Jan Kończak

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====== Creating a shell script ======
===== (Non)interactive mode =====

The login shell in a terminal emulator runs in interactive mode.
\\
When the shell executable is invoked with a file as its argument, it runs in
non-interactive mode, and reads the commands from the file rather than the
standard input.
\\
Some diagnostics are not printed in the non-interactive mode.

~~Exercise.#~~
Edit a file //filename// so that in contains the text:
  date
  echo hello world
  date
Then, run ''/bin/bash //filename//''.

===== Shebang =====

[[https://en.wikipedia.org/wiki/Shebang_(Unix)|Shebang]] is the name for the
first line of a file whenever the line contains ''#!'' followed by a full path
to an executable (and optionally arguments).

A file that has execute permission and starts with a shebang is called a script.
When a script is executed, the executable indicated in the shebang is used to 
interpreted the file.

<small>For instance, when an executable file called //myFile// that starts with
the line ''#!/bin/prog_name myArg'' is executed by the standard POSIX
''execlp("./myFile", "./myFile", NULL);'' function, then the kernel,
after verifying the execute permissions and noticing the shebang, executes in
return ''execl("/bin/prog_name", "/bin/prog_name", "myArg", "./myFile", NULL);''.
</small>

**An executable file that starts with ''#!/bin/sh'' is a shell script.**\\
<small>(just like an executable file that starts with ''#!/usr/bin/python3'' is
a python script)</small>.

~~Exercise.#~~
Modify the file //filename// created for the last exercise so that it starts with 
''#!/bin/sh'' shebang.
Then, grant execute permission to the file (e.g., with ''chmod +x //filename//''). 
Finally, run the file with ''./filename''.

===== The . command =====

The shell has a ''[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_18|.]]'' command (the command is literally a single dot).
\\
The ''. //filename//'' executes the commands from the file //filename// in the
current shell (as if the contents of the file were typed to the standard input).
The file does not need to be executable.

Notice that ''./file'' (and ''sh file'') interprets the file in a new shell and
''. ./file'' (and ''. file'' if PATH contains a ''.'') executes the commands
from the file in the current shell.

====== Processing an input line by the shell ======

The shell, in that order:
|1. reads a line of input | ''echo ${Y} "$Z 2+3=$%%((2+3))%%" > file;  l 'baz bar'%%%%'' ||
|2. splits the line into tokens | ''echo'', ''${Y}'', ''"$Z 2+3=$%%((2+3))%%"'', ''>'', ''file'', '';'', ''l'' , ''%%'baz bar'%%'' ||
|3. parses tokens into commands |''echo'', ''${Y}'', ''"$Z 2+3=$%%((2+3))%%"'', ''>'', ''file''  | ''l'', ''%%'baz bar'%%'' |
|4. resolves aliases | ''echo'', ''${Y}'', ''"$Z 2+3=$%%((2+3))%%"'', ''>'', ''file'' |  ''ls'',  ''-alF'', ''--color=auto'', ''%%'baz bar'%%''|
|5. expands certain tokens | ''echo'', ''a'', ''b'',    ''c d 2+3=5'',                ''>'', ''file''| ''ls'',  ''-alF'', ''--color=auto'', ''baz bar'' |
|5. sets up redirections|''echo'', ''a'', ''b'',    ''c d 2+3=5''|…|
|6. executes the command|||
(cf. [[https://pubs.opengroup.org/onlinepubs/009604499/utilities/xcu_chap02.html#tag_02_01|POSIX documentation]])

===== Splitting line into commands =====

Commands can be separated by:
  * ''**;**'' – ''cmd ; …'' fully executes ''cmd'' and only then goes to the next command,
  * ''**&**'' – ''cmd & …'' starts ''cmd'' in background and immediately goes to the next command.
Each process returns an exit value. Value of 0 indicates success, non-zero value
indicates abnormal termination((Confer with ''int main(){…; return 0;}'' and the ''exit()'' function)). \\
Commands can be joined with logical operators that depending on the return values:
    * ''**&&**'' – ''a && b'' runs ''a'' and if ''a'' succeeds, ''b'' is run;  ''a && b'' returns success iff both ''a'' and ''b'' succeed,
    * ''**||**'' – ''a || b'' runs ''a'' and if ''a'' fails,    ''b'' is run;           ''a || b'' returns failure iff both ''a'' and ''b'' fail.

