by Craciun Dan | v0.2.8 r7 (Nov 17, 2014) | Latest Version
General Purpose Commands
whereis - locate the binary, source and manual page files for a command
whereis searches for binary, source and man pages in standard Linux places. (Location: /usr/bin/whereis /usr/bin/X11/whereis)
which - locate a command
which searches for a command in all directories included in PATH. (Location: /bin/which /usr/bin/which /usr/bin/X11/which)
apropos - search the manual page names and descriptions
apropos searches for a given pattern in manual page names and descriptions, returning a list of matches. (Location: /usr/bin/apropos /usr/bin/X11/apropos)
whatis - display manual page descriptions
whatis displays the name and short description (located in the NAME section of the respective manual page) about a given command. (Location: /usr/bin/whatis /usr/bin/X11/whatis)
whoami - print effective userid
whoami displays the username of the currently logged in user (Location: /usr/bin/whoami /usr/bin/X11/whoami)
id - print real and effective user and group IDs
id displays the current username and the groups it belongs to. (Location: /usr/bin/id /usr/bin/X11/id)
cp - copy files and directories
cp copies files and directories. To copy a directory, use cp -r. (Location: /bin/cp)
dd - convert and copy a file
dd copies a file converting it according to the operands. (Location: /bin/dd)
grep - print lines matching a pattern
grep shows all the lines that match a specific given pattern in its input. (Location: /bin/grep)
gzip - compress or expand files
gzip compresses files given as arguments reducing size drastically. (Location: /bin/gzip)
kill - send a signal to a process
kill sends various signals to processes. The default signal is TERM. (Location: /bin/kill)
less - file perusal filter for crt viewing
less is a pager that displays text in a file. It is similar with more, but it is more powerful. (Location: /bin/less)
ln - make links between files
ln creates hard links and symbolic links between files. (Location: /bin/ln)
tar - the GNU version of the tar archiving utility
tar stores and extracts files from an archive. (Location: /bin/tar)
touch - change file timestamps
touch creates an empty file if it doesn't exist or updates the access and modification time of a file if if already exists. (Location: /bin/touch)
uname - print system information
uname shows information about the system, like the kernel version, current date and time, CPU architecture. (Location: /bin/uname)
file - determine file type
file performs filesystem tests, magic tests and language tests and returns the first match it finds. (Location: /usr/bin/file /usr/bin/X11/file)
df - report file system disk space usage
df displays disk usage for all mounted filesystems on the system, showing total size, used size and free space. To see the sizes in human readable form, use df -h and to see information for a specific file system only, specify it as an argument to df (e.g. df -h /dev/sda1). (Location: /bin/df)
du - estimate file space usage
du shows the total size of all the directories, sub-directories and files in the current location. Use the -h switch to show human readable sizes. (Location: /usr/bin/du /usr/bin/X11/du)
bzip2 - a block-sorting file compressor
bzip2 compresses files offering very good compression sizes. (Location: /bin/bzip2)
chgrp - change group ownership
chgrp changes the group of each given file. (Location: /bin/chgrp)
chmod - change file mode bits
chmod changes file and directory permissions, as well as setting file mode bits like the sticky bit. (Location: /bin/chmod)
chown - change file owner and group
chown changes the user and/or group ownership of each given file. (Location: /bin/chown)
ls - list directory contents
ls lists files and directories as well as information about them. (Location: /bin/ls)
mkdir - make directories
mkdir creates directories if they don't already exist. Use mkdir -p to create directories recursively (e.g. mkdir -p $HOME/mydir/mysubdir). (Location: /bin/mkdir)
mv - move (rename) files
mv moves or renames files. (Location: /bin/mv)
ps - report a snapshot of the current processes
ps shows the running processes in the current shell. (Location: /bin/ps)
pwd - print name of current/working directory
pwd shows the current working directory. (Location: /bin/pwd)
rm - remove files or directories
rm removes files or directories. To remove a directory, use rm -r. (Location: /bin/rm)
mv - move (rename) files
mv moves or renames files. (Location: /bin/mv)
sed - stream editor for filtering and transforming text
sed is a powerful utility for manipulating text. (Location: /bin/sed)
sort - sort lines of text files
sort is a tool which allows to sort text. (Location: /usr/bin/sort)
uniq - report or omit repeated lines
uniq is a tool which will show repeated lines or discard them. (Location: /usr/bin/uniq)
Useful One-Liners
Show the Default Shell of the Current User
cat /etc/passwd | grep $USER | cut -d ":" -f 7
See the Most Used Commands in Bash History
history | awk '{print $2}' | awk 'BEGIN {FS="|"}{print $1}' | sort | uniq -c | sort -n | tail | sort -nr
Quickly Write a Text File (without an Editor)
cat > filename.txt
Then type in whatever you like. Press Ctrl+D when you're done. The file filename.txt will be overwritten if it exists. Another way to do it:
cat > filename.txt <<EOF
> input text
> goes here
Show the Currently Running Shell
echo $0
ps -p $$
List All Users Recognized by the System
cat /etc/passwd | cut -d ":" -f 1
Show the Most Used 20 Commands
history | awk '{ print $2 }' | sort | uniq -c | sort -nr | head -20
Searching for Files
Search Files for a Specific Text
find . -iname "*.txt" -exec grep -l "hello" {} +
This will search and display all the files ending in .txt in the current directory for the text hello.
Find Files Modified in the Last N Days
find . -iname "*" -mtime -2
This will find and display all the files which were modified in the last two days.
Find All Empty Files and Folders
find . -iname "*" -empty
System Administration
Mount an ISO image
sudo mount -o loop /path/to/file.iso /mount/point
Mounts file.iso at /mount/point. The mount directory /mount/point should be empty, otherwise the files that it contains will be hidden while the image is mounted (but not lost, they will reappear as soon as the image is unmounted).
Make a Bootable USB Flash Drive from an ISO Image
sudo dd bs=4096k if=/path/to/image.iso of=/dev/sdc
if stands for input file (the ISO image in this case), while of is the USB device, which in this case is /dev/sdc.
System Configuration
Keyboard Mapping with xev
xev is a small utility which prints contents of X events, so you can assign new key functions to the keyboard using xmodmap. Type xev to see key events and keycodes. Close the X window to close xev when you're done.
