Intro To 'false' Command In Linux

2023-05-21 - By Robert Elder

     I use the 'false' command which does absolutely nothing and then exits unsuccessfully:

false

     The 'false' command can be built into the shell or it can exist as a regular executable:

type -a false

false is a shell builtin
false is /usr/bin/false
false is /bin/false

Return Code 1 == 'Failure'

     Whenever you run the 'false' command, it will exit with an unsuccessful return code:

false

     You can verify this by echoing the dollar sign question mark variable and then observing the value of 1 which indicates an unsuccessful program exit:

echo $?

1

Use Case Of 'false' Command

     An example application of the 'false' command is to deny certain users the ability to access a shell environment.

     For example, on my machine the postgres user is configured to spawn the '/bin/bash' shell environment upon login:

cat /etc/passwd | grep postgres

postgres:x:126:133:PostgreSQL administrator,,,:/var/lib/postgresql:/bin/bash

     Whenever I use sudo to become the 'postgres' user:

sudo -u postgres -i

     this will work normally and run the '/bin/bash' shell as indicated in '/etc/passwd'.

     However, the 'tomcat' user is configured to launch the 'false' command as its login shell:

cat /etc/passwd | grep tomcat

tomcat:x:1001:1001::/opt/tomcat:/bin/false

     This fails immediately and prevents access to any form of login shell:

sudo -u tomcat -i

echo $?

1

     This is because the 'false' command simply exits immediately with an error code.

     And that's why the 'false' command is my favourite Linux command.

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