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If your system implements procfs interface, you can just check if there is a special file /proc/$PID/status:

if [ -e /proc/$PID/status ]; then echo "process exists" fi 

otherwise you can use ps program:

if [ -n "$(ps -p $PID -o pid=)" ] 

In the latter form, -o pid= is an output format to display only the process ID column with no header. The quotes are necessary for non-empty string operator -n to give valid result.

On systems that implement procfs interface such as Linux, you can just check if /proc/$PID exists:

if test -d /proc/"$PID"/; then echo "process exists" fi 

otherwise you can use ps program:

if [ -n "$(ps -p $PID -o pid=)" ] 

In the latter form, -o pid= is an output format to display only the process ID column with no header. The quotes are necessary for non-empty string operator -n to give valid result.

if [ -e /proc/$PID/status ]; then echo "process exists" fi 

or

if [ -n "$(ps -p $PID -o pid=)" ] 

In the latter form, -o pid= is an output format to display only the process ID column with no header. The quotes are necessary for non-empty string operator -n to give valid result.

If your system implements procfs interface, you can just check if there is a special file /proc/$PID/status:

if [ -e /proc/$PID/status ]; then echo "process exists" fi 

otherwise you can use ps program:

if [ -n "$(ps -p $PID -o pid=)" ] 

In the latter form, -o pid= is an output format to display only the process ID column with no header. The quotes are necessary for non-empty string operator -n to give valid result.

if [ -n "$PID" -a -e /proc/$PID ]; then echo "process exists" fi 

or

if [ -n "$(ps -p $PID -o pid=)" ] 

In the latter form, -o pid= is an output format to display only the process ID column with no header. The quotes are necessary for non-empty string operator -n to give valid result.

if [ -e /proc/$PID/status ]; then echo "process exists" fi 

or

if [ -n "$(ps -p $PID -o pid=)" ] 

In the latter form, -o pid= is an output format to display only the process ID column with no header. The quotes are necessary for non-empty string operator -n to give valid result.