Manage files and folders

$df -hT
Filesystem            Type   Size  Used Avail Use% Mounted on
/dev/mapper/rhel-root xfs     17G  4.1G   13G  25% /
/dev/vda1             xfs    960M  310M  650M  33% /boot
tmpfs                 tmpfs  1.9G     0  1.9G   0% /dev/shm
$findmnt
$lsblk
$mount | grep xfs

• df -hT shows each mounted file system, its type and how full it is.
• findmnt shows the mounts as a tree.
• lsblk shows the disks and partitions and where they are mounted.

Separate file systems for /boot, /var or /home protect the system: a full /var/log cannot fill /.

$ls /etc/*.conf
$ls -d /etc/[a-c]*
$cp /var/log/messages{,.bak}

The last line copies messages to messages.bak: {,.bak} makes two words.

1. Make a folder, also with the parent folders that do not exist yet.

   $mkdir /tmp/project
   $mkdir -p /tmp/project/app/logs

2. Move around.

   $cd /tmp/project/app
   $pwd
   /tmp/project/app
   $cd ..
   $cd -
   $cd

3. Remove folders: rmdir only when empty, rm -r with all content.

   $rmdir /tmp/project/app/logs
   $rm -r /tmp/project

cd .. = one folder up, cd - = back to the last folder, cd alone = your home folder.

$cp /etc/hosts /tmp/
$cp /etc/hosts /tmp/hosts.bak
#cp -r /etc/ssh /tmp/ssh-backup
#cp -a /var/www/html /backup/html
$cp -a /etc/skel/. ~/test/

• When the target is a folder, the file keeps its name inside it.
• -a (archive) = recursive + keep permissions, owner, times, links and SELinux context. Best for backups.
• /etc/skel/. copies the content of the folder, hidden files included.

$mv report.txt /tmp/
$mv report.txt report-2026.txt
$mv /tmp/old-logs /srv/archive/

Moving inside the same file system is instant (only the name changes). Between file systems, mv copies and then deletes.

Every file has an inode with the data and the permissions. A file name is just a pointer to an inode. ls -i shows the inode number, and the second column of ls -l shows how many names point to it.

1. Make a hard link and a soft link.

   $ln -s /etc/hosts ~/hosts-soft
   $echo test > data.txt
   $ln data.txt data-hard.txt

2. Look at them.

   $ls -li data.txt data-hard.txt ~/hosts-soft

3. Remove the original and see what happens.

   $rm data.txt
   $cat data-hard.txt
   test

A hard link only works inside one file system. Also, a normal user may not make a hard link to a file of another user (a safety rule of the kernel).

1. Create a compressed archive of a folder.

   #tar -czvf /root/etc-backup.tar.gz /etc

2. List what is inside without extracting.

   $tar -tvf /root/etc-backup.tar.gz

3. Extract it into another folder.

   #mkdir /tmp/restore
   #tar -xzvf /root/etc-backup.tar.gz -C /tmp/restore

4. Extract only one file.

   #tar -xzvf /root/etc-backup.tar.gz -C /tmp/restore etc/hosts

tar removes the leading / from the names, so extracting never overwrites the real /etc by accident (unless you extract in /).

$gzip big.log
$ls big.log*
big.log.gz
$gunzip big.log.gz
$gzip -k big.log
$zcat /var/log/messages-*.gz | grep -i error

-k keeps the original file. zcat, bzcat and xzcat read a compressed file without unpacking it.

$find /etc -name "*.conf"
#find / -user student
#find / -size +100M
#find /var/log -mtime -1
$find /home -type d -name ".ssh"

Copy all files of a user to another folder:

#mkdir /root/student-files
#find / -user student -type f -exec cp --parents {} /root/student-files/ \; 2>/dev/null

• The FHS places files by purpose: /etc (settings), /var (changing data, logs), /home (users), /usr (programs), /tmp (temporary files).
• An absolute path starts at /; a relative path starts at the current folder.
• Wildcards: * (any text), ? (one character), [abc] (one of these characters).
• cp -a keeps owner, permissions and time stamps; mv moves or renames; rm -r deletes folders.
• A hard link is a second name for the same inode (same file system only); a symbolic link points to a path and can cross file systems.
• tar -czf makes a gzip archive, tar -xf unpacks, tar -tf lists; -C picks the target folder.
• find searches by name, size, owner, time and type, and can run a command on every file with -exec.