Assign New Keyboard Keys
xmodmap -e "keycode 94 = backslash bar"
xmodmap can be used to assign to values to keys, so for example pressing the \ key on UK keyboards will have another effect. The above example will make the key to the right of LShift to be \| on a UK keyboard.
Encode FLAC/WAV to Ogg
oggenc -b 192 filename.flac
You will need to install the vorbis-tools package first.
Encode WAV to MP3
lame -b 192 filename.wav
You will need to install the lame package first.
Split FLAC/WAV with CUE
cuebreakpoints cue_file.cue | shnsplit audio_file.flac
You will need to install the cuetools and shntool packages first.
Batch Resize JPG Files
for i in *.jpg; do convert $i -resize 528x "${i//./_resized.}"; done
Replace 528x with the desired size in pixels. This specifies the new width, aspect ratio of the original image will be preserved, the original images will be kept and the resized ones will be renamed as origname_resized.jpg. You will need to install the imagemagick package first.
Create ISO Images from Files/Folders
genisoimage -o ouput_file.iso input_directory
You will need to install the genisoimage package first.
Create ISO Images from CDs/DVDs
dd if=/dev/cdrw of=$HOME/output_file.iso
Replace /dev/cdrw with your device file.
Create ISO Images from Audio CDs
cat /dev/cdrw > $HOME/audio_file.iso
Replace /dev/cdrw with your device file.
Basic Notions
The Shell
A shell is a command interpreter that can accept commands from the stdin like the keyboard or from a file, called a script. A shell reads command lines, one by one, performs the necessary substitutions, execute the commands and returns the result to the user.
command [option] [argument]
This is the general form of a command, where:
  • command is the command to execute, usually a program, script or alias located inside directories such as /bin and /usr/bin
  • [option] is an option or group of options to pass to the program; options tell the program how to output or interpret various information (e.g. show or don't show hidden files); options may have a short form (e.g. -h) or a long form (e.g. --human-readable) and may be grouped together (e.g. instead of -a -h you may use -ah)
  • [argument] is the argument given to the program, for example in ls -l /etc, -l is an option and /etc is an argument, telling the ls command to list files inside the /etc directory
Most commands (but not all) may be issued without any options or arguments, in which case the program will use its default behavior. For example ls issued by itself without any parameters will list the file names in the current working directory, whichever that may be.
Filesystem Hierarchy
The following shows the standard filesystem hierarchy on a Linux system, according to the Filesystem Hierarchy Standard:
  • /bin - essential user command binaries (e.g. bash, bzip2, cat, chmod, chown, cp, date, df, echo, grep, kill, less, ln, ls, nano, pwd, rm, sed, tar, touch, which, uname)
  • /boot - static files of the boot loader
  • /dev - device files
  • /etc - host-specific system configuration
  • /home - user home directories (optional)
  • /lib - essential shared libraries and kernel modules
  • /media - mount point for removable media
  • /mnt - mount point for a temporarily mounted filesystem
  • /opt - add-on application software packages
  • /root - home directory for the root user (optional)
  • /sbin - system binaries
  • /srv - data for services provided by this system
  • /tmp - temporary files
  • /usr - user commands, include files, libraries, documentation etc
  • /var - logs, cache data
In addition to these, most distributions may include the following directories:
  • /proc
  • /sys
 rwx     r-x     r-x
 111     101     101
  7       5       5
File Types
The first bit in permissions can be:
  • - for a regular file
  • d for a directory
  • l for a symbolic link
  • c for a special file
  • s for a socket
  • p for a named pipe
  • b for a block device
Basic Commands
ls - list directory contents
This command lists information about files in a directory. It may or may not take options and arguments. For example, ls without any arguments will list the file names in the current working directory, while ls -a /etc will list all the files inside the /etc directory, including hidden files (preceded by '.') and virtual files (. and ..).
$ ls -l /etc
total 1244
drwxr-xr-x  3 root   root      4096 iul 15 12:39 acpi
-rw-r--r--  1 root   root      2981 iul 15 12:30 adduser.conf
Options like -a or -l can be nested together, like ls -lh /etc, which will list the files inside /etc using the long listing format (-l) and showing human-readable sizes (-h). Several options include:
  • -a, --all do not ignore entries starting with . (list all files, including the hidden ones)
  • -h, --human-readable with -l, print sizes in human readable format
  • -X sort alphabetically by entry extension
(Location: /bin/ls)
cd - change the shell working directory
This command changes the current working directory. For example cd /etc will change the current working directory to /etc.
$ pwd
$ cd /etc
$ pwd
$ cd $HOME
$ pwd
In the above example you can see a few examples of using cd. After each time the cd command is issued, pwd will print the current working directory to reflect the changes. $HOME is an environment variable which expands to the home directory of the current user (in this case /home/embryo). Without arguments, cd will change the directory to the home directory of the current user. (Location: cd is a Bash builtin)
Bash Tips
Bash Keyboard Shortcuts
Keyboard shortcuts are very important since they provide fast editing capabilities. They are of really great help when working with the shell. Here is a list of keyboard shortcuts to use in Bash:
  • ^F (Ctrl+F) move cursor one character to the right
  • ^B (Ctrl+B) move cursor one character to the left
  • ^A (Ctrl+A) move cursor to the start of the line
  • ^E (Ctrl+E) move cursor to the end of the line
  • ^U (Ctrl+U) delete all text to the left of the cursor
  • ^K (Ctrl+K) delete all text to the right of the cursor
  • ^P (Ctrl+P) bring up the previous command in history
  • ^N (Ctrl+N) bring up the next command in history
  • ^H (Ctrl+H) delete one character to the left
  • ^L (Ctrl+L) clear the terminal
  • ^R (Ctrl+R) reverse search
  • ^C (Ctrl+C) end a running program
  • ^Z (Ctrl+Z) suspend a running program
  • ^D (Ctrl+D) exit the current shell
  • Alt+F move cursor one word to the right
  • Alt+B move cursor one word to the left
  • Tab command or filename completion
Start Bash in Debug Mode
bash -x
Print the Remaining Arguments of a Script Starting at a Specified Position
echo "${@:N}"
Will echo all the remaining arguments passed to a script, starting with Nth argument. Take the following script, called

