# Bash Cheatsheet

A cheatsheet for linux shell bash.

Ratan Kulshreshtha published on
20 min, 3878 words

Categories: Cheatsheet

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
LANG=en_US.UTF-8
LC_CTYPE=en_US.UTF-8
LESS=-R

$echo$AWS_HOME


### 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:

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


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

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


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


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