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UNIX Shell Part-II in 10 mins

Basic Unix Shell Commands

Summary

command Function Example Syntax -OPTIONS
echo display a line of text echo [VARIABLE OR string]
cat concatenate files and print on the standard output cat [FILENAME] -n (show line numbers)
head output the first part of files head -[n] [FILENAME] -[n OR # of lines]
tail output the last part of files tail -[n] [FILENAME] -[n OR # of lines]
more file viewing (only down scrolling allowed with space key) more [FILENAME]
less file viewing (both UP and DOWN scrolling permitted ) less [FILENAME]
sort sort lines of text files sort [FILENAME or STDIN]
diff compare files line by line diff [FILENAME1] [FILENAME2] -u (unified)
wc count: print newline, word, and byte counts for each file wc [FILENAME1] -lwcm
w Show who is logged on and what they are doing. w
who show who is logged on who
which locate a command which [COMMAND_NAME]
whereis locate the binary, source, and manual page files for a command whereis [COMMAND_NAME OR BINARY_FILE]
whatis display one-line manual page descriptions whatis [COMMAND_NAME]
find search for files in a directory hierarchy find [DIR] [-name] "[STRING/WILDCARD]" -name
grep print lines matching a pattern grep [-OPTION] "[PATTERN]" [FILE] -i (ignore case)
zip list, test and extract compressed files in a ZIP archive zip [ZIPFILENAME] [FILE1] [FILE2] [DIR1] -r (compress DIR)
unzip compress or expand files (also look at gunzip) unzip [ZIP FILENAME] [FILENAME2] -l (list) -d (to new DIR)
ping send ICMP ECHO_REQUEST to network hosts ping [SERVER/ HOST/ URL/ IP]
curl transfer a URL curl [URL]
wget The non-interactive network downloader. Download file wget [URL] -c -b
top display Linux processes top
htop interactive process viewer htop
kill send a signal to a process kill [PID]
> ; >> directs standard output to file; append to file if it already exists [COMMAND1] > [FILENAME]
| pipe operator: pipe; takes standard output of cmd1 as standard input to cmd2 [COMMAND1] | [COMMAND2]

find - Searching Files & Directories

Write (or redirect) output from a command to a file. 

# Syntax: 
find [DIR] [-name] "[STRING/WILDCARD]"  # note the quotes on STRING/ WILDCARD

# Use find from the command line to locate a specific file by name or extension. The following example searches for *.err files in the /home/username/ directory and all sub-directories:

find /home/username/ -name "*.err"

find . -name testfile.txt       # Find a file called testfile.txt in current and sub-directories.
find /home -name *.jpg          # Find all .jpg files in the /home and sub-directories.
find . -type f -empty           # Find an empty file within the current directory.
find /home -user exampleuser -mtime 7 -iname ".db"  # Find all .db files (ignoring text case) modified in the last 7 days by a

Advanced usage : find locates all files in the hierarchy starting at the current directory and fully recursing into the directory tree. In this example, find will delete all files that end with the characters .bak:

find . -name "*.bak" -delete

Redirection

Write (or redirect) output from a command to a file.
> with write to the file which >> will append to the file if it already exists. 

# Syntax 
[COMMAND] > [FILENAME]

# Redirection
python hello.py > output.txt   # write stdout to (file)
python hello.py >> output.txt  # append stdout to (file) if file exists

# pipe and redirection together 
history | tail -20 > myhistory.txt   # saves history last 20 line to file (myhistory.txt)

| - Pipes

Takes standard output of COMMAND1 as standard input to COMMAND2

cat old.py | less

echo $history | more

history | grep -i "whatis"

history | grep -i "chmod"

cat new.py | grep -i import

# pipe and redirection together 
history | tail -20 > myhistory.txt   # saves history last 20 line to file (myhistory.txt)

Environment Variables

An example of an environment variable is the OSTYPE variable. The value of this is the current operating system you are using. Type

echo $OSTYPE

More examples of environment variables are

USER (your login name)
HOME (the path name of your home directory)
HOST (the name of the computer you are using)
ARCH (the architecture of the computers processor)
DISPLAY (the name of the computer screen to display X windows)
PRINTER (the default printer to send print jobs)
PATH (the directories the shell should search to find a command)
history (histroy of all the command run in SHELL)

To output a VARIABLE use a $ infront of the VARIABLE with and echo command

echo $USER

Exercise

  1. Get contents of URL https://towshif.com and find all lines starting with < and save them to a file strip-html.txt. Output the last 5 lines of the file.

  2. Find all names and paths of python files (.py) in your home directory and save it into a file all-py.txt. Count the number of files you found and output them. Now from the file output all the lines that contain the word 'setup' and save it into another file all-setup-py.txt. Count the number of files with word 'setup' in their path.

  3. Find all the commands in your history that contains a 'http' link.

  4. This link http://www.gutenberg.org/files/15/15.txt has a text file which contains a list of files for the legendary novel Moby Dick.

    • Read this file from shell,
    • find all the occurrencec of ".txt" filenames in it,
    • then sort them in reverse order and save to moby-list.txt.
    • In this file find the count of the string ahab. Ans is 13.

References

Answers:

# Ans 1: 
curl https://towshif.com | grep -i "<" > ~/code/krazykoder/strip-html.txt
tail -5 ~/code/krazykoder/strip-html.txt

# Ans 2:
find ~ -name "*.py" > ~/code/krazykoder/allpy.txt
wc -l  ~/code/krazykoder/allpy.txt
cat  ~/code/krazykoder/allpy.txt | grep -i "setup" > ~/code/krazykoder/all-setup-py.txt
wc ~/code/krazykoder/all-setup-py.txt

# Ans 3: 
history | grep -i 'http'

# And 4: 
curl http://www.gutenberg.org/files/15/15.txt | grep -i ".txt" | sort --reverse > moby-list.txt
grep -io "ahab" moby-list.txt | wc -l