Master Essential Linux Commands: cat, chmod, chown, cp, and More

cat

Usage: cat [-AbeEnstTuv] [--help] [--version] FILENAME Description: Displays the contents of a file or concatenates multiple files to standard output (the output can be redirected to create a combined file).

Common options and examples:

# cat test1 -n
1  1111
2
3  22222
4  333333

# cat test2 -b
1  1111
2  3333

# cat test1 test2 -n > test3
# cat test3
1  1111
2
3  22222
4  333333
5  1111
6
7  3333

chattr

Usage:

chattr [-RV] [-v<version>] [+/-/=<attribute>] FILE...

Description: Changes file hidden attributes on ext2/ext3/ext4 file systems.

Attribute modes:

a – allow only append operations.

b – do not update the file's last access time.

c – compress the file on disk.

d – do not allow the file to be deleted.

i – immutable; the file cannot be modified.

s – secure deletion.

S – synchronous updates.

u – undelete protection.

Common options and examples:

# chattr -RV +i test
Flags of test set as ----i-----------
# lsattr test
----i----------- test

# chattr -RV -i test
Flags of test set as ----------------
# lsattr test
---------------- test

# chattr -RV =idS test
Flags of test set as --S-i-d---------
# lsattr test
--S-i-d--------- test

chmod

Usage: chmod [-cfRv] [--help] [--version] [<mode>] FILE... or chmod [-cfRv] [--reference=<ref>] FILE... Description: Changes file or directory permissions.

Permission symbols:

u – user (owner)

g – group

o – others

a – all (u,g,o)

r – read (4)

w – write (2)

x – execute (1)

- – no permission (0)

Common options and examples:

# chmod -Rv +w test
mode of 'test' retained as 0755 (rwxr-xr-x)

# chmod -Rv a-w test
mode of 'test' changed from 0755 to 0555 (r-xr-xr-x)

# chmod -Rv a=w test
mode of 'test' changed from 0555 to 0222 (-w--w--w-)

# chmod --reference sh test
# ls -d test sh
drwxr-xr-x. 2 root root 4.0K test
drwxr-xr-x. 2 root root 4.0K sh

chown

Usage:

chown [-cfhRv] [--dereference] [--reference=<ref>] USER[:GROUP] FILE...

Description: Changes the owner and/or group of a file or directory.

Common options and examples:

# chown -Rc gwx.gwx test
changed ownership of 'test' from root:root to gwx:gwx
# ls -d test
drwxr-xr-x. 2 gwx gwx 30 Nov 4 16:04 test

# chown -hc gwx.gwx test1
changed ownership of 'test1' from root:root to gwx:gwx
# ls -l test1
lrwxrwxrwx. 1 gwx gwx 4 Nov 4 16:02 test1 -> test/

# chown --reference=/home/gwx/sh/back-etc.sh test
# ls -d test
drwxr-xr-x. 2 gwx gwx 30 Nov 4 16:04 test

cp

Usage: cp [-abdfilpPrRsuvx] SOURCE DEST Description: Copies files or directories.

Common options and examples:

# cp -aiv test test1
'test' -> 'test1'
'test/test1' -> 'test1/test1'
'test/test2' -> 'test1/test2'

# cp -sv sh.tar.xz sh.tar.xz.ln
'sh.tar.xz' -> 'sh.tar.xz.ln'
# ls -l sh.tar.xz.ln
lrwxrwxrwx. 1 root root 9 Nov 4 16:40 sh.tar.xz.ln -> sh.tar.xz

# cp -d sh.tar.xz.ln sh1.tar.xz
# ls -l sh1.tar.xz
lrwxrwxrwx. 1 root root 9 Nov 4 16:41 sh1.tar.xz -> sh.tar.xz

# cp -l sh.tar.xz sh2.tar.xz
# ls -l sh2.tar.xz
-rw-r--r--. 2 root root 2.7K sh2.tar.xz

ln

Usage: ln [-bdfinsv] TARGET LINK_NAME or ln [-bdfinsv] TARGET... DIRECTORY Description: Creates hard or symbolic links.

Common options and examples:

# ln -s nginx-1.11.5.tar.gz nginx
# ln nginx-1.11.5.tar.gz nginx1
# ls -l nginx*
lrwxrwxrwx. 1 root root 19 Nov 4 17:06 nginx -> nginx-1.11.5.tar.gz
-rw-r--r--. 2 root root 935K nginx1

locate

Usage: locate [-d DATABASE] PATTERN... Description: Searches for files using a local filename database (updated with updatedb).

Common options and examples:

# locate nginx-1.11.5.tar.gz
/root/nginx-1.11.5.tar.gz

lsattr

Usage: lsattr [-adlRvV] FILE... Description: Displays file hidden attributes.

Common options and examples:

# lsattr -a
---------------- ./.
---------------- ./..
---------------- ./test1
---------------- ./test2

# lsattr -d
---------------- .

# lsattr -R test
---------------- test/test1
---------------- test/test2
---------------- test/test.ln

mv

Usage: mv [-bfiuv] SOURCE DEST Description: Moves or renames files or directories.

Common options and examples:

# mv ../passwd .
# ls -l passwd
-rw-r--r--. 1 root root 3.0K passwd

# mv test.ln test.mv
# ls -l test.mv
-rw-r--r--. 2 root root 16 Nov 4 16:03 test.mv

rm

Usage: rm [-dfirv] FILE... Description: Removes files or directories.

Common options and examples:

# rm -dirv test1
rm: descend into directory 'test1'? y
rm: remove regular file 'test1/test1'? y
removed 'test1/test1'
rm: remove regular file 'test1/test2'? y
removed 'test1/test2'
rm: remove directory 'test1'? y
removed directory: 'test1'

tee

Usage: tee [-ai] FILE... Description: Reads from standard input and writes to both standard output and files.

Common options and examples:

# tee -ai test1
(test1 now contains appended lines)

touch

Usage: touch [-acfm] [-d DATE] [-r REFERENCE] [-t TIME] FILE... Description: Changes file timestamps; creates the file if it does not exist.

Common options and examples:

# touch -at 201611041700.25 test1
# ls -u test1
-rw-r--r--. 2 root root 24 Nov 4 17:00 test1

# touch -c gghgag   (no file created)

# touch -mt 201611010000 test1
# ls -l test1
-rw-r--r--. 2 root root 24 Nov 1 00:00 test1

# touch -r test1 test2
# ls -l test2
-rw-r--r--. 1 root root 9 Nov 1 00:00 test2

umask

Usage: umask [-S] [MASK] Description: Sets the default permission mask applied when creating new files.

Common options and examples:

# umask
0022
# umask -S
u=rwx,g=rx,o=rx

whereis

Usage: whereis [-bfmsu] FILE... Description: Locates the binary, source, and manual page files for a command.

Common options and examples:

# whereis -b ls
ls: /usr/bin/ls

which

Usage: which FILE... Description: Searches the directories listed in the $PATH environment variable for executable files.

Common options and examples:

# which ls
ls is aliased to `ls -hF --color'
ls is /usr/bin/ls