Linux Top 20 useful Shell Commands

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Linux command line examples from practice with 20 commonly used commands

You don’t want to see a complete list here, but there may be a start for some of the common Linux commands, useful for sysops and admins.

1. Translate ASCII character set to octal, decimal, and hexadecimal

$ man ascii
man page ascii characters
Illustration: man ascii

2. Output calendar to console

$ cal
Calendar view 6 months with week number, cal -n 6 -w

Calendar view 6 months with week number

$ cal -n 6 -w

3. Edit binary hexadecimal in VIM

$ vi file
 :%!xxd
 ..
 :%!xxd -r
 :wq

4. Compare two files

$ diff -w file1.txt file2.txt
2c2.3
< Jacqueline --> C. Meier
> Jacqueline Mayer

5. Find content in files recursively using find

$ find . -type f -print0 | xargs -0 grep "mojito"

6. Send a file as e-mail

$ mail -s subject jacque@unblog.ch < notes.txt

7. Download multiple URLs from list

$ cat download-urls.txt | xargs wget -c

8. Remove duplicate lines with awk

$ awk '!($0 in array) { array[$0]; print }' temp

Line output of /etc/passwd with the same uid and gid

$ awk -F ':' '$3==$4' /etc/passwd

9. Conversion from Windows/DOS (CR/LF) to Unix (LF) format

$ sed 's/.$//' filename
# Text Search and Replace
$ sed -e s/dog/cat/g file.txt > file.new
# Deletes all blank lines
$ sed '/^$/d' filename
# Deletes spaces at the end of each line
$ sed 's/ *$//' filename

10. Recursive String Search in Files

$ grep -r "ramsch" *

11. How many CPMs does my computer have?

$ grep processor /proc/cpuinfo | wc -l

12. View Free Memory

$ cat /proc/meminfo | grep MemFree
$ free -m

13. View active processes

$ ps -ef
$ ps aux
$ vmstat 5 10

14. Create directory tree (tree)

$ ls -R | grep ":$" | sed -e 's/:$//' -e 's/[^-][^\/]*\//--/g' -e 's/^/   /' -e 's/-/|/'

15. Output Asterisk PBX Applications

$ asterisk -rx "show applications" | awk '{print $1}' | sed -n -e "s/:"//p

16. Number of MTA emails identified by SPAM

$ cat /var/log/maillog | egrep -c '(Alert\!)|(identified spam)|(reject\:)'

17. How long does the computer run

$ uptime

18. Set time and date

$ date -s "Dec 12 18:30:00 2014"

19. Who am I and the effective user ID, UID and GID output

$ whoami
$ id

20. Output list of most recently logged in users

$ last -a

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