Advanced Commands for Linux users¶
Advanced commands provide greater customization and controls in more specialized situations once you become familiar with basic commands.
Objectives: In this chapter, future Linux administrators will learn:
some useful commands not covered in the previous chapter.
some advanced commands.
user commands, Linux
Knowledge: 
Complexity:
Reading time: 20 minutes
uniq command¶
The uniq command is a very powerful command, used with the sort command, especially for log file analysis. It allows you to sort and display entries by removing duplicates.
To illustrate how the uniq command works, let's use a firstnames.txt file containing a list of first names:
antoine
xavier
steven
patrick
xavier
antoine
antoine
steven
Note
uniq requires the input file to be sorted before running because it only compares consecutive lines.
With no argument, the uniq command will not display identical lines that follow each other in the firstnames.txt file:
$ sort firstnames.txt | uniq
antoine
patrick
steven
xavier
To display only the rows that appear only once, use the -u option:
$ sort firstnames.txt | uniq -u
patrick
Conversely, to display only the lines that appear at least twice in the file, use the -d option:
$ sort firstnames.txt | uniq -d
antoine
steven
xavier
To simply delete lines that appear only once, use the -D option:
$ sort firstnames.txt | uniq -D
antoine
antoine
antoine
steven
steven
xavier
xavier
Finally, to count the number of occurrences of each line, use the -c option:
$ sort firstnames.txt | uniq -c
3 antoine
1 patrick
2 steven
2 xavier
$ sort firstnames.txt | uniq -cd
3 antoine
2 steven
2 xavier
xargs commands¶
The xargs command allows the construction and execution of command lines from standard input.
The xargs command reads whitespace or linefeed delimited arguments from standard input, and executes the command (/bin/echo by default) one or more times using the initial arguments followed by the arguments read from standard input.
A first and simplest example would be the following:
$ xargs
use
of
xargs
<CTRL+D>
use of xargs
The xargs command waits for an input from the standard stdin input. Three lines are entered. The end of the user input is specified to xargs by the keystroke sequence CTRL+D. xargs then executes the default command echo followed by the three arguments corresponding to the user input, namely:
$ echo "use" "of" "xargs"
use of xargs
It is possible to specify a command to be run by xargs.
In the following example, xargs will run the command ls -ld on the set of folders specified in the standard input:
$ xargs ls -ld
/home
/tmp
/root
<CTRL+D>
drwxr-xr-x. 9 root root 4096 5 avril 11:10 /home
dr-xr-x---. 2 root root 4096 5 avril 15:52 /root
drwxrwxrwt. 3 root root 4096 6 avril 10:25 /tmp
In practice, the xargs command executed the ls -ld /home /tmp /root command.
What happens if the command to be executed does not accept multiple arguments, such as with the find command?
$ xargs find /var/log -name
*.old
*.log
find: paths must precede expression: *.log
The xargs command attempted to execute the find command with multiple arguments behind the -name option, which caused find to generate an error:
$ find /var/log -name "*.old" "*.log"
find: paths must precede expression: *.log
In this case, the xargs command must be forced to execute the find command several times (once per line entered as standard input). The -L option followed by an integer allows you to specify the maximum number of entries to be processed with the command at one time:
$ xargs -L 1 find /var/log -name
*.old
/var/log/dmesg.old
*.log
/var/log/boot.log
/var/log/anaconda.yum.log
/var/log/anaconda.storage.log
/var/log/anaconda.log
/var/log/yum.log
/var/log/audit/audit.log
/var/log/anaconda.ifcfg.log
/var/log/dracut.log
/var/log/anaconda.program.log
<CTRL+D>
To specify both arguments on the same line, use the -n 1 option:
$ xargs -n 1 find /var/log -name
*.old *.log
/var/log/dmesg.old
/var/log/boot.log
/var/log/anaconda.yum.log
/var/log/anaconda.storage.log
/var/log/anaconda.log
/var/log/yum.log
/var/log/audit/audit.log
/var/log/anaconda.ifcfg.log
/var/log/dracut.log
/var/log/anaconda.program.log
<CTRL+D>
Case study of a backup with a tar based on a search:
$ find /var/log/ -name "*.log" -mtime -1 | xargs tar cvfP /root/log.tar
$ tar tvfP /root/log.tar
-rw-r--r-- root/root 1720 2017-04-05 15:43 /var/log/boot.log
-rw-r--r-- root/root 499270 2017-04-06 11:01 /var/log/audit/audit.log
The special feature of the xargs command is that it places the input argument at the end of the called command. This works very well with the above example since the files passed in will form the list of files to be added to the archive.
