shell scripting grep command

Question

I want to find whether a string contains a forward slash using "grep" command. It is easy at the beginning, and I wrote the following script.

foo=someone/books
if [ `echo "$foo" | grep '/'` ]
then
    echo "has forward slash"
fi

however, the problem is in the corner, if I set the variable "foo" to the following string,

foo="someone/books in stack"

The above script will be failed since there is "space" in variable foo, when the command expands, the condition in if statement is as below.

grep '/' someone/books in stack

Because of "space", the above "grep" command has too many arguments that is illegal. FYI, I try to solved this problem using case statement:

case $foo in
    */*)
        echo "has forward slash"
        ;;
    *)
        ;;
esac

However, I do not want to use case statement since it's verbose. So how could I solved this problem using grep command or others?

actually the problem does not exist the way you describe it - if you pipe a text with spaces into grep, it will work just fine, because it'll never expand it onto greps command line like you described. Verify again that echo "stuff with/ spaces" | grep '/' actually works fine — Yefim Dinitz
– Yefim Dinitz, Commented Nov 12, 2012 at 15:35
then you must be using the strangest grep I've ever seen... — Yefim Dinitz
– Yefim Dinitz, Commented Nov 12, 2012 at 15:49
@Yefim Dinitz In fact, if I write this command into script, it will not worked find. however if I write this command in the terminal, it will work find, So why? — eleven
– eleven, Commented Nov 12, 2012 at 15:52

dogbane · Accepted Answer · 2012-11-12 15:55:03Z

2

You are not using the if-statement correctly. The command in the if-condition needs to be quoted so that it becomes a single string when expanded. Like this:

if [ "`echo "$foo" | grep '/'`" ]
then
    echo "has forward slash"
fi

Or even better is if you check the return code of grep in your if-condition:

if $(echo "$foo" | grep -q '/')
then
    echo "has forward slash"
fi

You can also do away with the grep and use this instead:

foo="someone/books in stack"
if [[ "$foo" == */* ]]
then
  echo "has forward slash"
fi

edited Nov 12, 2012 at 15:55

answered Nov 12, 2012 at 15:47

dogbane

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1 Comment

eleven Over a year ago

I know. But what about /bin/sh rather than /bin/bash

Gabriel S. · Accepted Answer · 2012-11-12 15:56:52Z

0

foo="someone/books with spaces"
bar=`echo $foo | grep '/'`
if [ $? -eq 0 ]
then
    echo "$foo has forward slash"
fi

This works for me.

If you don't want to resort to using exit status this way, I suggest you take a look at dogbane's reply for a more correct answer.

answered Nov 12, 2012 at 15:56

Gabriel S.

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Comments

glenn jackman · Accepted Answer · 2012-11-12 17:42:41Z

0

There's nothing wrong with being verbose -- it often aids maintainability. However, you can terse it up: You don't need every newline, nor do you need the default branch if you do nothing in it.

case "$foo" in
    */*) echo "has forward slash" ;;
esac

If you insist on using grep, use the return status from it and not the output:

if echo "$foo" | grep -q /; then
    echo "has forward slash"
fi

answered Nov 12, 2012 at 17:42

glenn jackman

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