The command line is something every developer should learn and implement into their daily routine. In this post we’ll look at the Bash Shell (Bourne Again SHell), which is a command-line interface (CLI) and is currently the most widely used shell.

Basic Operations

Shell Operations

a. export

Displays all environment variables. If you want to get details of a specific variable, use echo $VARIABLE_NAME.

export

Example:

$ export
AWS_HOME=/Users/adnanadnan/.aws
LANG=en_US.UTF-8
LC_CTYPE=en_US.UTF-8
LESS=-R

$ echo $AWS_HOME
/Users/adnanadnan/.aws

b. whatis

whatis shows description for user commands, system calls, library functions, and others in manual pages

whatis something

Example:

$ whatis bash
bash (1)             - GNU Bourne-Again SHell

c. whereis

whereis searches for executables, source files, and manual pages using a database built by system automatically.

whereis name

Example:

$ whereis php
/usr/bin/php

d. which

which searches for executables in the directories specified by the environment variable PATH. This command will print the full path of the executable(s).

which program_name

Example:

$ which php
/c/xampp/php/php

e. clear

Clears content on window.

File Operations

a. cat

It can be used for the following purposes under UNIX or Linux.

• Display text files on screen
• Copy text files
• Combine text files
• Create new text files

cat filename
cat file1 file2
cat file1 file2 > newcombinedfile
cat < file1 > file2 #copy file1 to file2

b. chmod

The chmod command stands for "change mode" and allows you to change the read, write, and execute permissions on your files and folders. For more information on this command check this link.

chmod -options filename

c. chown

The chown command stands for "change owner", and allows you to change the owner of a given file or folder, which can be a user and a group. Basic usage is simple forward first comes the user (owner), and then the group, delimited by a colon.

chown -options user:group filename

d. cp

Copies a file from one location to other.

cp filename1 filename2

Where filename1 is the source path to the file and filename2 is the destination path to the file.

e. diff

Compares files, and lists their differences.

diff filename1 filename2

f. file

Determine file type.

file filename

Example:

$ file index.html
 index.html: HTML document, ASCII text

g. find

Find files in directory

find directory options pattern

Example:

$ find . -name README.md
$ find /home/user1 -name '*.png'

h. gunzip

Un-compresses files compressed by gzip.

gunzip filename

i. gzcat

Lets you look at gzipped file without actually having to gunzip it.

gzcat filename

j. gzip

Compresses files.

gzip filename

k. head

Outputs the first 10 lines of file

head filename

l. lpq

Check out the printer queue.

lpq

Example:

$ lpq
Rank    Owner   Job     File(s)                         Total Size
active  adnanad 59      demo                            399360 bytes
1st     adnanad 60      (stdin)                         0 bytes

m. lpr

Print the file.

lpr filename

n. lprm

Remove something from the printer queue.

lprm jobnumber

o. ls

Lists your files. ls has many options: -l lists files in 'long format', which contains the exact size of the file, who owns the file, who has the right to look at it, and when it was last modified. -a lists all files, including hidden files. For more information on this command check this link.

ls option

Example:

$ ls -la
rwxr-xr-x   33 adnan  staff    1122 Mar 27 18:44 .
drwxrwxrwx  60 adnan  staff    2040 Mar 21 15:06 ..
-rw-r--r--@  1 adnan  staff   14340 Mar 23 15:05 .DS_Store
-rw-r--r--   1 adnan  staff     157 Mar 25 18:08 .bumpversion.cfg
-rw-r--r--   1 adnan  staff    6515 Mar 25 18:08 .config.ini
-rw-r--r--   1 adnan  staff    5805 Mar 27 18:44 .config.override.ini
drwxr-xr-x  17 adnan  staff     578 Mar 27 23:36 .git
-rwxr-xr-x   1 adnan  staff    2702 Mar 25 18:08 .gitignore

p. more

Shows the first part of a file (move with space and type q to quit).

more filename

q. mv

Moves a file from one location to other.

mv filename1 filename2

Where filename1 is the source path to the file and filename2 is the destination path to the file.