Using the example of the cp command, to copy a list of files in a directory, this list of files will be added at the end of the command... but what the cp command expects at the end of the command is the destination. To do this, use the -I option to put the input arguments somewhere else than at the end of the line.
$ find /var/log -type f -name "*.log" | xargs -I % cp % /root/backup
The -I option allows you to specify a character (the % character in the above example) where the input files to xargs will be placed.
yum-utils package¶
The yum-utils package is a collection of utilities, built for yum by various authors, which make it easier and more powerful to use.
Note
While yum has been replaced by dnf in Rocky Linux 8, the package name has remained yum-utils, although it can be installed as dnf-utils as well. These are classic YUM utilities implemented as CLI shims on top of DNF to maintain backwards compatibility with yum-3.
Here are some examples of these utilities:
repoquerycommand
The repoquery command is used to query the packages in the repository.
Examples of use:
-
Display the dependencies of a package (it can be a software package that has been installed or not installed), equivalent to
dnf deplist <package-name>repoquery --requires <package-name> -
Display the files provided by an installed package (does not work for packages that are not installed), equivalent to
rpm -ql <package-name>$ repoquery -l yum-utils /etc/bash_completion.d /etc/bash_completion.d/yum-utils.bash /usr/bin/debuginfo-install /usr/bin/find-repos-of-install /usr/bin/needs-restarting /usr/bin/package-cleanup /usr/bin/repo-graph /usr/bin/repo-rss /usr/bin/repoclosure /usr/bin/repodiff /usr/bin/repomanage /usr/bin/repoquery /usr/bin/reposync /usr/bin/repotrack /usr/bin/show-changed-rco /usr/bin/show-installed /usr/bin/verifytree /usr/bin/yum-builddep /usr/bin/yum-config-manager /usr/bin/yum-debug-dump /usr/bin/yum-debug-restore /usr/bin/yum-groups-manager /usr/bin/yumdownloader … -
yumdownloadercommand:
The yumdownloader command downloads RPM packages from the repositories. Equivalent to dnf download --downloadonly --downloaddir ./ package-name
Note
This command is very useful to quickly build a local repository of a few rpms!
Example: yumdownloader will download the samba rpm package and all its dependencies:
$ yumdownloader --destdir /var/tmp --resolve samba
or
$ dnf download --downloadonly --downloaddir /var/tmp --resolve samba
| Options | Comments |
|---|---|
--destdir |
The downloaded packages will be stored in the specified folder. |
--resolve |
Also downloads the package dependencies. |
psmisc packages¶
The psmisc package contains utilities for managing system processes:
pstree: thepstreecommand displays the current processes on the system in a tree-like structure.killall: thekillallcommand sends a kill signal to all processes identified by name.fuser: thefusercommand identifies thePIDof processes that use the specified files or file systems.
Examples:
$ pstree
systemd─┬─NetworkManager───2*[{NetworkManager}]
├─agetty
├─auditd───{auditd}
├─crond
├─dbus-daemon───{dbus-daemon}
├─firewalld───{firewalld}
├─lvmetad
├─master─┬─pickup
│ └─qmgr
├─polkitd───5*[{polkitd}]
├─rsyslogd───2*[{rsyslogd}]
├─sshd───sshd───bash───pstree
├─systemd-journal
├─systemd-logind
├─systemd-udevd
└─tuned───4*[{tuned}]
# killall httpd
Kill processes (option -k) that access the /etc/httpd/conf/httpd.conf file:
# fuser -k /etc/httpd/conf/httpd.conf
watch command¶
The watch command regularly executes a command and displays the result in the terminal in full screen.
The -n option allows you to specify the number of seconds between each execution of the command.
Note
To exit the watch command, you must type the keys: CTRL+C to kill the process.
Examples:
- Display the end of the
/etc/passwdfile every 5 seconds:
$ watch -n 5 tail -n 3 /etc/passwd
Result:
Every 5.0s: tail -n 3 /etc/passwd rockstar.rockylinux.lan: Thu Jul 1 15:43:59 2021
sssd:x:996:993:User for sssd:/:/sbin/nologin
chrony:x:995:992::/var/lib/chrony:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
- Monitoring the number of files in a folder:
$ watch -n 1 'ls -l | wc -l'
- Display a clock:
$ watch -t -n 1 date