===== Print nth line from file =====

<code bash>
sed 'Nq;d' file.txt
</code>

===== Prepend text to beginning of file =====

<code bash>
echo "new content" | cat - file.txt > temp
mv temp file.txt
</code>

<code bash>
sed -i '1s/^/new content\n/' file.txt
</code>

===== Remove lines containing only whitespace =====

<code bash>
sed -i '/^\s*$/d' file.txt
</code>

===== Delete nth line from file =====

<code bash>
# N should be the target line number
sed -i 'Nd' file.txt
</code>

===== Replace an entire line in file by line number =====

<code bash>
# N should be the target line number
sed -i 'Ns/.*/replacement-line/' file.txt
</code>

===== Heredoc =====

<code bash>
cat << EOF > file.txt
The current working directory is $PWD.
You are logged in as $(whoami).
EOF
</code>

===== Plain-print the difference between two files =====

Suppose we have two files: ''%%packages.fedora%%'' and ''%%packages%%''.

''%%packages.fedora%%'':

<code bash>
autossh
bash-completion
bat
bc
borgmatic
bzip2
cmake
curl
diff-so-fancy
diffutils
dnf-plugins-core
</code>

''%%packages%%'':

<code bash>
bash-completion
bc
bzip2
curl
diffutils
dnf-plugins-core
</code>

To plain-print the lines that exist in ''%%packages.fedora%%'' but do not exist in ''%%packages%%'':

<code bash>
comm -23 <(sort packages.fedora) <(sort packages)
</code>

Output:

<code output>
autossh
bat
borgmatic
cmake
diff-so-fancy
</code>

  * The ''%%comm%%'' command compares two sorted files line by line.
  * The ''%%-23%%'' flag is shorthand for ''%%-2%%'' and ''%%-3%%''.
  * ''%%-2%%'' suppresses column 2 (lines unique to ''%%packages%%'').
  * ''%%-3%%'' suppresses column 3 (lines that appear in both files).

===== Split large text file into smaller files with equal number of lines =====

<code bash>
split -l 60 bigfile.txt prefix-
</code>

===== Loop through lines of file =====

<code bash>
while read line; do
    echo "$line";
done </path/to/file.txt
</code>

===== Use grep to find URLs from HTML file =====

<code bash>
cat urls.html | grep -Eo "(http|https)://[a-zA-Z0-9./?=_%:-]*"
</code>

===== Use Awk to print the first line of ps aux output followed by each grepped line =====

To find all cron processes with ''%%ps aux%%'':

<code bash>
ps aux | awk 'NR<2{print $0;next}{print $0 | grep "cron"}' | grep -v "awk"
</code>