Also it can be used for rename a file.

mv old_name new_name

r. rm

Removes a file. Using this command on a directory gives you an error. rm: directory: is a directory To remove a directory you have to pass -r which will remove the content of the directory recursively. Optionally you can use -f flag to force the deletion i.e. without any confirmations etc.

rm filename

s. tail

Outputs the last 10 lines of file. Use -f to output appended data as the file grows.

tail filename

t. touch

Updates access and modification time stamps of your file. If it doesn't exists, it'll be created.

touch filename

Example:

$ touch trick.md

Text Operations

a. awk

awk is the most useful command for handling text files. It operates on an entire file line by line. By default it uses whitespace to separate the fields. The most common syntax for awk command is

awk '/search_pattern/ { action_to_take_if_pattern_matches; }' file_to_parse

Lets take following file /etc/passwd. Here's the sample data that this file contains:

root:x:0:0:root:/root:/usr/bin/zsh
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync

So now lets get only username from this file. Where -F specifies that on which base we are going to separate the fields. In our case it's :. { print $1 } means print out the first matching field.

awk -F':' '{ print $1 }' /etc/passwd

After running the above command you will get following output.

root
daemon
bin
sys
sync

For more detail on how to use awk, check following link.

b. cut

Remove sections from each line of files

example.txt

red riding hood went to the park to play

show me columns 2 , 7 , and 9 with a space as a separator

cut -d " " -f2,7,9 example.txt

riding park play

c. echo

Display a line of text

display "Hello World"

echo Hello World

Hello World

display "Hello World" with newlines between words

echo -ne "Hello\nWorld\n"

Hello
World

d. egrep

Print lines matching a pattern - Extended Expression (alias for: 'grep -E')

example.txt

Lorem ipsum
dolor sit amet,
consetetur
sadipscing elitr,
sed diam nonumy
eirmod tempor
invidunt ut labore
et dolore magna
aliquyam erat, sed
diam voluptua. At
vero eos et
accusam et justo
duo dolores et ea
rebum. Stet clita
kasd gubergren,
no sea takimata
sanctus est Lorem
ipsum dolor sit
amet.

display lines that have either "Lorem" or "dolor" in them.

egrep '(Lorem|dolor)' example.txt
or
grep -E '(Lorem|dolor)' example.txt

Lorem ipsum
dolor sit amet,
et dolore magna
duo dolores et ea
sanctus est Lorem
ipsum dolor sit

e. fgrep

Print lines matching a pattern - FIXED pattern matching (alias for: 'grep -F')

example.txt

Lorem ipsum
dolor sit amet,
consetetur
sadipscing elitr,
sed diam nonumy
eirmod tempor
foo (Lorem|dolor)
invidunt ut labore
et dolore magna
aliquyam erat, sed
diam voluptua. At
vero eos et
accusam et justo
duo dolores et ea
rebum. Stet clita
kasd gubergren,
no sea takimata
sanctus est Lorem
ipsum dolor sit
amet.

Find the exact string '(Lorem|dolor)' in example.txt

fgrep '(Lorem|dolor)' example.txt
or
grep -F '(Lorem|dolor)' example.txt

foo (Lorem|dolor)

f. fmt

Simple optimal text formatter

example: example.txt (1 line)

Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.

output the lines of example.txt to 20 character width

cat example.txt | fmt -w 20

Lorem ipsum
dolor sit amet,
consetetur
sadipscing elitr,
sed diam nonumy
eirmod tempor
invidunt ut labore
et dolore magna
aliquyam erat, sed
diam voluptua. At
vero eos et
accusam et justo
duo dolores et ea
rebum. Stet clita
kasd gubergren,
no sea takimata
sanctus est Lorem
ipsum dolor sit
amet.

g. grep

Looks for text inside files. You can use grep to search for lines of text that match one or many regular expressions, and outputs only the matching lines.

grep pattern filename

Example:

