Essential-Linux-Commands

In this post we will see essential commands, I think which are very useful for developers in Linux.

Changing User

Here we use su command to change user in Linux. See man page for su for more details

$ su - userName

Environment variables

We use use env command to see the environment variables. See man page for env for more details

$ env

Changing Directory

We use cd command to change directory. See man page for cd for more details

To change to home directory of current user

$ cd ~

To change to previous working directory

The following command use OLDPWD environment variable to know the previous working directory.

$ cd -

To change to root directory

$ cd /

Transfer Folder/Files from one Host to other Host

Here we use scp command to transfer files across hosts. See man page for scp for more details

scp command to copy a file

$ scp user1@host1IPAddr:absolutePath1/fileName user2@host2IPAddr:absolutePath2

scp command to copy all files in a folder

$ scp user1@host1IPAddr:absolutePath1/* user2@host2IPAddr:absolutePath2

scp command to copy folder

$ scp -r user1@host1IPAddr:absolutePath1 user2@host2IPAddr:absolutePath2

Finding locations of a file

Here we use find command to find the location/path where a file exists. See man page for find for more details

find command to find a file from root folder

$ find -name fileName /

find command to find a file from current folder

$ find -name fileName .

Search for a string recursively in all files and show it’s occurences in files with line numbers

Here we use grep command to search for a string recursively in all files and show it’s occurences in files with line numbers. See man page for grep for more details

$ grep -rn "word_to_search" /path

Finding which executable current environment referring to

Here we use which command to find the location of the executable used in current environment. which command looks for PATH variable to find the location of the executable used in current environment. See man page for which for more details

$ which python

Finding location of executable

Here we use whereis command to find the locations of the executable used. where command looks for common Linux locations, PATH, MANPATH variable to find the locations of the executable. See man page for whereis for more details

$ whereis python

Get details of Linux distribution

Here we use lsb_release command to details of Linux distribution like version. See man page for lsb_release for more details

$ lsb_release -a                                                                                                                  
No LSB modules are available.                                                                                                                          
Distributor ID: Ubuntu                                                                                                                                 
Description:    Ubuntu 18.04.3 LTS                                                                                                                     
Release:        18.04                                                                                                                                  
Codename:       bionic

Get File Information like type, 32 bit/64 bit, processor architecture, stripped/unstripped

Here we use file command to get file Information like type, 32 bit/64 bit, processor architecture, stripped (like removal of debug symbols)/unstripped. See man page for file for more details

$ file libc-2.23.so
libc-2.23.so: ELF 64-bit LSB shared object, x86-64, version 1 (GNU/Linux), dynamically linked, interpreter /lib64/l, BuildID[sha1]=1ca54a6e0d76188105b12e49fe6b8019bf08803a, for GNU/Linux 2.6.32, stripped

$ file libc-2.23.so
libc-2.23.so: ELF 32-bit LSB shared object, Intel 80386, version 1 (GNU/Linux), dynamically linked, interpreter /lib/ld-, BuildID[sha1]=9a6b57c7a4f93d7e54e61bccb7df996c8bc58141, for GNU/Linux 2.6.32, stripped

Get the processes using a file

Here we use lsof to know the processes using a file. See man page for lsof for more details

$ lsof /lib/x86_64-linux-gnu/libc-2.23.so
COMMAND    PID       USER  FD   TYPE DEVICE SIZE/OFF   NODE NAME
systemd      1       root mem    REG  253,1  1868984 528009 /lib/x86_64-linux-gnu/libc-2.23.so
systemd-u  283       root mem    REG  253,1  1868984 528009 /lib/x86_64-linux-gnu/libc-2.23.so
dhclient   662       root mem    REG  253,1  1868984 528009 /lib/x86_64-linux-gnu/libc-2.23.so

To know dependent shared objects

Here we use readelf to know the dependent shared objects of a shared object or executable. See man page for readelf for more details

$ readelf -d /bin/ls | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libselinux.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]