Private Island Networks Inc.

Linux Command Line Tips

Various Notes on Using and Managing a Linux System from the Command Line

Table of Contents


  • A line that start with a "$" denotes a command line entry in a user shell / terminal.
  • '#' can denote two things: root shell or comment. If a line starts with '$', then '#' and what follows is a comment.
  • Most examples will work equally well on an Ubuntu desktop or an embedded Linux system built with Yocto (assuming the right packages are installed).
  • Most commands have many options. Refer to the man pages for more information.

Command Line Basics

Let's determine our login and information about the machine we're using:

$ whoami

$ grep postgres /etc/passwd
# uid: 123, gid: 130, home directory: /opt/pgsql/data, shell: /bin/bash

$ uname -a
Linux <hostname> 4.15.0-64-generic #73-Ubuntu SMP <date> x86_64 x86_64 x86_64 GNU/Linux

$ hostname

# info about our distribution 
$ lsb_release -a
Distributor ID:	Ubuntu
Description:	Ubuntu 18.04.3 LTS
Release:	18.04
Codename:	bionic

Annoying Bell

Turn off the bell you hear while working on the command line

edit /etc/inputrc:

# do not bell on tab-completion
set bell-style none


Quickly build a searchable cscope database / index of your C project:

$ find -L . -name '*.[chS]' > cscope.files
$ cscope -b -q -k
$ cscope -d


date uses the following format: [MMDDhhmm[[CC]YY][.ss]]

# # assume it's noon on April 22 2019:
# date 042212002019
Mon Apr 22 12:00:00 EDT 2019

# # check how timezone is set:
# ls -l /etc/localtime 
lrwxrwxrwx 1 root root 27 Apr 22 10:12 /etc/localtime -> /usr/share/zoneinfo/EST5EDT

# # change it to central:
# ln -f -s /usr/share/zoneinfo/CST6CDT /etc/localtime
# date 
Mon Apr 22 11:01:45 CDT 2019

Additional notes on date and timezone:

  • Use ntp daemon to automatically update date and time
  • The TZ setting can also be modified in /etc/profile

Basic Disk Operations

Tasks for creating a new drive partition mounting, testing, and fixing it:

# # format partitions on an unmounted drive:
# fdisk /dev/sda

# # create a filesystem on the third SDA partition:
# mke2fs /dev/sda3

# # mount a partition at /mnt/drive:
# mkdir -p /mnt/drive
# mount /dev/sda3 /mnt/drive

# # test and fix disk errors on the partition (unmount first)
# e2fsck /dev/sda3 

# # or to automatically fix errors and be verbose:
# e2fsck -vy /dev/sda3


Control environment variables from the command line:

$ export var1="privacy"	 # note there are no spaces

$ echo $var1		# return the value of "var1"

$ set | grep var1

$ echo "I like my $var1"
I like my privacy

$ unset var1		# remove the environemnt variable

$ echo $var1

$ set | grep var1

Exit Status of a Program

If you're familiar with 'C' programming on Linux, you probably realize that many programs will return a meaningful value upon exit. You can poll this value on the command line as shown with a simple example below.

$ cd /	  # cd to root of file system
$ ls 
bin boot ...

$ echo $?

$ ls -l does-not-exist
ls: cannot access 'does-not-exist': No such file or directory

$ echo $?

$ man ls
  Exit status:
       0      if OK,

       1      if minor problems (e.g., cannot access subdirectory),

       2      if serious trouble (e.g., cannot access command-line argument).


Non-trivial examples using a very powerful but non-intuitive tool:

$ # find *.py files and copy to a temp directory
$ find . -name '*.py' | xargs -I{} cp {} ~/temp

$ # delete *.js files that aren't minified: 
$ find . -path '*.js' ! -path '*.min.js' | xargs rm

$ # find my js files but exclude those in node_modules and build
find . -path '*.js' ! -path "./node_modules/*" ! -path "./build/*"

$ # Find source files in a C project and include symbolic links
$ find -L  . -name '*.[chS]'

$ # same as above but include cpp files
$ find -L  . -name '*.[chS]' -or -name '*.cpp'

$ # Search book directory tree for a pdf title, but exclude files that start with a '.'
$ find . -name '[^.]*.pdf' | grep -i <part of title>

$ # Let's do the same as above but prune books in the oreilly directory
$ find . -path "./oreilly*" -prune -o -name '[^.]*.pdf'

$ # Sort all files within a directory structure by date & time; print newest first
$ find . -type f -printf '%TY-%Tm-%Td %TT %p\n' | sort -r


Some useful grep options for searching through files:

$ # search recursively for "elf" as a word and case insensitive
$ grep -iwR elf .

$ # do the same thing but exclude binary files 
$ grep -iwIR elf .

$ # find "elf" when it's at the end of a line, note use of regex here
$ grep -iwR 'elf$' .

$ # use expanded / full regexp to search for .html or .htm in file
$ grep -E "\.html?" <file>

useful grep command line options:

  • -i: case insensitive
  • -n: output line number
  • -w: select only those lines containing matches that form whole words
  • -I: exclude binary files
  • -R: search into the directories below

Kill a process

Suppose our thunderbird app isn't responding, and we want to kill it:

$ ps -e | grep thunderbird
 3120 pts/1    00:01:01 thunderbird
$ sudo kill -9 3120
$ ps -e | grep thunderbird

$ # but with an app that we launched, there's an easier way:
$ pgrep thunderbird
$ pkill thunderbird 
$ pgrep thunderbird


Show the shared libraries used by the run-time linker:

$ ldconfig -p
1334 libs found in cache `/etc/' (libc6,x86-64) => /usr/lib/x86_64-linux-gnu/ (libc6,x86-64) => /usr/lib/x86_64-linux-gnu/

# grep for a particular library
$ ldconfig -p | grep libwebkit (libc6,x86-64) => /usr/lib/x86_64-linux-gnu/

Note that the "so" suffix stands for "shared object"

Print the shared object dependencies of an executable:

ldd /bin/ping (0x00007ffc81891000) => /lib/x86_64-linux-gnu/ (0x00007faaa4d47000) => /lib/x86_64-linux-gnu/ (0x00007faaa4b14000) => /usr/lib/x86_64-linux-gnu/ (0x00007faaa48de000) => /lib/x86_64-linux-gnu/ (0x00007faaa46c3000) => /lib/x86_64-linux-gnu/ (0x00007faaa42d2000)
	/lib64/ (0x00007faaa5180000)

Note that is the aforementioned run-time linker


Note: syslog can usually be found at /var/log

# # write to the kernel log:
# echo Hello World > /dev/kmsg

# tail -n 1 /var/log/syslog
Apr 22 12:24:39 kernel: <12>Hello World

# # write to syslog and stderr with specific tag
# logger -s -t 'tag' 'hello world again'
<13>Apr 22 12:28:28 tag: hello world again

# tail -n 1 /var/log/syslog 
Apr 22 12:28:28 tag: hello world again

man pages

Use the man pages effectively:

$ # search for socket man pages
$ man -f socket		
socket (7)           - Linux socket interface
socket (2)           - create an endpoint for communication
Socket (3perl)       - networking constants and support functions

$ # list socket(7)
$ man -S 7 socket

$ # search for man pages with socket in the short description:
$ man -k socket
Socket (3perl)       - networking constants and support functions
accept (2)           - accept a connection on a socket
accept4 (2)          - accept a connection on a socket

$ # walk through each man page for "socket":
$ man -a socket

$ # Don't forget about whatis
$ whatis socket
socket (7)           - Linux socket interface
socket (2)           - create an endpoint for communication

Processor Affinity

Run a command on a particular CPU core

$ # Use a bit mask to specify the CPU 
$ # example: cat the cpu stats using CPU 1 (the second core) 
$ taskset 2 cat /proc/cpuinfo	

Python from command line

Execute Python commands without entering interpreter shell:

$ python3 -c "print(hex(0x400<<8))"

$ python3 -c "import math; F=10**6; print(2*math.pi*F)"

$ python3 -c "import site; print(site.getsitepackages())"
['/usr/local/lib/python3.6/dist-packages', ...]

User and Group ID

This example is run from a freshly installed embedded system as root.

# # return our groups:
# groups 

# # get our real and effective user & group IDs
# id
uid=0(root) gid=0(root) groups=0(root)

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