$ grep admin /etc/passwd
_kadmin_admin:*:218:-2:Kerberos Admin Service:/var/empty:/usr/bin/false
_kadmin_changepw:*:219:-2:Kerberos Change Password Service:/var/empty:/usr/bin/false
_krb_kadmin:*:231:-2:Open Directory Kerberos Admin Service:/var/empty:/usr/bin/false

You can also force grep to ignore word case by using -i option. -r can be used to search all files under the specified directory, for example:

$ grep -r admin /etc/

And -w to search for words only. For more detail on grep, check following link.

h. nl

Number lines of files

example.txt

Lorem ipsum
dolor sit amet,
consetetur
sadipscing elitr,
sed diam nonumy
eirmod tempor
invidunt ut labore
et dolore magna
aliquyam erat, sed
diam voluptua. At
vero eos et
accusam et justo
duo dolores et ea
rebum. Stet clita
kasd gubergren,
no sea takimata
sanctus est Lorem
ipsum dolor sit
amet.

show example.txt with line numbers

nl -s". " example.txt

      Lorem ipsum
      dolor sit amet,
     3. consetetur
     4. sadipscing elitr,
     5. sed diam nonumy
     6. eirmod tempor
     7. invidunt ut labore
     8. et dolore magna
     9. aliquyam erat, sed
    10. diam voluptua. At
    11. vero eos et
    12. accusam et justo
    13. duo dolores et ea
    14. rebum. Stet clita
    15. kasd gubergren,
    16. no sea takimata
    17. sanctus est Lorem
    18. ipsum dolor sit
    19. amet.

i. sed

Stream editor for filtering and transforming text

example.txt

Hello This is a Test 1 2 3 4

replace all spaces with hyphens

sed 's/ /-/g' example.txt

Hello-This-is-a-Test-1-2-3-4

replace all digits with "d"

sed 's/[0-9]/d/g' example.txt

Hello This is a Test d d d d

j. sort

Sort lines of text files

example.txt

f
b
c
g
a
e
d

sort example.txt

sort example.txt

a
b
c
d
e
f
g

randomize a sorted example.txt

sort example.txt | sort -R

b
f
a
c
d
g
e

k. tr

Translate or delete characters

example.txt

Hello World Foo Bar Baz!

take all lower case letters and make them upper case

cat example.txt | tr 'a-z' 'A-Z'

HELLO WORLD FOO BAR BAZ!

take all spaces and make them into newlines

cat example.txt | tr ' ' '\n'

Hello
World
Foo
Bar
Baz!

l. uniq

Report or omit repeated lines

example.txt

a
a
b
a
b
c
d
c

show only unique lines of example.txt (first you need to sort it, otherwise it won't see the overlap)

sort example.txt | uniq

a
b
c
d

show the unique items for each line, and tell me how many instances it found

sort example.txt | uniq -c

    3 a
    2 b
    2 c
    1 d

m. wc

Tells you how many lines, words and characters there are in a file.

wc filename

Example:

$ wc demo.txt
7459   15915  398400 demo.txt

Where 7459 is lines, 15915 is words and 398400 is characters.

Directory Operations

a. cd

Moves you from one directory to other. Running this

$ cd

moves you to home directory. This command accepts an optional dirname, which moves you to that directory.

cd dirname

b. mkdir

Makes a new directory.

mkdir dirname

You can use this to create multiple directories at once within your current directory.

mkdir 1stDirectory 2ndDirectory 3rdDirectory

You can also use this to create parent directories at the same time. For instance, if you wanted a directory named 'project1' in another subdirectory at '/samples/bash/projects/', you could run:

mkdir /samples/bash/projects/project1

If any of these directories did no already exist, they would be created as well.

c. pwd

Tells you which directory you currently are in.

pwd

SSH, System Info & Network Operations

a. bg

Lists stopped or background jobs; resume a stopped job in the background.

b. cal

Shows the month's calendar.

c. date

Shows the current date and time.

d. df

Shows disk usage.

e. dig

Gets DNS information for domain.

dig domain

f. du

Shows the disk usage of files or directories. For more information on this command check this link

du [option] [filename|directory]

Options:

• -h (human readable) Displays output it in kilobytes (K), megabytes (M) and gigabytes (G).
• -s (supress or summarize) Outputs total disk space of a directory and supresses reports for subdirectories.

