How to resize and/or reduce image file size of a JPEG

How to resize and/or reduce image file size of a JPEG:

convert $inputjpeg  -resize 25% -quality 75% $outputjpeg

Create an animated .gif from existing images using the Gimp

1. Open as Layers. In the open dialog, select the images that will become the frames of the animation
2. Export as .gif

How to remove unprintable characters from a file

cat <file> | tr -dc '[\011\012\015\040-\176]'

Creating a PEM format ssh credentials file

ssh-keygen

openssl rsa -in ./id_rsa -outform pem > ./id_rsa.pem

PAM: a simple RGB+ALPHA file format

To create images with transparencies without too much scaffolding, use the netpgm-associated format PAM, and then use pamtotiff.

#include 

int main() {
    
    printf("P7\n");
    printf("WIDTH 256\n");
    printf("HEIGHT 256\n");
    printf("DEPTH 4\n");
    printf("MAXVAL 255\n");
    printf("TUPLTYPE RGB_ALPHA\n");
    printf("ENDHDR\n");

    unsigned int row,col;
    for (row=0; row<256; row++) {
    for (col=0; col<256; col++) {
        
        printf("%c", (char)col);         // RED
        printf("%c", (char)(255-col));   // GREEN
        printf("%c", (char)row);         // BLUE
        printf("%c", (char)(255-row));   // OPACITY

    }}
    
    return 0;

}

Changing the location specified by java.io.tmpdir before running a Java program

On some systems, java.io.tmpdir is hard-coded to /tmp/. This can be changed with, e.g.:

export _JAVA_OPTIONS=-Djava.io.tmpdir=/mnt/tmp/whatever

Change a running process to nohup

1. put the process in the background (CTRL-Z)
2. make sure it's still running though (bg)
3. run disown -h

This doesn't really change to nohup, but it does prevent the job from receiving the SIGHUP signal when you exit the shell. Use the -a option to affect all jobs.

According to http://linux.about.com/library/cmd/blcmdl1_disown.htm:

disown [-ar] [-h] [jobspec ...]
    Without options, each jobspec is removed from the table of active jobs. If the -h option is given, each jobspec is not removed from the table, but is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP. If no jobspec is present, and neither the -a nor the -r option is supplied, the current job is used. If no jobspec is supplied, the -a option means to remove or mark all jobs; the -r option without a jobspec argument restricts operation to running jobs. The return value is 0 unless a jobspec does not specify a valid job.