Idnan/bash-guide
A guide to learn bash
| repo name | Idnan/bash-guide |
| repo link | https://github.com/Idnan/bash-guide |
| homepage | |
| language | |
| size (curr.) | 81 kB |
| stars (curr.) | 9414 |
| created | 2017-04-01 |
| license | |
Table of Contents
- Basic Operations
1.1. File Operations
1.2. Text Operations
1.3. Directory Operations
1.4. SSH, System Info & Network Operations
1.5. Process Monitoring Operations - Basic Shell Programming
2.1. Variables
2.2. Array
2.3. String Substitution
2.4. Functions
2.5. Conditionals
2.6. Loops - Tricks
- Debugging
1. Basic 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.
1.1. 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:
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
1.2. 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
1. Lorem ipsum
2. 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.
1.3. 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
1.4. 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
1.5. 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 &
2. 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
2.1. 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
2.2. 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
2.3 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
2.4. 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.
2.5. 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
2.6. 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
3. 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
4. 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
Contribution
Translation
License