Commands can be grouped by:
  * ''**{** … **}**'' – groups commands to be executed in this shell, \\ //Warning//: commands in ''{ … }'' must be terminated by a '';'' or a ''&'' \\ //Warning//: a space must follow the ''{'' and precede the ''}''.
  * ''**(**…**)**'' – runs a new shell (subshell) and executes the commands in the subshell
To understand the difference, consider the following commands executed from the ''/home/student'' directory:
  * ''{ cd /tmp; pwd; }; pwd'' – enters ''/tmp'' and outputs ''/tmp'' twice,
  * ''(cd /tmp; pwd); pwd'' – runs a subshell, enters ''/tmp'' and outputs ''/tmp'' in the subshell, the subshell terminates, the shell outputs ''/home/student''.

~~Exercise.#~~
Run the commands ''pwd'' and ''ls'' from the same line.

~~Exercise.#~~
Write a line that will run the command ''cd Desktop'' and if it succeeds,
will run a ''ls'' command.

===== Job control =====

When a ''&'' terminates a command, then the process is run in background.

At a time, at most one process can run in foreground, but any number of
processes may run in background or be suspended.
\\
One may suspend a foreground process by pressing ''Ctrl+z''. 

The list of processes controlled by the shell, that includes the background
processes and suspended processes, can be printed by the ''**jobs**'' command.

To move a background or suspended process to the foreground, one has to issue
the ''**fg**'' command. To continue a suspended process in the background, one
can issue the ''**bg**'' command.
\\
With no arguments, the ''fg'' and ''bg'' target the most recently suspended
process, and if there are no suspended process, then the most recently
backgrounded process.

<small> The ''fg'', ''bg'' and ''kill'' shell commands accept a job specifiers
to select a particular process by its number, name, …. </small>

~~Exercise.#~~
Run a command ''ping put.poznan.pl''. Suspend it with ''Ctrl+z''. Inspect the
jobs table with ''jobs''. Put the command back in foreground and terminate it
with ''Ctrl+c''.

~~Exercise.#~~
Run a command ''ping put.poznan.pl &''. Inspect the jobs table with ''jobs''.
Put the command back in foreground and suspend it. Then, issue a command that
will continue it in background.

===== Shell expansions =====

The shell //[[https://pubs.opengroup.org/onlinepubs/009604499/utilities/xcu_chap02.html#tag_02_06|expands]]//
the following tokens:
  - the tilde in ''~'', ''~/…'' (and ''…:~'' / ''…:~/…'' in variable assignment) is expanded to home directory of the current user, \\ the tilde followed by a username (in contexts as above), e.g., ''~user'', is expanded to home directory of the user,
  - ''$//NAME//'' and ''${…}'' are substituted by a variable/parameter value 
  - ''$%%((…))%%'' is substituted by the result of arithmetic calculations((POSIX requires only integer arithmetic)),
  - ''$(…)'' and ''`…`'' are substituted by the standard output of given commands
  - patterns (text including ''*'', ''?'' and ''[…]'') are expanded to filenames wherever possible


Inside double quotes only expansions 2-4 are performed.
\\
Inside single quotes no expansions are performed.

**Importantly, the results of expansions 2-4 are split again into tokens whenever
the expansions are not double-quoted.**

===== Parameters / variables =====

==== Environment variables – overview ====

[[https://en.wikipedia.org/wiki/Environment_variable|Environment variables]]
are key-value pairs associated with a process.
Their names must not contain the equals sign (''='').
\\
The usual naming convention is that the names start with a capital letter and
contain only capital letters, numbers and underscore.
\\
In the shell, the characters ''a-zA-Z0-9_'' are valid in variable names, and the
variables must not start with a number.

**Each process has a separate set of environment variables.**
Upon ''fork'', the environment variables are copied. Upon ''exec…'',
the variables are either retained or replaced with a given set (cf. ''execv'' vs ''execve'').
\\
<small>The C language functions ''[[https://en.cppreference.com/w/c/program/getenv|getenv]]''
and ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/setenv.html|setenv]]''
provide access to the environment variables.</small>

==== Parameters and variables in shell ====

For the shell, the name
[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05|parameter]]
denotes either script/function arguments or some special variables, while the name
[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_03|variable]]
denotes an ordinary shell variable (that is closely related to an environment variable).
\\
**The shell variables marked as //exported// are passed to child processes as
their environment variables**. One can export a variable called //NAME// by
the means of ''**export** //NAME//'' command.

To set a variable, one has to write:
**''//NAME//=//value//''** 
\\
<small>
//Warning//: ''NAME =value''  attempts to run the program ''NAME'' with argument ''=value''. \\
//Warning//: ''NAME = value'' attempts to run the program ''NAME'' with arguments ''='' and ''value''. \\
//Warning//: ''NAME= value''  attempts to run the program ''value'' with environment variable ''NAME'' set to empty value.

One may unset the variable with ''unset //NAME//''.
</small>

The list of all variables can be obtained by the ''set'' command (with no arguments),
and the list of all exported variables can be obtained by the ''env'' command
(with no arguments).

**The shell substitutes ''$//X//'' and ''${//X//}'' with the value of the
variable //X//.**
\\
<small>
Moreover the following expansions are performed:
  * ''${#X}'' → the length of the value (in characters)
  * ''${X:-expr}'' → value of ''X'' if ''X'' is set and nonempty, ''expr'' otherwise
  * ''${X:+expr}'' → expr if ''X'' is set and nonempty, empty value otherwise
  * ''${X:number}'' → value of ''X'' with first ''number'' of characters skipped
  * ''${X::number}'' → the first ''number'' of characters of ''X'''s value 
  * ''${X:skip:length}'' → the first ''length'' characters starting from ''skip+1'' of the ''X'''s value
  * ''${X%pattern}'' → ''X'''s value with the shortest match of the ''pattern'' removed from the beginning \\ ''${X<nowiki>%%</nowiki>pattern}''  → ''X'''s value with the longest match of the ''pattern'' removed from the beginning
  * ''${X#pattern}'' → ''X'''s value with the shortest match of the ''pattern'' removed from the end \\ ''${X##pattern}'' → ''X'''s value with the longest match of the ''pattern'' removed from the end
</small>

~~Exercise.#~~
Print the value of a variable ''//VAR//'' using the ''echo'' command.
Then, set the variable ''//VAR//'' to //text//.
\\
Finally, print the value of the variable ''//VAR//'' again.

~~Exercise.#~~
Assign the value of ''//VAR//'' to a variable ''//OTHER//'' and run ''echo''
that outputs the value of the ''//OTHER//'' variable.

~~Exercise.#~~
Assign the value ''5'' to a variable named ''SIZE''. \\ Then, construct
a command that uses the variable ''SIZE'' to print ''The file size is //5//MB'' 

<small>
~~Exercise.#~~
Set the variable ''PROG'' to ''ls'' and the variable ''ARG'' to ''/tmp''.
Then run in the shell ''$PROG $ARG''.
</small>

=== Positional and special parameters ===

Arguments of a script or a function are referred to by ''**${1}**'', ''**${2}**'',
''**${3}**'', …. \\
The first nine arguments can also be referred to by  ''$1'', ''$2'', ''$3'', …, ''$9''.
Moreover, the following substitutions are performed:
| **''$#''** | the number of arguments |
| ''$*'' \\ ''$@'' | all arguments split into single words |
| ''"$*"'' | all arguments as a single token |
| **''"$@"''** | all arguments, each being a separate token |

