Writing scripts can be very useful and saves lots of your time but it can be a bit of a pain, but there are some useful commands that can be used to make life easier. Here are some of the commands I use frequently.

1. Redirecting output to a file
   to redirect the output to a file, use the > operator. if the file does not exist, it will be created. to append to a file, use the >> operator.

   For example

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    ❯ echo "Hello World" > hello.txt
   
    ❯ cat hello.txt
    Hello World
   

   this will create a file named hello.txt with the content “Hello World”. if the file already exists, the content of the file will be overwritten by “Hello World”

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    ❯ echo "123" >> hello.txt
   
    ❯ cat hello.txt
    Hello World
    123
   

   this will append “123” to the file hello.txt. if the file does not exist, it will be created with the content “123”

2. Redirecting the stdin/stdout and stderr
   File descriptors:
   • 0 : standard input (stdin)
   • 1 : standard output (stdout)
   • 2 : standard error (stderr)

   You can redirect these based on the file descriptor you want to use.

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    ❯ cat hello.txt 1>success.txt 2>error.txt
   
    ❯ cat success.txt
    Hello World
    123
   

   this will redirect the standard output to success.txt and the standard error to error.txt. In this case, the error.txt will be empty because the cat command did not encounter any errors.

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    ❯ cat no-file.txt 1>success.txt 2>error.txt
   
    ❯ cat error.txt
    cat: no-file.txt: No such file or directory
   

   In this case, we will get an error message because the file no-file.txt does not exist. success.txt will be empty because the cat command did not print anything to the standard output. And the error.txt will contain the error message.

3. Piping the output to another command
   To pipe the output of one command to the input of another command, use the | operator.

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    ❯ echo "Hello World" | wc -c
    12
   

   this will use the output of the echo command as input to the wc -c command and print the number of characters in “Hello World”.

4. Read from a file
   To print the contents of a file, use the cat command.

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    ❯ cat hello.txt
    Hello World
    123
   
    ❯ cat -n hello.txt
        1   Hello World
        2   123
   

   this will print the content of the file hello.txt. Adding -n flag will print the line number.

5. Searching text
   To search for a text in a file, use the grep command.

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    ❯ grep -i "hello" hello.txt
    Hello World
   

   this will search for the text “hello” in the file hello.txt. Adding -i will ignore the case.

   You can also use grep with output from another command using pipe.
   For example.

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    ❯ kubectl get po | grep -i "grafana"
    grafana-59b7f4d459-ksw62                1/1     Running   0              24d
   
    ❯ kubectl describe po grafana-59b7f4d459-ksw62 | grep  -E -i "image|port"
    Image:          grafana/grafana:7.2.1
    Image ID:       docker.io/grafana/grafana@sha256:733842cca5bd9bcab1eb795da264863a8245402ff3ac8ff17e274334bb32c692
    Ports:          3000/TCP, 3000/TCP
    Host Ports:     0/TCP, 0/TCP
   

   will search for the text “image” or “port” in the output of the kubectl describe po grafana-59b7f4d459-ksw62 command. Adding -E will allows us to search for multiple items using | as a separator.

6. Extracting parts of a string
   To extract parts of a string, use the cut command.

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    ❯ echo "Hello World" | cut -c 1-5
    Hello
   
    ❯ cut -c 1-5 hello.txt
    Hello
    123
   

   will extract the 1-5 character of the string “Hello World” or the first 5 characters of each line in the file hello.txt.

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    ❯ pwd
    /Users/guyzsarun/Desktop
   
    ❯ pwd | cut -f 4 -d "/"
    Desktop
   

   -f 4 will extract the 4th field of the string “/Users/guyzsarun/Desktop”, -d "/" will use / as the separator.

7. Awk command will be used to extract parts of a string line by line

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    ❯ docker -v
    Docker version 20.10.17, build 100c701
   
    ❯ docker -v | awk '{print $3,$5}'
    20.10.17, 100c701
   

   this will print the version of docker and the build number splitting the string by spaces. $3 will print the version and $5 will print the build number.

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    ❯ java --version
    openjdk 18.0.2 2022-07-19
    OpenJDK Runtime Environment Homebrew (build 18.0.2+0)
    OpenJDK 64-Bit Server VM Homebrew (build 18.0.2+0, mixed mode, sharing)
   

   For this example, if we want to get only the version from the first line, we can specify the line number using NR==1.

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    ❯ java --version | awk 'NR==1 {print $2}'
    18.0.2
   

   or we can search for terms that we want. for example, matching the line with the word VM and select the second field.

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    ❯ java --version | awk '/VM/ {print $2}'
    64-Bit
   

8. Translating command

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    ❯ echo "Hello World" | tr "H" "h"
    hello World
   

   this will translate the string “Hello World” to “hello World” by replacing the “H” with “h”.

   or we can convert the whole string from lowercase to uppercase

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    ❯ tr "[:lower:]" "[:upper:]" < hello.txt
    HELLO WORLD
    123
   
    or
   
    ❯ tr "[a-z]" "[A-Z]" < hello.txt
    HELLO WORLD
    123
   

   We can also use tr command to delete characters from a string by using the -d flag.

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    ❯  echo "Hello World" | tr -d "[Wr]"
    Hello old
   

   You can specify multiple characters to delete by putting them in brackets. In the example, we deleted the “W” and “r” characters.

   We can also use SET to delete or update multiple characters from a string.

   • [:alpha:] : All letters
   • [:digit:] : All digits
   • [:upper:] : All uppercase letters
   • [:lower:] : All lowercase letters

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    ❯ tr -d "[:digit:]" < hello.txt
    Hello World
   
   

   this will print out the content from hello.txt without any digits.