Shell – Check for process and kill if running

processpsshell

I am trying to find if a process is running or not, and if it is runing then I want to call a script or print something. I tried with the below few ways, but it gives me syntax error or command not found error.

APP_ID = ps -eaf | grep -i `whoami` | grep -i <process_name> | grep -i java | awk '{print$2}'

if[''!= '${APP_ID}'] then
    echo "Stopping instance $APP_ID"

I get the result as below:

test.sh: line 15: APP_ID: command not found
test.sh: line 17: if[!= ${APP_ID}]: command not found
Stopping instance

What is wrong in the above script? And if it has some errors, then why does it print the echo in the if condition?

Also the below script fails saying test.sh: line 3: [ps: command not found

if [ps -eaf | grep -i `whoami` | grep -i <process_name> | grep -i java]; then
    echo 'stop'
fi

Best Answer

Your syntax has many problems:

remove spaces around "=" when setting a variable

wrong:
```
APP_ID = value
```
right:
```
APP_ID=value
```
to run a program and put its output into a variable, you need $(...) (preferred in bash, not available in sh) or backticks `command` (supported in bash and sh). (and remember that when assigning a variable, quotes are not needed, but in other cases they are important: command "$(command)")

wrong:
```
APP_ID=command
```
right:
```
APP_ID=$(command)
```
add spaces around everything when using "[", and you need a semicolon or newline after the "]". This is because "[" is a bash builtin command, and the non-builtin one is also known as "test" (see man test), which just like other commands, takes its arguments separated by space:

wrong:
```
if[x!= y] then echo hello world fi
```
right:
```
if [ x != y ]; then echo hello world; fi
```
use double quotes, not single quotes when you want to expand a variable

wrong:
```
if [ '' != '${APP_ID}' ]; then
```
right:
```
if [ '' != "${APP_ID}" ]; then
```
Also for the above example, you can use -n (non-empty) instead of comparing to empty string
```
if [ -n "${APP_ID}" ]; then
```
And for the ps example, you don't need grep and awk:
```
APP_ID=$(ps -C whoami --no-header --format 'pid')
```

And so here is the fixed up script:

APP_ID=$(ps -C whoami --no-header --format 'pid')

if [ -n "${APP_ID}" ]; then
    echo "Stopping instance $APP_ID"
fi

Related Solutions

Bash – How to view the output of a running process in another bash session

If all you want to do is spy on the existing process, you can use strace -p1234 -s9999 -e write where 1234 is the process ID. (-s9999 avoids having strings truncated to 32 characters, and write the system call that produces output.) If you want to view only data written on a particular file descriptor, you can use something like strace -p1234 -e trace= -e write=3 to see only data written to file descriptor 3 (-e trace= prevents the system calls from being loged). That won't give you output that's already been produced.

If the output is scrolling by too fast, you can pipe it into a pager such as less, or send it to a file with strace -o trace.log ….

With many programs, you can divert subsequent output with a ptrace hack, either to your current terminal or to a new screen session. See How can I disown a running process and associate it to a new screen shell? and other linked threads.

Note that depending on how your system is set up, you may need to run all these strace commands as root even if the process is running under your user with no extra privileges. (If the process is running as a different user or is setuid or setgid, you will need to run strace as root.) Most distributions only allow a process to trace its children (this provides a moderate security benefit — it prevents some direct malware injection, but doesn't prevent indirect injection by modifying files). This is controlled by the kernel.yama.ptrace_scome sysctl.

Bash – How to do a grep on remote machine and print out the line which contains those words

If you are open to other formats, consider:

inc="hello|animal|atttribute|metadata"
exc="timeout|runner" 
ssh machineB "grep -E '$inc' path/ptd.log | grep -vE '$exc'"

Faster Alternative

If your log files are large and you are grepping for fixed words, as opposed to fancy regular expressions, you may want to consider this approach:

inc='hello
animal
atttribute
metadata'

exc='timeout
runner'

ssh office "grep -F '$inc' ptd.log | grep -vF '$exc'"

By putting each word on a separate line, we can use grep's -F feature for fixed strings. This turns off regex processing, making the process faster.