++++ Examples |
<html><pre style="line-height:1em">
~ $ bash -s &quot;the_first_argument&quot; &quot;the second argument&quot;
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> echo $#
2
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> ls $*
ls: cannot access &apos;the_first_argument&apos;: No such file or directory
ls: cannot access &apos;the&apos;: No such file or directory
ls: cannot access &apos;second&apos;: No such file or directory
ls: cannot access &apos;argument&apos;: No such file or directory
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> ls $&#64;
ls: cannot access &apos;the_first_argument&apos;: No such file or directory
ls: cannot access &apos;the&apos;: No such file or directory
ls: cannot access &apos;second&apos;: No such file or directory
ls: cannot access &apos;argument&apos;: No such file or directory
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> ls &quot;$*&quot;
ls: cannot access &apos;the_first_argument the second argument&apos;: No such file or directory
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> ls &quot;$&#64;&quot;
ls: cannot access &apos;the_first_argument&apos;: No such file or directory
ls: cannot access &apos;the second argument&apos;: No such file or directory
</pre></html>
++++

Other special parameters include:
| **''$0''** | within script, ''$0'' holds the name of the script |
| **''$$''** | process identifier of the shell |
| **''$?''** | exit status of the last command |
| **''$!''** | process identifier of the last backgrounded process |

++++ Examples |
<html><pre style="line-height:1em">
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> sleep 1m &amp;
[1] 21738
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> sleep 1s
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> echo $!
21738
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> echo $?
0
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> false
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> echo $?
1
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> python &lt;&lt;&lt; &apos;exit(123)&apos;
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> echo $?
123
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> echo $$
21731
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> ps
  PID TTY          TIME CMD
21731 pts/9    00:00:00 bash
21738 pts/9    00:00:00 sleep
21742 pts/9    00:00:00 ps
</pre></html>
++++

~~Exercise.#~~
Write a shell script that prints its first and third argument.

~~Exercise.#~~
Write a shell script that prints its name and the number of its arguments.


=== Selected standard variables ===

|''PATH''|Colon-separated list of directories where binaries are looked up \\ Notice: it's the ''exec…p'' system call that looks up an executable files with a matching name in these paths |
|''HOME''|Path to the home directory of the current user|
|''PS1'' \\ ''PS2''|The main [[https://en.wikipedia.org/wiki/Command-line_interface#Command_prompt|prompt]] \\ The prompt for next line of multi-line commands|
|''PWD''|Current working directory|
|''EDITOR''|Default text editor|
|''LOGNAME'' / ''USER'' \\ ''UID''|Name of the current user\\ (Numeric) user identifier of the current user|
|''LANG''|Language, region and character encoding; for instance ''de_CH.UTF-8'' means german, Switzerland and  UTF-8 encoding|
|''RANDOM''|A random number (generated upon each access)|

~~Exercise.#~~
Set the variable ''LANG'' to the value ''ja_JP.UTF-8'' and run the program ''date''.
\\
Then, change the value of ''LANG'' to ''de_DE.UTF-8'' and run the command ''rm -rf /root/.ssh/nope''.

~~Exercise.#~~
Display the value of the ''PS1'' variable. Then, change it.

<small>
~~Exercise.#~~
Try to run the ''lspci'' command. Then modify the ''PATH'' variable so that it
additionally contains the paths ''/sbin'' and ''/usr/sbin''. Retry the ''lspci''
command.
</small>

===== Arithmetic expansion =====

The shell can do simple integer maths – it substitutes ''**$%%((%%**…**))**''
with the result of an expression contained in the parentheses.
\\
The shell understands assignments and operators similar to those in C (cf.
[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html#tag_17_01_02_01|table of operators]]).
\\
Inside ''%%$((…))%%'' one can leave out the ''$'' preceding variable names, e.g., ''%%X=$((X+2))%%''.

~~Exercise.#~~ Set the value of ''X'' and ''Y'' to some two digit numbers, and then:
\\       • increment ''X'' by 1,
\\       • increment ''Y'' by 2,
\\       • set ''Z'' to the result of ''X'' multiplied by ''Y'',
\\       • calculate the reminded of dividing ''Z'' by 128.

===== Command substitution =====

The shell substitutes for ''**$(**//command//**)**'' (and ''`//command//`'') the
standard output of the //command//.
\\
Notice that first the commands contained in ''$(…)'' execute, then the shell
replaces ''$(…)'' with the standard output (and if the ''$(…)'' was unquoted,
the result are split into tokens), and only afterwards the line containing
''$(…)'' is run.

