Chapter 2 Basic Commands and Directory Hierarchy

The Bourne Shell: /bin/sh

(2.1 and further)

A shell is a program that runs commands. Shell scripts are text files that contain a sequence of shell commands (cfr. windows .bat-files).

Linux uses an enhanced version of the Bourne shell called bash or the "Bourne-again" shell. A shell window is called a terminal.

You can change your shell with chsh.

Shell Globbing (Wildcards)

(2.4 and further)

• Anything with *
• Just one with ?

If you don't want the shell to expand a glob in a command, enclose the glob in single quotes.

Note: shell performs expansions before running commands, and only then.

ls wont display dotfiles, ls -a will show them.

You can run into problems with globs because .* matches . and ... You may want to use something as .^. or .?? to get all dot files except the current and parent directories.

Environment and Shell Variables

(2.8 and further)

Assign with $FOO=bar and access with $FOO. An environment variable is a variable not specific to the shell. If you want this use export FOO.

The Command Path

(2.9)

PATH is a special environment variable that contains the command path (=list of system directories that the shell searches when trying to locate a command). You can change this as follows PATH=dir:$PATH or PATH=$PATH:dir.

Special characters

(2.10)

Shell Input and Output

(2.14)

command > file and command >> file to append.

set -C to avoid clobbering in bash.

ls /fffffff > f 2> e will send standard output to f and standard error to e. The number 2 specifies the stream ID that the shell modifies. 1 is output (default), 2 is error. You can also send the standard error to the same place as stdout with the >& notation. So to send both to f we use ls /fffffff > f 2>&1.

Listing and manipulating processes

kill pid to kill a pid. To freeze use kill -STOP pid and to unfreeze or continu use kill -CONT pid. CTRL+C is the same thing as performing a kill with the INT(interrupt) signal.

A TSTP signal can be send by pressing CTRL+Z, then start the process again by entering fg or bg. Use jobs to see if you have still processes hanging.

Use COMMAND & to put it in background.

File modes and Permissions

(2.17)

-rw-r--r-- shows that the file (- no d) is readable, writable foor user, only readable for group and only readable for others.

To modify permissions use chmod. For example chmod go+r file or chmod 644 file. The numbers are combinatons of (4,2,1) where 4 is reading, 2 is writing and 1 is executing, therefore 644 is read/write for user, and read for both group and others. 711 is read/write/execute for user, execute for both group and others.

A symbolic link can be created with ln -s target linkname and is essentialy a shortcut to another file.

Archiving and Compressing Files

• gunzip file.gz to unzip or gzip file to compress.
• tar cvf archive.tar file1 file2 to create an archive.
• tar xvf archive.tar to unpack the tar.
• tar ztvf file.tar.gz to do both.

Note: .tgz is the same as .tar.gz (the suffix was meant to fit into FAT filesystems).

Linux Directory Hierarchy Essentials

• /bin contains ready to run programs
• /dev device files
• /etc system configuration directory
• /home personal directories for regular users
• /lib libraries for executables (shared)
• /proc provides system statistics
• /sys similar to /proc, it provides a device and system interface
• /sbin system executables (mostly system management)
• /tmp temporary folders & files (used as workspace)
• /usr contains large directory hierarchy
• /var variable subdirectory where programs record runtime information.
• /boot kernel boot loader files
• /media attachment point for removable media
• /opt for third-party software

Used commands & shortcuts in this chapter

• echo, cat, ls, man, info, ps (auxw)
• cp, mv, touch, rm, cd
• mkdir, rmdir
• grep (-i: case-insensitive ; -v: inverts ; -E: egrep )
• less (more), pwd (-P: physical (avoid symlinks)), diff (-u: numbers), file
• find, locate (note: locate works with index)
• head, tail, sort
• CTRL-D (stops current standard input entry and return to shell)
• CTRL-C (terminates a program)
• CTRL-B -F -P -N (move cursor left, right, up, down)
• CTRL-A -E (beginning/end of line)

Extra (slides + notities)

(Slides)

shell:

• user interface (CLI)
• command interpreter (interactief)
• uitvoering scripts (niet-interactief)

Soorten shells: Bourne (sh -> bash) op Linux, Korn (ksh) op System V, Joy (csh) op BSD en Falstad (zsh).

Om een lijst van beschikbare shells op te vragen gebruik het commando chsh -l. chsh -s kan gebruikt worden om de shell te wijzigen. Dit kan terminaal zijn voor root!

declare -p geeft een overzicht van alle shell variabelen. Het-x attribuut wijst op ee environment variabele. env geeft een overzicht van de environment.

Shell options zijn zogenaamde on/off switches. -:on, +:off. Voorbeeld: set +o errexit of set -o noglob. Om de ingestelde opties te bekijken gebruik echo $SHELLOPTS of echo $-.

Sommige shell opties kunnen enkel via shopt opgevraagd of ingesteld worden. shopt -s ... stelt iets in, shopt -u ... schakelt het uit. echo $BASHOPTS kan je gebruiken om ingestelde shopt opties te bekijken.

Sommige uitvoerbare programma's zijn bash builtin (door bash geïmplementeerd) de rest wordt extern genoemd. Builtin is doorgaans efficiënter.

which [-a] ..., type [-a] [-t] ... kan men gebruiken om te ontrafelen of programma's builtin of extern zijn.

echo ${BASH_CMDS[...]} voor hashed. enable [-a] [-n] ... ... kan men gebruiken om builtins te disablen of enablen.

Weak quotes zijn dubbele quotes. Hierbinnen behouden de speciale tekens hun betekenis. Single quotes zijn strong quotes en dus letterlijk. $'..' worden POSIX quotes genoemd en behouden eveneens hun speciale betekenis.

I/O kanalen:

• STDIN 0< of <
• STDOUT 1> of >, 1>> of >>, 1>| of >|
• STDERR 2>, 2>>, 2>&1, >&, &>

Note: niet te verwarren met file descriptors.

Help:

• man
• info
• help
• /usr/share/doc/*

Filters:

• cat
• tac
• rev
• wc
• head
• tail
• nl
• cut
• od
• hexdump
• sort
• uniq
• fold
• fmt
• strings
• tr
• grep
• perl
• xargs

File Modes and Permissions:

• ls -l
• chown, chgrp en chmod
• umask en umask -S

Manipulating Processes:

• ps
• echo $$
• pstree
• kill
• pgrep
• pkill -signal
• sleep
• suspend
• jobs
• % of fg en bg
• wait

TODO uitleg setuid, setgid, sticky bit

TODO vb'n van:

• declare -x, declare +x, echo ${}, declare -p, unset, export
• man -k (apropos), man -f (whatis)
• stat, comm, paste, join, split
• echo [-e] [-n]
• (s)locate
• find
• | xargs, -exec|ok, | bash
• filters
• kills, sleep, ..