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Advanced Commands for Linux users

In this chapter you will learn some advanced commands for Linux.


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 because it only compares consecutive lines.

Without an 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, you must 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 as is the case 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>

If we wanted to be able to specify both arguments on the same line, we would have to 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.

Now, if we take the example of the cp command and want 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, we 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 (in our example the % character) where the input files to xargs will be placed.

yum-utils package

The yum-utils package is a collection of utilities from different authors for yum, 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 usage:

  • repoquery command:

The repoquery command queries the repositories.

Examples of use:

  • Knowing the dependencies of an uninstalled package:
repoquery --requires <package>
  • Know the files provided by a non-installed package:
$ 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
…
  • yumdownloader command:

The yumdownloader command downloads RPM packages from the repositories.

Note

This command is very useful to quickly build a local repository of a few rpm!

Example: yumdownloader will download the repoquery rpm package and all its dependencies:

$ yumdownloader --destdir /var/tmp -- resolve repoquery
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: the pstree command displays the current processes on the system in a tree-like structure.
  • killall: the killall command sends a kill signal to all processes identified by name.
  • fuser: the fuser command identifies the PID of 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/passwd file 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
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