Files and Directories

Create, copy, and remove

mkdir - Creates a directory. Use it when you need a new folder for a project. -p creates any missing parent directories along the path, and it does not complain if the directory already exists.

mkdir -p /home/devopscowboy/projects/ansible-lab/playbooks

touch - Creates an empty file, or updates the timestamp if that file already exists. Use it when a file has to exist before you edit it, or when a program only checks that the name is there.

touch notes.md

cat - Prints a whole file to the screen. Use it for a short file, such as /etc/hosts. On a long file the text scrolls past. Use less for those.

cat /etc/hosts

less - Opens a file one screen at a time. Use it for logs and anything longer than a page. The space bar moves forward, q quits, and /pattern searches. On RHEL, general system messages are in /var/log/messages.

less /var/log/messages

cp - Copies a file. The first path is the original and the second is the copy. Use it to keep a backup before you edit. -r copies a directory and everything inside it. -a does that and also keeps permissions, ownership, and times.

cp notes.md notes.md.bak
cp -r playbooks playbooks-copy

mv - Moves a file or directory. That is also how you rename one: the original name is gone, and nothing is left in the old place. Use it to change a name or to put a file in a different folder. If the destination name already exists, mv replaces that file.

mv web.yml site.yml
mv notes.md docs/notes.md

rm - Deletes a file. There is no trash folder, and you cannot undo it. -r deletes a directory and everything inside it. Run ls on the path first so you can see the names you are about to remove.

rm notes.md
rm -r old-playbooks

rm -rf means recursive and force. It does not ask, and it deletes a whole tree. Do not point it at /, and do not run it on a path you have not read back to yourself. RHEL’s root shell is often set up to ask before rm, cp, and mv. A normal user’s shell is not. Do not count on the question. Prefer rm -r without -f while you are learning.

Links

ln -s creates a symbolic link. The first path is the target, the second is the link name. readlink -f prints the path with every symlink resolved. A hard link is a second name for the same file. Hard links stay on one filesystem, and a directory cannot be a hard link.

ln -s /opt/app/releases/v2 /opt/app/current
readlink -f /opt/app/current
ln /etc/passwd /tmp/passwd.name

Read a file

less /var/log/messages
head -n 20 file
tail -n 50 -f /var/log/secure
wc -l file

On RHEL, general system messages go to /var/log/messages, and SSH and sudo go to /var/log/secure. less is the pager: /pattern searches, n finds the next hit, q quits. tail -f follows new lines. tail -F keeps following when the file is rotated and recreated. The journal is the other half of RHEL logging, on Logs and the journal.

Search

grep -n 'pattern' file
grep -RIn 'pattern' .
find . -name '*.log'
find / -name 'sshd_config' 2>/dev/null

grep -n prints the line number. -R searches through directories, -I skips binary files, and the n numbers each hit. find -name matches a file name. 2>/dev/null hides the permission errors find / prints for directories you cannot read.

Archives

tar -czf archive.tar.gz directory
tar -cjf archive.tar.bz2 directory
tar -tzf archive.tar.gz
tar -xzf archive.tar.gz -C /opt

-c creates, -x extracts, -t lists, and -f is the archive name. -f has to be followed by that name. -z is gzip and -j is bzip2. -C /opt extracts into that directory instead of the current one.

Slice a text file

sed -n '10,20p' file
sed -i.bak 's/old/new/g' file
awk -F: '{print $1}' /etc/passwd
cut -d: -f1 /etc/passwd
grep -Ev '^\s*(#|$)' /etc/ssh/sshd_config

sed -n '10,20p' prints that line range and nothing else. sed -i.bak edits in place and keeps file.bak. On RHEL, GNU sed also allows -i with no suffix. awk -F: and cut -d: -f1 both print the first colon-separated field. In /etc/passwd that field is the username.

grep -Ev drops blank lines and lines whose first non-space character is #. It does not strip a comment that follows a setting.