For Linux users who are familiar with commands like cp, mv, and ls, here are some commands I found useful but took some time to master:

cat

Most often, cat is used to display file contents, such as cat {your file}. However, it can also be used to concatenate two files and create a new file.

• To concatenate two files:
  • cat {file1} {file2} > {merged file}
• To create a new file and write to it:
  • cat > {your file}
  • It will accept your input and write it to the file.
  • Some automation scripts use this command to create files, like:

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    cat > testFile << EOF {your content} EOF

grep

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grep -R SELINUX /etc/*

• -i: Ignore case.
• -R: Recursively search in subdirectories.
• -v: Invert match, output lines that don’t contain the keyword.

cut

This command is useful for quickly extracting specific content from files with a fixed format, such as /etc/passwd. In this file, each line is separated by colons (":").

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root:x:0:0:root:/root:/bin/bash
vagrant:x:1000:1000::/home/vagrant:/bin/bash

Using cut on this file:

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cut -d: -f1 /etc/passwd

• -d specifies the delimiter, and we use “:” to indicate that the delimiter is a colon.
• -f1 indicates that we want to extract the first field after splitting, which is the username.

The output will be:

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root
vagrant

awk

When the separator is more complex or variable, you can use awk. For the example mentioned above, you can use awk to achieve the same result:

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

• -F specifies the field separator. {print $1} specifies to print the first field after splitting.

sed

sed is used for text substitution. It operates on streams and doesn’t overwrite the original file. For example:

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echo "this is a book." > test
# Create a sample text
sed 's/book/dog/g' test
> this is a dog.
# It replaces "book" with "dog" in the text.
# 's' stands for search.
# 'g' stands for global.
# You can replace 'test' with '*' to change multiple files.

cat test
> this is a book.
# The original file remains unchanged.

# To overwrite, you can add -i
sed -i 's/book/dog/g' test
cat test
> this is a dog.

Redirection

By default, the output of Linux commands is displayed on the screen, but you can redirect the output to a file. Here are some key points:

• > will overwrite the target, while >> appends.
• 1 is stdout, and 2 is stderr.
• You can use & to redirect all output.

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# When the target is stdout, you don't need to specify 1.
ls >> tmpfile

# 'lss' is a non-existent command that will produce an error. You can redirect its stderr output.
lss 2>> tmpfile

# The '&' symbol redirects both stdout and stderr.
ls &>> tmpfile
lss &>> tmpfile

Pipe

Use the pipe | to pass the output of one command as input to another.

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# Count the lines by passing the output of 'ls' to 'wc'.
ls | wc -l

# Extract the 'Mem' column from the output of 'free'.
free | grep -i Mem