Section 11.
Shell programming 2.
"There are
two different areas of functionality in shells. First is
interactive use and the second is scripting. Much of the
debate about shells has focused on interactive use only, For example, tcsh is an acceptable shell for interactive use but
practically unusable for scripting."
David Korn - 2001.
11. Shell programming 2.
11.1. Shell scripts.
A shell script is a series of
commands, lists or pipes executed in a shell
non-interactively. A shell script does not need to be saved
to a file. e.g.
sa101$ if [ -f /etc/shadow ];then echo "got it! Now what?";\
else echo "Search me!";fi
The above is a perfectly valid
shell script. Note that no file was required. We can
re-execute the script by using the shell history.
NB. the escape character
(\) is used to indicate that the line continues without a
line feed, you should not include it when typing this script
at the terminal.
11.2. Shell history.
The shell can keep a record of
your previous commands. There are various mechanisms for
accessing the shell history. Bash can use both the old csh !
history syntax, emacs or vi. The editor for shell history
can be set with the environment variable FC_EDIT.
If you know vi the easy
way to access history is to set the shell option
vi.
sa101$ set -o vi
Having set the option
<esc>k puts you into vi mode with the last
command issued, available as if at the bottom of a text
file. We can now use the usual vi commands to edit the line
or just hit return to re-execute it.
Hitting k will step us
back through the history (i.e. go up the history file),
j will step us back down the lines of the history
file.
11.3. Exercise
sa101$ mkdir test
sa101$ cd test
sa101$ touch file1
sa101$ set -o vi
sa101$ ^[k
sa101$ k
sa101$ f1r2
sa101$ ls -l
NB: ^[ is the notation
for a keyboard <esc> key.
If you are using vi mode
you can search the history for a string with the same
command that is used in vi and in less. i.e.
/<string>.
sa101$ history
...
468 vi train_news
469 sudo cp /var/tmp/t train_news
470 vi train_news
471 sudo bash
472 pwd
474 cd ~
475 pwd
476 cd clients
477 cd fulford/clients
478 ls
479 cd po*
480 cd sa101
481 make xhtml
482 sudo make xhtml
483 pwd
484 ls
485 d
486 cd /usr/local/web
487 cd cl
488 set -o vi
489 vi train_news
490 vi sa101_news
491 sudo bash
492 history
493 set -o vi
494 make xhtml
495 history
sa101$ set -o vi
sa101$ ^[K/web
sa101$ cd /usr/local/web
After issuing the
/<string> command it is possible to step back
through each instance of the string <string> by
hitting the next key (n).
11.4. History substitution.
A list of recent commands issued
can be obtained with the history command. This will
list the number of previous commands set by the environment
variable HISTSIZE. The default value for the
bash shell is 500. (This can be changed by users
setting a different value in their home directory
.profile.)
The commands are numbered and
can be re-executed by using the ! character, also
known as bang or shriek.
E.g.
sa101$ history 5
492 man history
493 history|wc
494 history|less
495 history -5
496 history 5
sa101$ !493
history|wc
497 1626 492910
This is known as
history substitution and can be incorporated
into shell scripts.
If you are going to use
history frequently, it is useful to use the
alias command to reduce the keystrokes required to
obtain the history list.
sa101$ alias h=history
sa101$ h 5
502 ls -l 1*
503 date
504 grep fulford /etc/passwd
505 df
506 h 5
11.5. Exercises.
Read the
man page for history.
Read the
man page for bash and find the references to
HISTSIZE, HISTFILE and
HISTFILESIZE.
Set the value
of HISTSIZE in $HOME/.profile.
Use the table
of commands and metanotation below to revise and consolidate
your learning so far.
11.6. Tools and metanotation:
