Linux command line usage tutorial

Shell key shortcuts

Command-line editing

Handled by readline library compiled into bash shell; also available as frontend rlwrap / rline.

Terminal shortcuts

Exercises:

• Try out Ctrl-R, come back with Alt- >.
• Execute yes, pause, continue and abort.

Basic commands

Common options

Exercises:

• Run ls -R, pstree, cd .., cd -
• Why is cd a shell command?
• What is the name of the very first process?

Documentation

Manual page sections (see man man): 1) user commands, 2) system calls, 3) library functions, 5) configuration files, 8) administration commands (etc.). Nomenclature: "see ls (1)". Explicitly give section or display manual pages from all sections:

man 1 ls       # view manual page about ls from section 1
man 1 intro    # view introduction to manual page section 1
man -a kill    # view manual page from all sections
man -k kill    # search headlines

Manual pages are displayed with less by default. Key summary:

Exercises:

• Display manual pages for UTF-8, signal (7), the passwd file and the printf C function.
• What options does the cd command accept?
• Which option makes grep act recursively?

Input / output redirection

Commands useful in pipes

Examples

ls -1 | wc -l    # number of files and subdirectories
ps -lf | cat     # prevent right cutoff

Exercises:

• View the output of the following pipe by piping it into less: factor `seq 1 2000` | awk '{ if(NF==2) { print $2; }}' What does it do?
• Output the number of hyphens in an arbitrary string. (Use echo and tr, among others)

Access permissions

UNIX file permissions: Read, write and execute for file owner, group and others.

Frequent use: Restricting access to devices by assigning a dedicated group to the device file and disabling access for others:

crw-rw---- 1 root lp 6, 0 2011-11-07 09:27 /dev/lp0

Exercises:

• Create two files with touch file1 file2
• Adjust permissions so that others in the group can read only one of the files
• Try to read both files of someone else (using cat or less)
• Without experimenting, find a device file you may not read (other than /dev/lp0)

Installing software

Distribution packages

Ubuntu offers ~37000 packages -- enough for most purposes.

Package managers differ between distros. Ubuntu has the high-level command-line tool aptitude. Syntax:

aptitude [<options>] <command> <arguments>

Compile from source

Compile as normal user:

• Download from project page (not from arbitrary server)
• Unpack archive: tar xzf file.tgz or tar xjf file.tar.bz2 or tar xJf file.tar.xz
• cd into package directory
• ./configure for this system (./configure --help for options)
• Dependencies and their header files (development packages) may need to be installed
• make to build

Install as root:

• make install in package directory after build

Exercise: Print out (exclusively) the Ubuntu packages that start with "abc" (Hint: \< is the regular expression for a left word boundary)

Configuration information

Configuration files

Global configuration files are text files in /etc. Notable config files:

See section 5 manual pages for documentation. Edit with any text editor as root, except for passwd and friends (see below).

Environment variables

Like shell variables, they are dereferenced with "$" in the shell.

Listing environment variables:

printenv

To set an environment variable globally, add something like the following to a file ending .sh in /etc/profile.d: (For per-user settings, put the line in ~/.profile or ~/.bashrc.)

export PATH="new/path:$PATH"

Exercises:

• How do you enable X11 forwarding for SSH?
• Which environment variable(s) contain your user name?

User administration

adduser --ingroup users newuser  # give --ingroup to prevent usergroup creation
adduser newuser othergroup       # additional group membership

Administrator access

/etc/sudoers syntax:

<user / %group>  <host(s)> = ( <target user> )  <command(s)>
thisuser ALL = (root) ALL          # allow thisuser to do anything as root
%admin ALL = (root) ALL            # allow admin group to gain root privileges
thisuser ALL = (otheruser) opera   # allow thisuser to run opera as otheruser

File systems

No drive letters as in Windoze, but one large directory tree. File systems are made available at designated directories called mount points.

Informational commands and files

Mounting and unmounting a file system: (root only, except with the "user" option in /etc/fstab)

mount /dev/sdc1 /mnt/disk   # mount <device representing partition> <mount point>
umount /mnt/disk            # historical misspelling

Device /dev/sdc1 = sd SCSI disk, c third disk, 1 first partition.

Device files (traditionally in /dev) map to Linux device drivers. File-like access is usually possible:

Symbolic links refer to other files by name. Create with:

ln -s path/to/target link_name

Use ls -F to identify special files, and ls -l or readlink to see target.

Exercise: List all symlinks in /usr/bin with the help of ls -l and grep.

More commands

Editors

In order of decreasing suitability for dummies.

Finding and identifying files

find . -iname \*ircam\*   # Search current dir for file names containing "ircam" (case insensitive)
grep -irl ircam .         # Search current dir for files containing the string "ircam"

Science and engineering

Exercises:

• Find configuration files related to Samba
• Output pi to 100 significant digits

Creating new commands

Interpreter scripts

If the first line of a text file is #!/path/to/interpreter and it has its executable bits set, the system will execute it by calling the interpreter with the script file name as an argument (and following command line arguments as applicable).

Shell scripts

Interpreter line is #!/bin/sh, or #!/bin/bash if bash-specific features are required.

Command line arguments are $1, $2 and so on; number of arguments is $#. Use "$1" to preserve spaces; "$*" is all parameters as one string, "$@" is all parameters as separate strings. Further shell variables are created by assignment: varname=value.

Control flow:

if [ -n "$var" ]; then do_that ; fi             # help if;  help test
for var in 1 2 foo bar ; do echo $var ; done    # help for
output_files | while read f ; do cat $f ; done  # help while; help read

Shell wrapper scripts for argument handling

#!/bin/sh
bc -l <<EOF
($1 - 32)*5/9
EOF

Exercises:

• Create a command that creates a directory and descends into it (mkdir and cd)
• Output all squares from 1 to 100 from the shell (use seq)

Further reading

man bash | wc -l      # ...and all worth reading
5718

UNIX tutorial with book recommendations

Introductory UNIX course with exercises

Comprehensive guide to programming the shell bash

List of useful command-line programs sorted by topic

Alphabetical list of shell commands and frequently used utilities

TOS / Impressum