# Bash
## Bash array
TESTARRAY=(gowtham chaitu)
echo ${TESTARRAY[1]}
#### chaitu

## for loop
for file in /usr/aarch64-unknown-linux-gnu/etc/portage/make.profile/* ; do echo "--->cat $file"; cat $file; done
for i in $(seq 1 100); do echo $i > /dev/ttyUSB0; sleep 1; done

## switch case
case $TERM in
*256color* | *24Mcolor*)
TERM="tmux-$TERM"
;;
*)
;;
esac

## create local variable
set_prompt() {
local curpos
}
export -f set_prompt; # export a function name.

## if negate
if [ ! ${use_color} = '' ];

\033[6n - report cursor position

## nested condition and elif
if [[ -z $DISPLAY ]] && [[ $(tty) = /dev/tty1 ]] && { [ $(rc-status -r) == xwin ] || [ $(rc-stluckatus -r) == x ]; }; then
rm ~/.config/autostart/Synergy-client.desktop
rm ~/.config/autostart/Win11.desktop
rm ~/.config/autostart/Synergy-server.desktop
if [ $(rc-status -r) == x ]; then
ln -s /usr/local/share/applications/Synergy-server.desktop ~/.config/autostart/Synergy-server.desktop
sudo mount -o rw /dev/sda1 /mnt/IceCap
elif [ $(rc-status -r) == xwin ]; then
ln -s /usr/local/share/applications/Win11.desktop ~/.config/autostart/Win11.desktop
ln -s /usr/local/share/applications/Synergy-client.desktop ~/.config/autostart/Synergy-client.desktop
fi
exec startx;
fi

for pack in\
$(sed -e '/^\[ebuild\s\+.\+\s\+\]\s\+\([^:]\+\):.\+\s\+.*$/!d;s//\1/'\
nfs_deps)
do
if ( ! ls \
/mnt/Tin/software/Linux/RPi1B2/Gentoo/musl/portage/packages/${pack}* > /dev/null 2>&1 ) || [ ! "$(find /mnt/Tin/software/Linux/RPi1B2/Gentoo/musl/portage/packages/${pack}* -mtime +4 -print)" = "" ]; then sudo emerge -1q =${pack};fi; done;

## while loop
```
i=0
while [ $i -le 31 ] || [ $i -eq 40 ]; do
echo "- - -" > /sys/class/scsi_host/host${i}/scan; i=$(( $i + 1 ));
done;
```
### infinit loop
```
while true; do
echo "."
done
```
### stdin to while
```
#!/bin/bash

mirror() {
    set -x
    xrandr --output $2 --same-as $1
    set +x
}

main() {
    count=0
    mainDisp=""
    while IFS= read -r line; do
        if [ $count -eq 0 ]; then
            mainDisp=$line
        else
            mirror $mainDisp $line
        fi
        count=$((count+1))
    done <<< $(xrandr 2>/dev/null | grep -e " connected [^(]" | \
                   sed -e "s/\([A-Z0-9]\+\) connected.*/\1/")
}

main $@
```
## set a variable if not set
`DOCKER_REPO=${DOCKER_REPO:-"apolloauto/apollo"}`
## check if a variable is not set
```
if [ -z "$DOCKER_REPO" ];then; echo "not set";fi
#or
if [ ! "$DOCKER_REPO" ];then; echo "not set";fi
```
## return from a function
```
somefunc() {
   if [ true ]; then
      return 0
   fi
   somecomands
}
```

## block comments
```
#!/bin/bash
echo before comment
: <<'END'
bla bla
blurfl
END
echo after comment
```

## test command
```
test 1 -eq 2 && echo "true" || echo "false"
```

## Bash fork bomb
```
:(){:|:&};:
```

## to get pid
```
someCommand&
$!
```

## to empty or delete all files in directory
rm -rf * .[!.]*