Bash Advanced Topics

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Overview

The topics below cover various advanced bash shell usage that can be applied to NST when working at the command line. See the Advanced Bash-Scripting Guide for an excellent document on advanced shell scripting.

Extended Bash Completion

The bash shell's completion functionality can be extended to external programs.

systemctl Bash Completion

The systemd-units RPM package contains a bash completion script ("/etc/bash_completion.d/systemctl-bash-completion.sh") for the systemctl command which allows controlling of the systemd system daemon and service manager. Bash completion extensions for the systemctl command can be quite useful due to its long command arguments. To enable it, source the script in at the bash shell:

[root@probe ~]# . /etc/bash_completion.d/systemctl-bash-completion.sh
[root@probe ~]# 

To use it, lets say to get the status of a serial device (systemctl status serial-getty@ttyS0.service), perform the following:

systemctl stat<tab>

Substitute the "<tab>" for the tab key. This will result in:

systemctl status

Now the next part of the command:

systemctl status seria<tab>

Again substitute the "<tab>" for the tab key. This will result in the full command:

systemctl status serial-getty@ttyS0.service

***Note: If more than one completion value is found, then hitting the tab key will not complete and fill in a value. In this case one may use the <esc> + = key sequence (i.e., The Escape key followed by the Equal Sign key) to find out the possible completion values.

Starting the Bash Shell for a '/sbin/nologin' User

One can use the su command to run the bash shell for a user that has a "/sbin/nologin" login shell. This can be handy to see if a user has specific system permissions. Below is an example for the "apache" user:

[root@probe ~]# cat /etc/passwd | grep apache
apache:x:48:48:Apache:/var/www:/sbin/nologin
[root@probe ~]# su - apache
This account is currently not available.
[root@probe ~]# su - apache -s /bin/bash
[apache@probe ~]$ id -a
uid=48(apache) gid=48(apache) groups=48(apache),0(root),486(backuppc),499(nagios)
[apache@probe ~]$