option), just suppress the 'DEVICE=':
Code:
```
sed -n '/^DEVICE=/s,^DEVICE=,,p' file.txt
```

Warning: I did test this code, but only a little.

Farzan Mufti · 02-03-2009, 11:41 AM

If you wanted to use grep's pattern matching capability you could use
if (( `echo $variable | grep -c "PATTERN"` > 0 ))
then
CODE
fi

A better way could be to use Bash's pattern matching using =~ operator.
if [[ $variable =~ Regular Exp ]]
then
CODE
fi

weibullguy · 02-03-2009, 12:15 PM

Quote:

Originally Posted by archtoad6

Who was the 6th Marx brother?

Manfred, I think.

archtoad6 · 02-04-2009, 06:15 AM

, no it was Greppo, I hid the answer here (& in my OP) in a background matching color tag -- highlight it to read it; or did you know that?

... Of course you knew that, quoting the post displayed the hidden answer & the method of hiding it.