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I would like to have a bash script that checks if a file has more than # amount of lines but i have not yet got it working right and I'm not so sure on how to do it. I've never used bash before.

right now i use: linesStr=$(cat log | wc -l) to get the amount of lines in the file (expect it to be a string). when echo'ing it gives me the number 30 which is correct.

but since its most likely a string it doesnt do the if-statement, so i need to have linesStr converted into a int called linesInt.

I also have the feeling the if-statement itself is not done correctly either.

#!/bin/bash

linesStr=$(cat log | wc -l)
echo $linesStr

if [$linesStr > 29]
    then echo "log file is bigger than 29 lines"
    #sed -i 1d log
fi

I would appreciate if anyone can give me a simple beginners solution.

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3
  • You can just do wc -l < log. Commented Feb 11, 2017 at 23:09
  • 1
    wc -l < log returns "30 log" and $(cat log | wc -l) just gives "30" so I'd prefer to use that instead Commented Feb 11, 2017 at 23:31
  • 1
    Sorry, that was a bug in my original comment. wc -l log prints 30 log, but wc -l < log just prints 30. Commented Feb 12, 2017 at 1:04

4 Answers 4

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11
  1. No need for cat.
  2. Lack of spaces around [ and ].
  3. Use a numeric comparison operator instead of the redirect operator.

Here is a working script.

#!/bin/bash

linesStr=$( wc -l < log )

if [[ "$linesStr" -gt "29" ]]; then
    echo Foo
fi
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10 Comments

No need for awk.
Try it without.
With your script i get: -bash: test.bash: line 15: syntax error near unexpected token fi' -bash: test.bash: line 15: fi'
why using awk when you can do something like this wc -l < log
Either with wc or with awk , we can avoid sub shells and pipes: awk 'END {print NR}' log is the same as wc -l <log = prints the number of lines directly.
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2

your if block of code is wrong if [$linesStr > 29] there should be a space after [ and before ]

#!/bin/bash

linesStr=$(wc -l < log )
echo $linesStr

if [[ $lineStr -gt 29 ]];then 
    echo "log file is bigger than 29 lines"
fi

it is advisable that you always use [[ ]] with an if statement rather than using [ ]. Whenever you want to compare integers dont use > or <, use -gt -ge -lt -le. And if you want to do any form of mathematical comparison it is advisable that you use (( )).

(( lineStr > 29 )) && {
    # do stuff
}

you should also note that you don't need the bash comparison operators or getting the value of a variable with $ when using (( ))

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1 Comment

Comparing numbers in bash (man bash - arithmetic evaluation) can be done with (( )) for hard coded numbers but with variables there is a bug: If one of the variables holds not valid number (i.e text) the result of arithmetic evaluation is incorrect. Test it with : a=4;b="3";(($a>$b)) && echo "ok" || echo "not ok" (this works fine) and then repeat the test by assigning b="asd".
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There are no string or integer types to convert. The problem is that you're using the wrong comparison operator. For numeric comparison use if [ $linesStr -gt 29 ]. Read man bash section CONDITIONAL EXPRESSIONS for available operators.

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1 Comment

The other problem is that [ is a command (and ] is an argument of that command), so you need spaces around [ and ]. [$linesStr will try to run a command named [30.
0 
(( $(wc -l < log) > 29 )) && echo too long
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1 Comment

While this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference.

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