Technical: August 2022

Saturday, August 13, 2022

Bash - How to Check if a File or Directory Exists in Mac/Unix

How to Check if a File or Directory Exists using Bash?

Using IF statement

FILENAME=/etc/conf.yml
if [ -f "$FILENAME" ]; then
echo "$FILENAME exists."
else
echo "$FILENAME does not exist."
fi

Without using IF statement

[[ -f /etc/conf.yml ]] && echo "$FILENAME exists."

See below link for more details

https://linuxize.com/post/bash-check-if-file-exists/#check-if-directory-exist

Bash/Unix/Mac - File permissions

File permissions - Bash/Unix/Mac

Format is 421
4 - read
2 - write
1 - execute

The following table shows each permission is assigned a value.
Number Octal Permission Representation Ref
0 No permission ---
1 Execute permission --x
2 Write permission -w-
3 Execute and write permission: 1 (execute) + 2 (write) = 3 -wx
4 Read permission r--
5 Read and execute permission: 4 (read) + 1 (execute) = 5 r-x
6 Read and write permission: 4 (read) + 2 (write) = 6 rw-
7 All permissions: 4 (read) + 2 (write) + 1 (execute) = 7 rwx

Number	Octal Permission Representation	Ref
0	No permission	---
1	Execute permission	--x
2	Write permission	-w-
3	Execute and write permission: 1 (execute) + 2 (write) = 3	-wx
4	Read permission	r--
5	Read and execute permission: 4 (read) + 1 (execute) = 5	r-x
6	Read and write permission: 4 (read) + 2 (write) = 6	rw-
7	All permissions: 4 (read) + 2 (write) + 1 (execute) = 7	rwx

$ chmod 755 filename.txt
$ chmod 622 filename2.txt
$ chmod

Shows how much memory is free
$ watch free -h

Bash Command Quick Reference

Bash Command Quick Reference

ls => list of files in a folder / directory
$ ls

List of files with permission (long format)
$ ls -l

List of all files in a directory

$ ls -a

cd => change directory

$ cd <folder>

Go back to home

$ cd

Or

$ cd ~

Create or make a directory or folder

$ mkdir Junk

Locate or search a file.

$ locate filename

To update system db
$ sudo updatedb

"Which" command is used to find other commands
$ which cal
(cal represents calendar command)

Use "history" command to display the history of all the commands you used
$ history

To know what the command does (use whatis command)
$ whatis cal

Copy a file use cp command (cp sourcefile newfile)
$ cp ~/.bashrc bashrc

Move a file (this overwrites and deletes the source file)
$ mv sourcefile targetfile.bash

Remove a file (rm command)

Cat command - "cat" command is used to read the file and display in the monitor
$ cat file1 file2
(merge file1 and file 2 content and shows in the screen)

$ cat >> file2
Append text

$ cat > file3
(new file)

$cat

Nano =>
$ nano

Copy the list of files and access from root directory and write it to a file called out.txt
$ ls -al / > out.txt

To see the output of a file used below command
$ less out.txt

Sudo => if we need to run a command with elevated privileges / access.
$ sudo updatedb

$ sudo -s
Exit

$ su - cindy
(This command will switch user to cindy and do necessary work)

$ exit
logout

This appends the output to all.txt
cat *.txt >> all.txt
This overwrites all.txt
cat *.txt > all.txt