Frequently Repeated Question/Request/Problems

NevemTeve · 03-12-2019, 12:06 AM

Q: How do I handle options in bash-script?
A: An example:

Code:

while [[ "$1" =~ ^- ]]; do 
    Opt="$1"
    shift
    ... process Opt
done

Q: What would be a non-bash-only solution?
A: You can use 'case':

Code:

while true; do
    case "$1" in
    -*) ;;
    *)  break
    esac
    Opt="$1"
    shift
    ... process Opt
done

NevemTeve · 03-19-2019, 11:47 PM

Q: I wish to process data from an environment variable. What are some wrong ways to do it?
A: Let's see some:

Code:

MyVar='n e m o'; echo $MyVar | sed 's/n/N/' => N e m o
MyVar='n  e  m  o'; echo $MyVar | sed 's/n/N/' => N e m o BAD
MyVar='n  e  m  o'; echo "$MyVar" | sed 's/n/N/' => N  e  m  o
MyVar='-n'; echo "$MyVar" | sed 's/n/N/' => (empty) BAD
MyVar='-n'; printf '%s\n' "$MyVar" | sed 's/n/N/' => -N

So the last one is the safe solution, just don't forget: printf (unlike echo) doesn't automatically append newline-character.

Q: Is there a simpler, bash-only, non-POSIX way?
A: Try this:

Code:

MyVar='n  e  m  o'; sed 's/n/N/' <<<"$MyVar" => N  e  m  o

Mind you, this method does append a newline.

NevemTeve · 03-20-2019, 12:16 AM

Q: My enemies created a file with a new-line (or other control-character between 0x00..0x1f or 0x7f..0x9f) in the filename. What can I do with it?
A: You can delete it with the Midnight Commander (F8). Or in shell, somehow put its name into an environment variable, then you can rename it:

Code:

Var=$'evil\nfile\rname'
mv -- "$Var" "$(printf '%s' "$Var" | tr '\000-\037\177-\237' _ )"
ls -l evil_file_name

Note: In windows-12** or utf8, character-codes 0x80..0x9f are legal.

NevemTeve · 02-16-2020, 09:10 AM

Q: I seem to have permission problems in my PHP-script. How do I debug it?
A: Use functions like posix_getcwd, stat, posix_getuid, posix_getgid. Example:

PHP Code:



<?php
    function printfilestat ($fname) {
        $st= stat ($fname);
        if (! $st) {
            printf ("\n%s: 'stat' failed\n", $fname);
            return;
        }
        $userdata= posix_getpwuid ($st['uid']);
        $groupdata= posix_getgrgid ($st['gid']);
        printf ("file '%s': owner=%d(%s) group=%d(%s) access=%o\n",
            $fname,
            $st['uid'], $userdata['name'],
            $st['gid'], $groupdata['name'],
            $st['mode']);
    }

    printf ("<PRE>\n");
    $uid= posix_getuid ();
    $userdata= posix_getpwuid ($uid);
    $gid= posix_getgid ();
    $groupdata= posix_getgrgid ($gid);
    $cwd= posix_getcwd ();

    printf ("posix_pwuid=%d(%s) posix_getgid=%d(%s)\n",
        $uid, $userdata['name'],
        $gid, $groupdata['name']);
    printf ("posix_getcwd=%s\n", $cwd);

    printfilestat ($cwd);
    printfilestat (__FILE__);
    printfilestat (dirname (__FILE__));

    printf ("</PRE>\n");
?>

example output:

Code:

posix_pwuid=33(www-data) posix_getgid=33(www-data)
posix_getcwd=/local/home/projects/public_html
file '/local/home/projects/public_html': owner=1000(projects) group=1000(devel) access=40755
file '/local/home/projects/phptest/web/access.php': owner=1000(projects) group=1000(devel) access=100755
file '/local/home/projects/phptest/web': owner=1000(projects) group=1000(devel) access=40755