Example:

du -sh pictures
1.4M pictures

g. fg

Brings the most recent job in the foreground.

h. finger

Displays information about user.

finger username

i. jobs

Lists the jobs running in the background, giving the job number.

j. last

Lists your last logins of specified user.

last yourUsername

k. man

Shows the manual for specified command.

man command

l. passwd

Allows the current logged user to change their password.

m. ping

Pings host and outputs results.

ping host

n. ps

Lists your processes.

ps -u yourusername

Use the flags ef. e for every process and f for full listing.

ps -ef

o. quota

Shows what your disk quota is.

quota -v

p. scp

Transfer files between a local host and a remote host or between two remote hosts.

copy from local host to remote host

scp source_file user@host:directory/target_file

copy from remote host to local host

scp user@host:directory/source_file target_file
scp -r user@host:directory/source_folder target_folder

This command also accepts an option -P that can be used to connect to specific port.

scp -P port user@host:directory/source_file target_file

q. ssh

ssh (SSH client) is a program for logging into and executing commands on a remote machine.

ssh user@host

This command also accepts an option -p that can be used to connect to specific port.

ssh -p port user@host

r. top

Displays your currently active processes.

s. uname

Shows kernel information.

uname -a

t. uptime

Shows current uptime.

u. w

Displays who is online.

v. wget

Downloads file.

wget file

w. whoami

Return current logged in username.

x. whois

Gets whois information for domain.

whois domain

Process Monitoring Operations

a. kill

Kills (ends) the processes with the ID you gave.

kill PID

b. killall

Kill all processes with the name.

killall processname

c. &

The & symbol instructs the command to run as a background process in a subshell.

command &

d. nohup

nohup stands for "No Hang Up". This allows to run command/process or shell script that can continue running in the background after you log out from a shell.

nohup command

Combine it with & to create background processes

nohup command &

Basic Shell Programming

The first line that you will write in bash script files is called shebang. This line in any script determines the script's ability to be executed like a standalone executable without typing sh, bash, python, php etc beforehand in the terminal.

#!/usr/bin/env bash

Variables

Creating variables in bash is similar to other languages. There are no data types. A variable in bash can contain a number, a character, a string of characters, etc. You have no need to declare a variable, just assigning a value to its reference will create it.

Example:

str="hello world"

The above line creates a variable str and assigns "hello world" to it. The value of variable is retrieved by putting the $ in the beginning of variable name.

Example:

echo $str   # hello world

Array

Like other languages bash has also arrays. An array is a variable containing multiple values. There's no maximum limit on the size of array. Arrays in bash are zero based. The first element is indexed with element 0. There are several ways for creating arrays in bash which are given below.

Examples:

array[0]=val
array[1]=val
array[2]=val
array=([2]=val [0]=val [1]=val)
array=(val val val)

To display a value at specific index use following syntax:

${array[i]}     # where i is the index

If no index is supplied, array element 0 is assumed. To find out how many values there are in the array use the following syntax:

${#array[@]}

Bash has also support for the ternary conditions. Check some examples below.

${varname:-word}    # if varname exists and isn't null, return its value; otherwise return word
${varname:=word}    # if varname exists and isn't null, return its value; otherwise set it word and then return its value
${varname:+word}    # if varname exists and isn't null, return word; otherwise return null
${varname:offset:length}    # performs substring expansion. It returns the substring of $varname starting at offset and up to length characters

String Substitution

Check some of the syntax on how to manipulate strings

${variable#pattern}         # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest
${variable##pattern}        # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest
${variable%pattern}         # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest
${variable%%pattern}        # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest
${variable/pattern/string}  # the longest match to pattern in variable is replaced by string. Only the first match is replaced
${variable//pattern/string} # the longest match to pattern in variable is replaced by string. All matches are replaced
${#varname}     # returns the length of the value of the variable as a character string

Functions

As in almost any programming language, you can use functions to group pieces of code in a more logical way or practice the divine art of recursion. Declaring a function is just a matter of writing function my_func { my_code }. Calling a function is just like calling another program, you just write its name.

function name() {
    shell commands
}

Example:

#!/bin/bash
function hello {
   echo world!
}
hello

function say {
    echo $1
}
say "hello world!"

