Day 4 - Viewing and Editing Files in the Terminal

Workflows become faster when files can be viewed and edited without leaving the terminal. This lesson covers less, head, tail, basic search, real time log viewing, safe edits with nano, and an introduction to Vim for future growth.

Prerequisites

• Day 1 through Day 3 completed
• Terminal with Bash
• A user account with sudo

Fast file previews

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# print a whole file
cat /etc/os-release

# first and last lines
head -n 20 /var/log/syslog 2>/dev/null || head -n 20 /var/log/messages 2>/dev/null
head -n 3 ~/.bashrc

tail -n 20 ~/.bashrc

# follow a growing file in real time
sudo tail -f /var/log/auth.log 2>/dev/null || sudo tail -f /var/log/secure 2>/dev/null

Notes:

• head shows the first lines, tail shows the last lines.
• tail -f keeps the file open and prints new lines as they are written.
• Press Ctrl C to stop tail -f.

less for viewing with search and navigation

Start less on a file.

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less /etc/ssh/ssh_config

Navigation keys:

• Space or PageDown move forward one screen
• b or PageUp move backward
• g jump to top, G jump to bottom
• line numbers: 100g goes to line 100
• search forward with /pattern, next with n, previous with N
• search backward with ?pattern
• quit with q

Open compressed logs without manual decompression when lesspipe is enabled.

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less /var/log/syslog.1.gz 2>/dev/null || less /var/log/messages-*.gz 2>/dev/null

Pipe into less to page long output.

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journalctl -u ssh 2>/dev/null | less
ps aux | sort -k4 -nr | less

Color friendly paging with -R preserves ANSI colors.

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grep --color=always -n "PermitRootLogin" /etc/ssh/sshd_config 2>/dev/null | less -R

Set the default pager.

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printf '\nexport PAGER=less\n' >> ~/.bashrc
source ~/.bashrc

Grep basics with paging

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# show matching lines with line numbers
grep -n "^Port" /etc/ssh/sshd_config 2>/dev/null | less -R

# recursive search under a directory
grep -R "TODO" ~/projects | less -R

# case insensitive and whole word
grep -niw "error" /var/log/syslog 2>/dev/null | less -R

ripgrep (rg) is a faster alternative available in many repos, but standard grep is widely installed.

Editing safely with nano

Nano is a simple editor that is friendly for new users.

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nano ~/playground/day4/notes.txt

Common keys (shown at the bottom in nano):

• Ctrl O write file, Enter confirm
• Ctrl X exit
• Ctrl W search, Alt W search next
• Ctrl K cut line, Ctrl U paste
• Alt G go to line and column

Create and edit a config style file.

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mkdir -p ~/playground/day4
nano ~/playground/day4/app.conf

Example contents:

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[server]
port=8080
log_level=info

Enable line numbers and soft wrap by default with a user config.

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mkdir -p ~/.config
printf "set linenumbers\nset softwrap\n" > ~/.config/nanorc

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printf '\nexport EDITOR=nano\nexport VISUAL=nano\n' >> ~/.bashrc
source ~/.bashrc

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sudoedit /etc/ssh/sshd_config

Vim introduction for long term growth

Vim is powerful and widely available. A minimal starter is enough for this series.

Start the built in tutorial.

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vimtutor

Open a file.

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vim ~/playground/day4/demo.txt

Modes:

• Normal mode for navigation and commands
• Insert mode for typing text
• Visual mode for selections

Basic commands:

• i enter Insert mode, Esc return to Normal mode
• :w write file, :q quit, :wq write and quit, :q! quit without saving
• movement: h left, j down, k up, l right, w next word, b previous word, 0 start of line, $ end of line
• search: /text then n next, N previous
• replace current line example: :%s/old/new/g for whole file
• undo u, redo Ctrl R

Open multiple files and switch between them.

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vim a.txt b.txt
:bn     " next buffer
:bp     " previous buffer
:args   " list files

Optional minimal config to make Vim friendlier.

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cat > ~/.vimrc <<'EOF'
set number
set ignorecase smartcase
set incsearch
set hlsearch
set expandtab shiftwidth=2 tabstop=2
syntax on
EOF

Real time log viewing patterns

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# follow one file
sudo tail -f /var/log/auth.log 2>/dev/null | ts 2>/dev/null || sudo tail -f /var/log/secure 2>/dev/null

# follow by unit with systemd
journalctl -u ssh -f 2>/dev/null

# follow kernel messages
journalctl -k -f 2>/dev/null

ts (from moreutils) prepends timestamps if installed.

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sudo apt install -y moreutils 2>/dev/null || sudo dnf install -y moreutils 2>/dev/null

Compare changes after edits

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# copy a file, make a safe change, then compare
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak 2>/dev/null || true
sudoedit /etc/ssh/sshd_config
sudo diff -u /etc/ssh/sshd_config.bak /etc/ssh/sshd_config | less -R

Exit codes and verification:

• diff returns 0 when files are identical, 1 when they differ, and above 1 on error.
• After editing service configs, test syntax if a tool supports it, then restart the service with systemctl in a later lesson.

Troubleshooting

• Permission denied when editing system files. Use sudoedit instead of running the editor as root.
• E212: Can't open file for writing in Vim. Write with :w !sudo tee % only when sudoedit is not an option.
• less shows garbled output. Add -R when piping colored output.
• Terminal does not support the clipboard. Use the editor's own copy and paste or a terminal that supports clipboard integration.
• tail -f shows nothing. Confirm that the log path matches the distribution. Debian and Ubuntu often use /var/log/syslog and /var/log/auth.log. RHEL and Fedora use /var/log/messages and /var/log/secure.

Up next: Day 5 - Users, Groups, and Sudo

Day 5 covers users, groups, and sudo. It explains account data in /etc/passwd and /etc/group, how to create users, and how to grant limited administrative rights with sudo while keeping the system safe.