How to check size of a file using Bash?

Question

[ -n file.txt ] doesn't check its size, it checks that the string file.txt is non-zero length, so it will always succeed.

If you want to say "size is non-zero", you need [ -s file.txt ].

To get a file's size, you can use wc -c to get the size (file length) in bytes:

file=file.txt
minimumsize=90000
actualsize=$(wc -c <"$file")
if [ $actualsize -ge $minimumsize ]; then
    echo size is over $minimumsize bytes
else
    echo size is under $minimumsize bytes
fi

In this case, it sounds like that's what you want.

But FYI, if you want to know how much disk space the file is using, you could use du -k to get the size (disk space used) in kilobytes:

file=file.txt
minimumsize=90
actualsize=$(du -k "$file" | cut -f 1)
if [ $actualsize -ge $minimumsize ]; then
    echo size is over $minimumsize kilobytes
else
    echo size is under $minimumsize kilobytes
fi

If you need more control over the output format, you can also look at stat. On Linux, you'd start with something like stat -c '%s' file.txt, and on BSD/Mac OS X, something like stat -f '%z' file.txt.

It surprises me that no one mentioned stat to check file size. Some methods are definitely better: using -s to find out whether the file is empty or not is easier than anything else if that's all you want. And if you want to find files of a size, then find is certainly the way to go.

I also like du a lot to get file size in kb, but, for bytes, I'd use stat:

size=$(stat -f%z $filename) # BSD stat

size=$(stat -c%s $filename) # GNU stat?

alternative solution with awk and double parenthesis:

FILENAME=file.txt
SIZE=$(du -sb $FILENAME | awk '{ print $1 }')

if ((SIZE<90000)) ; then 
    echo "less"; 
else 
    echo "not less"; 
fi

If your find handles this syntax, you can use it:

find -maxdepth 1 -name "file.txt" -size -90k

This will output file.txt to stdout if and only if the size of file.txt is less than 90k. To execute a script script if file.txt has a size less than 90k:

find -maxdepth 1 -name "file.txt" -size -90k -exec script \;

If you are looking for just the size of a file:

$ cat $file | wc -c
> 203233

stat appears to do this with the fewest system calls:

$ set debian-live-8.2.0-amd64-xfce-desktop.iso

$ strace stat --format %s $1 | wc
    282    2795   27364

$ strace wc --bytes $1 | wc
    307    3063   29091

$ strace du --bytes $1 | wc
    437    4376   41955

$ strace find $1 -printf %s | wc
    604    6061   64793