echo "${@:3}"
If ran as ./ ab cd ef gh ij kl, the output will be:
ef gh ij kl
Floating-Point Arithmetic Examples
echo "5/2" | bc -l
echo | awk '{ print 5/2 }'
perl -e 'print 5/2'
Bash Builtins
(( expression ))
Evaluate expression value.
Execute commands from a file in the current shell. Example:
. $HOME/.bashrc
Null command. No effect, the command does nothing.
Evaluate conditional expression. This command is the same as the test builtin, but the last argument must be a a ] character to match the opening [. Example:
if [ -f /bin/bash ]; then
  echo "File /bin/bash exists."
Execute conditional command.
Define or display aliases. Example:
alias rmf='rm -f'
Display or manipulate the history list.
Execute commands based on conditional. Example:
if [ $VAR -gt 10 ]; then
  echo "$VAR is greater than 10."
elif [ $VAR -lt 10 ]; then
  echo "$VAR is less than 10."
  echo "$VAR is 10."
Display status of jobs.
Send a signal to a job. The following commands do the same thing, sending the SIGKILL signal to the process with the PID of 1550:
kill -9 1550
kill -SIGKILL 1550
kill -KILL 1550
Evaluate arithmetic expressions.
Other Commands
qdbus: Show Amarok Metadata Info
qdbus org.kde.amarok /Player GetMetadata
qdbus: Change Amarok Volume
qdbus org.kde.amarok /Player VolumeSet 40
qdbus: Play/Pause Amarok
qdbus org.kde.amarok /Player PlayPause
Ubuntu/Mint Useful Tips and One-Liners
APT: Upgrade the System
sudo apt-get update && sudo apt-get dist-upgrade
APT: Add a PPA Repository
sudo add-apt-repository ppa:USERNAME/PPA_NAME
APT: Show Package Info
apt-cache show PACKAGE
APT: Clean Up Package Cache
sudo apt-get clean
APT: Clean Up Packages No Longer Available
sudo apt-get autoclean
APT: Install a Program's Dependencies
sudo apt-get build-dep <PACKAGE>
The source repositories (lines starting with deb-src inside your /etc/apt/sources.list file) should be enabled.
APT: Search for Packages by Pattern
apt-cache search <PATTERN>
This will search for packages which contain PATTERN in their name or description.
APT: List Dependencies of a Package
apt-cache depends <PACKAGE>
This will list the package names on which PACKAGE depends on.
APT: List All Packages that Depend on a Package
apt-cache rdepends <PACKAGE>
This will list the package names which depend on PACKAGE.
APT: Fix Broken Dependencies
sudo apt-get -f install
DPKG: Install a DEB Package
sudo dpkg -i PACKAGE.deb
DPKG: Remove a Manually Installed DEB Package
sudo dpkg -r PACKAGE.deb
DPKG: Forcibly Remove an Installed Package
sudo dpkg --purge --force-all PACKAGE
DPKG: List All Installed Packages
dpkg --get-selections
This will list all the packages installed on a system using APT.
DPKG: List Packages That Install a Certain File
dpkg -S <FILENAME>
This will list all packages that will install FILENAME. The pattern FILENAME will be matched for files containing it in their name.
DPKG: List Installed Files by a Package
DPKG: List Contents of a DEB Package
dpkg -c <FILE>
DPKG: Show the Control File of a DEB Package
dpkg -f <FILE>
Get a List of Every Installed Package
dpkg -l | tr -s ' ' '#' | cut -f2 -d"#"
Show Mint Release Info
lsb_release -a
GSettings: Disable Overlay Scrollbars in Ubuntu
gsettings set com.canonical.desktop.interface scrollbar-mode normal
Standard C
#include <stdlib.h>

void *malloc (size_t size);
#include <stdlib.h>

void srand (unsigned int seed);
#include <sys/time.h>

int gettimeofday(struct timeval *tv, struct timezone *tz);
#include <stdio.h>

int fgetc(FILE *stream);
This function reads the next character from stream and returns it as an unsigned char cast to an int, or EOF on end of file or error.
while ((c = fgetc(stream)) != EOF) {
	array[i] = c;
#include <stdio.h>

char *fgets(char *s, int size, FILE *stream);
This function reads at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A terminating null byte ('\0') is stored after the last character in the buffer.
#include <stdio.h>

int fprintf (FILE *fd, const char *format, ...);
#include <stdio.h>

int fscanf (FILE *fd, const char *format, ...);
#include <stdio.h>

int sscanf (const char *str, const char *format, ...);
This function reads the input from the string point to by str and formats it according to format.
char str[5] = "1234";
int n;

if (sscanf(str, "%d", &n) != EOF) {
        fprintf(stdout, "%d", n);
#include <string.h>

char *strcpy(char *dest, const char *src);
This function copies the string pointed to by src, including the terminating null byte ('\0'), to the buffer pointed to by dest. The destination string dest must be large enough to receive the copy.
#include <string.h>

int strcmp(const char *s1, const char *s2);
This function compares two strings and returns an integer less than zero if s1 is found to be less than s2, equal to zero if s1 is found to match s2, or greater than zero if s1 is greater than s2.
System Calls
#include <fcntl.h>

int open (const char *name, int flags);
int open (const char *name, int flags, mode_t mode);
This function is used to open a file. The flags argument can be one of O_RDONLY, O_WRONLY or O_RDWR. The flags argument can be bitwise-ORed with one or more of several other values, like O_APPEND, O_TRUNC or O_CREAT. If O_CREAT is specified, the mode argument is also required.
fd = open("filename.txt", O_RDONLY | O_CREAT, 0644);
if (fd < 0) {
  perror("ERROR: open()");
#include <unistd.h>

int close (int fd);
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int select (int nfds,
            fd_set *read-fds,
            fd_set *write-fds,
            fd_set *except-fds,
            struct timeval *timeout);

FD_ZERO (fd_set *set);
FD_SET (int fd, fd_set *set);
FD_CLR (int fd, fd_set *set);
FD_ISSET (int fd, const fd_set *set);
#include <unistd.h>

ssize_t read (int fd, void *buffer, size_t size);
#include <unistd.h>

ssize_t write (int fd, const void *buffer, size_t size);
#include <stdio.h>

void perror (const char *message);