Check if a process ID was provided

Can you reverse engineer a bash script or command alias that produces the same output as the following, this command should show the virtual memory of the process with the given pid.

$ vmrss 17102
pid choosen: 17102 from 17102 vs
41.3164 MB
41.3164 MB
41.3164 MB
41.3164 MB
41.3164 MB
41.3164 MB
41.3164 MB
41.3164 MB
41.3164 MB
41.3164 MB

To create a Bash script or command alias that replicates the output you’ve shown, we need a script that accepts a process ID as an argument, reads the virtual memory size (VmRSS) from /proc/[pid]/status, and then prints it out multiple times in a specific format.

Here’s a basic version of such a script:

#!/bin/bash

# Check if a process ID was provided
if [ -z "$1" ]; then
    echo "Usage: $0 <pid>"
    exit 1
fi

pid=$1

# Check if the /proc/[pid]/status file exists
if [ ! -f "/proc/$pid/status" ]; then
    echo "Process with PID $pid not found."
    exit 1
fi

# Extract the VmRSS value
vmrss_kb=$(grep VmRSS /proc/$pid/status | awk '{print $2}')

# Check if VmRSS was found
if [ -z "$vmrss_kb" ]; then
    echo "Could not find VmRSS for process with PID $pid."
    exit 1
fi

# Convert KB to MB
vmrss_mb=$(echo "scale=4; $vmrss_kb/1024" | bc)

# Print the result 10 times
echo "pid choosen: $pid from $pid vs"
for i in {1..10}; do
    echo "$vmrss_mb MB"
done

As an alias

alias vmrss="function _vmrss(){\
  pid=$1;\
  vmrss_kb=$(grep VmRSS /proc/$pid/status | awk '\''{print $2}'\'');\
  vmrss_mb=$(echo "scale=4; $vmrss_kb/1024" | bc);\
  echo "pid choosen: $pid from $pid vs"; for i in {1..10}; do echo "$vmrss_mb MB"; done; };\
  _vmrss"

Usage

vmrss 17102

Please note that these scripts assume a Linux environment where the /proc filesystem is available and that bc (for basic calculations) is installed. This script might need modifications to work in different environments or to handle edge cases more gracefully.

What does VmRSS stand for

“VmRSS” stands for “Virtual Memory Resident Set Size.” In the context of operating systems like Linux, it refers to the amount of memory that a process has in physical RAM, excluding any memory that the process is using that has been swapped out to disk.

The VmRSS is an important metric when assessing the memory usage of a process, as it directly reflects the real amount of RAM that the process is currently using. This is in contrast to other memory metrics that may include memory shared with other processes or memory that is allocated but not actively used.

To summarize, VmRSS:

Represents the non-swapped physical memory a process is using. Is crucial for understanding the actual memory footprint of a process on the system’s RAM. Is often monitored for performance tuning and system administration tasks, especially in environments where memory usage is critical.

What does the ‘-z’ in the test command stand for?

In shell scripting, the -z option used within the test command or the [ ] construct is used to check if the length of a string is zero. It essentially tests whether a given string is empty.

For example:

str=""
if [ -z "$str" ]; then
    echo "The string is empty."
else
    echo "The string is not empty."
fi

In this script, if [ -z “$str” ] checks if the variable str contains an empty string. If str is empty, the condition evaluates to true, and the script will echo “The string is empty.” Otherwise, it will echo “The string is not empty.”

This check is particularly useful in scripts to verify that variables (often inputs or arguments) have been set and are not just empty strings.