When you run the above example the hello function will output "world!". The above two functions hello and say are identical. The main difference is function say. This function, prints the first argument it receives. Arguments, within functions, are treated in the same manner as arguments given to the script.

Conditionals

The conditional statement in bash is similar to other programming languages. Conditions have many form like the most basic form is if expression then statement where statement is only executed if expression is true.

if [ expression ]; then
    will execute only if expression is true
else
    will execute if expression is false
fi

Sometime if conditions becoming confusing so you can write the same condition using the case statements.

case expression in
    pattern1 )
        statements ;;
    pattern2 )
        statements ;;
    ...
esac

Expression Examples:

statement1 && statement2  # both statements are true
statement1 || statement2  # at least one of the statements is true

str1=str2       # str1 matches str2
str1!=str2      # str1 does not match str2
str1<str2       # str1 is less than str2
str1>str2       # str1 is greater than str2
-n str1         # str1 is not null (has length greater than 0)
-z str1         # str1 is null (has length 0)

-a file         # file exists
-d file         # file exists and is a directory
-e file         # file exists; same -a
-f file         # file exists and is a regular file (i.e., not a directory or other special type of file)
-r file         # you have read permission
-s file         # file exists and is not empty
-w file         # you have write permission
-x file         # you have execute permission on file, or directory search permission if it is a directory
-N file         # file was modified since it was last read
-O file         # you own file
-G file         # file's group ID matches yours (or one of yours, if you are in multiple groups)

file1 -nt file2     # file1 is newer than file2
file1 -ot file2     # file1 is older than file2

-lt     # less than
-le     # less than or equal
-eq     # equal
-ge     # greater than or equal
-gt     # greater than
-ne     # not equal

Loops

There are three types of loops in bash. for, while and until.

Different for Syntax:

for x := 1 to 10 do
begin
  statements
end

for name [in list]
do
  statements that can use $name
done

for (( initialisation ; ending condition ; update ))
do
  statements...
done

while Syntax:

while condition; do
  statements
done

until Syntax:

until condition; do
  statements
done

Tricks

Set an alias

Run nano ~/.bash_profile and add the following line:

alias dockerlogin='ssh www-data@adnan.local -p2222'  # add your alias in .bash_profile

To quickly go to a specific directory

Run nano ~/.bashrc and add the following line:

export hotellogs="/workspace/hotel-api/storage/logs"

Now you can use the saved path:

source ~/.bashrc
cd $hotellogs

Re-execute the previous command

This goes back to the days before you could rely on keyboards to have an "up" arrow key, but can still be useful. To run the last command in your history

!!

A common error is to forget to use sudo to prefix a command requiring privileged execution. Instead of typing the whole command again, you can:

sudo !!

This would change a mkdir somedir into sudo mkdir somedir.

Exit traps

Make your bash scripts more robust by reliably performing cleanup.

function finish {
  # your cleanup here. e.g. kill any forked processes
  jobs -p | xargs kill
}
trap finish EXIT

Saving your environment variables

When you do export FOO = BAR, your variable is only exported in this current shell and all its children, to persist in the future you can simply append in your ~/.bash_profile file the command to export your variable

echo export FOO=BAR >> ~/.bash_profile

Accessing your scripts

You can easily access your scripts by creating a bin folder in your home with mkdir ~/bin, now all the scripts you put in this folder you can access in any directory.

If you can not access, try append the code below in your ~/.bash_profile file and after do source ~/.bash_profile.

# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
    PATH="$HOME/bin:$PATH"
fi

Debugging

You can easily debug the bash script by passing different options to bash command. For example -n will not run commands and check for syntax errors only. -v echo commands before running them. -x echo commands after command-line processing.

bash -n scriptname
bash -v scriptname
bash -x scriptname