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up vote 6 down vote favorite
1

I want to read my files line by line every 5 seconds. This time I just tried one-line bash command to do this. And bash command is:

let X=1;while [ $X -lt 20 ];do cat XXX.file |head -$X|tail -1;X=$X+1;sleep 5;done

However I got the error like:

-bash: [: 1+1: integer expression expected

What's the problem? btw, why can't we do $X < 20? (Instead we have to do -lt, less than?)

thx

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And this doesn't address your question at all, but have you considered using the read operator, rather than that cat-head-tail thing? It would be simpler and much less IO-intensive. –  Malvolio Aug 18 '11 at 16:30

3 Answers 3

up vote 15 down vote accepted

Your assignment X=$X+1 doesn't perform arithmetic; if $X is 1, it sets it to the string "1+1". Change X=$X+1 to let X=X+1 or let X++.

As for the use of -lt rather than <, that's just part of the syntax of [ (i.e., the test command). It uses = and != for string equality and inequality -eq, -ne, -lt, -le, -gt, and -ge for numbers. As @Malvolio points out, the use of < would be inconvenient, since it's the input redirection operator.

Or consider using the equivalent (( expr )) construct rather than the let command. For example, let X++ can be written as ((X++)). At least bash, ksh, and zsh support this, though sh likely doesn't. I haven't checked the respective documentation, but I presume the shells' developers would want to make them compatible.

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up vote 1 down vote

I would use

X=`expr $X + 1`

but that's just me. And you cannot say $X < 20 because < is the input-redirect operator.

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Thx...I notice, there must be space among 'expr', '$X', '+',AND '1'...So why... –  wang Aug 18 '11 at 16:37
    
Is "because" sufficient answer? The shell's parser is not very sophisticated. It was written sometime around 1970. Do you remember 1970? I don't and I'm older than you. There have been a lot of updates to shell, of course, but nobody wants to break backward-compatibility. –  Malvolio Aug 18 '11 at 17:02
    
-> expr is a program and it expects operands and operators to be separate command-line arguments. –  svckr Mar 15 '13 at 11:37
up vote 1 down vote

The sum X=$X+1 should be X=$(expr $X + 1 )

You can also use the "<" for the comparison, but you have to write (("$X" < "20")) with the double parenthesis instead of [ $X -lt 20 ]

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