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Evaluating a string in shell script

rated 0 times [ 2] [ 0] / answers: 1 / hits: 11678 / 2 Years ago, wed, february 2, 2022, 6:15:43

I am following this bash shell scripting guide:

http://www.tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html

In the section Numeric Comparisons, it cites an example:

anny > num=`wc -l work.txt`



anny > echo $num

201



anny > if [ "$num" -gt "150" ]

More input> then echo ; echo "you've worked hard enough for today."

More input> echo ; fi

What seems to happen above is we store a string of commands in a bash variable and then we invoke echo on the variable. What seems to happen is the string is evaluated and the wc command is executed and returns the line count to the controlling terminal.

Ok, so I launch my terminal in Ubuntu 12.04 and try something similar:

$ touch sample.txt && echo "Hello World" > sample.txt

$ cat sample.txt

Hello World

$ num='wc -l sample.txt'

echo $num

wc -l sample.txt

Wait a second, that didn't evaluate the string and return the line count. That just echoed the string back to the terminal. Why did I get different results?

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edseager

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2 Years ago

answered 2 Years ago tionavocad · Accepted Answer

You need to use backticks to evaluate the expression.

$ num=`wc -l sample.txt`

$ echo $num

1 sample.txt

If you want to see only "1" in the output, use the command

$ num=`cat sample.txt | wc -l`

$ echo $num

1

And also works:

$ num=`wc -l < sample.txt`

$ echo $num

1

For additional information, see Differences between doublequotes " ", singlequotes ' ' and backticks ´ ´ on commandline?