Shell Scripting – How to Use grep on a Variable

grepshellstring

Let's say I have a variable

line="This is where we select from a table."

now I want to grep how many times does select occur in the sentence.

grep -ci "select" $line

I tried that, but it did not work. I also tried

grep -ci "select" "$line"

It still doesn't work. I get the following error.

grep: This is where we select from a table.: No such file or directory

Best Answer

Have grep read on its standard input. There you go, using a pipe...

$ echo "$line" | grep select

... or a here string...

$ grep select <<< "$line"

Also, you might want to replace spaces by newlines before grepping :

$ echo "$line" | tr ' ' '\n' | grep select

... or you could ask grep to print the match only:

$ echo "$line" | grep -o select

This will allow you to get rid of the rest of the line when there's a match.

Edit: Oops, read a little too fast, thanks Marco. In order to count the occurences, just pipe any of these to wc(1) ;)

Another edit made after lzkata's comment, quoting $line when using echo.

Related Solutions

Shell – Write variable containing large text with \n to a file with common shell interpreters. How to do it

You can make a here document and use cat:

$ cat <<EOT >output
$a
EOT
$ wc -l output
40000

I don't know exactly how powerful your Busybox's ash is (it's configurable), but that should work anywhere, even with no builtins at all. It expands the variable value into a quasi-file, which is then given to cat, rather than putting the value itself into the command line.

Since you seem to have literal "\n" in your string that you want to get rid of, you can also use string replacement during parameter subsitution. This can be configured out of Busybox's ash, in which case you'll have to go to sed. If you do have this (non-POSIX) extension, you can use ${var//\\\n/$NEWLINE}:

$ busybox ash
$ foo="x\ny\nz"
$ echo "$foo"
x\ny\nz
$ NEWLINE="
"
$ echo "${foo//\\\n/$NEWLINE}"
x
y
z

This is the same syntax as Bash; ${var/pattern/replacement}, where pattern is global if it starts with /. It seems your ash has a lot configured out of it, so this might not be there at all. In that case you're probably going to have to go with sed, although I guess judicious use of IFS and read would let you work around that and reconstruct a correct string.

Best Answer

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Shell – Write variable containing large text with \n to a file with common shell interpreters. How to do it

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