~~Exercise.#~~
Write a command that will print the text ''In the current directory there are N files'',
with N being replaced by the real count of the files in the current directory.

~~Exercise.#~~
The command ''date +%H_%M_%S'' outputs the current time.
Write a command that redirects the output of ''pstree -au'' to a file
''processes_TIME.log'' with the TIME substituted by the current time.

<small>
~~Exercise.#~~
Write one line that will:
\\       • assign the current time in nanoseconds (the output of ''date +%s%N'') to a variable ''START'',
\\       • run the command ''sleep 1s'',
\\       • assign the current time in nanoseconds to a variable ''END'',
\\       • output the time elapsed between ''START'' and ''END''
</small>

===== Aliases =====

[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_03_01|Aliases]]
are intended to provide user-defined alternative names for a command.
\\
Once a line is split into commands by the shell (but before other expansions are
done), the shell checks whether a word that is present where a command name is
expected matches any alias, and if so, the word is replaced by the value of the
alias.

<small>
For instance: some Linux distribution in default configuration files set up the
alias ''la'' to ''ls -la'' and the alias ''ls'' to ''ls --color=auto''.
\\
Typing ''la'' first turns that into ''ls''  ''-la'', and then ''ls'' is turned
into ''ls''  ''--color=auto'', yielding total of ''ls''  ''--color=auto''  ''-la''
</small>

To create a new alias, one should issue:
\\
''**alias** //word//=//value//''
\\
For instance: ''alias la="ls -la"''\\
<small>An alias can be removed by ''unalias //word//''.

The ''alias'' command with no arguments lists defined aliases.</small>

Aliases, just like variables, are set only for the current shell process.

~~Exercise.#~~ Create an alias called ''year'' that will translate to ''cal -my''.
Use the alias. Use the alias to display the calendar for 2025.

====== The read command ======

The ''**[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/read.html|read]]**''
command reads a line from the standard input.
\\
''read'' requires as arguments a list of variable names, e.g., ''read LINE'' or 
''read A B C''
\\
Once a line is read, it is split into words, and subsequent words are assigned
to subsequent variables in the list.
\\
If there are less words in the input line than the variables in the list, then
the remaining variables are set to an empty value.
\\
If there are more words in the input line than the variables in the list, then
the last variable in the list gets the extra text.

++++ Example: |
<html><pre style="line-height:1em">
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> read X Y Z
   one   &quot;two   three&quot;
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> echo &quot;X=&apos;$X&apos; Y=&apos;$Y&apos; Z=&apos;$Z&apos;&quot;
X=&apos;one&apos; Y=&apos;&quot;two&apos; Z=&apos;three&quot;&apos;
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> read X Y Z
   one
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> echo &quot;X=&apos;$X&apos; Y=&apos;$Y&apos; Z=&apos;$Z&apos;&quot;
X=&apos;one&apos; Y=&apos;&apos; Z=&apos;&apos;
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> read X Y Z
   one      two  three  four     five
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> echo &quot;X=&apos;$X&apos; Y=&apos;$Y&apos; Z=&apos;$Z&apos;&quot;
X=&apos;one&apos; Y=&apos;two&apos; Z=&apos;three  four     five&apos;
</pre></html>
++++

~~Exercise.#~~
Print the text ''What is your name: '', then read the name, and print the text
''Hello //name//!''.

~~Exercise.#~~
Read two numbers at a time and then output their sum.

====== The test command ======

The **''test''** command allows evaluating logical tests that involve strings,
numbers and files.
\\
Apart from the name ''test'', the command can also be invoked by the name **''[''**.
\\
There is a slight difference between ''test'' and **''[''**: the latter needs
an extra '']'' argument as the last argument.
\\
E.g., ''test "foo" != "foo"'' is identical to ''[ "foo" != "foo" ]''.

=== String-related tests ===

^  Expression  ^  True when  ^
|''[ "$X" ]'' \\ ''[ -n "$X" ]''| ''"$X"'' nas nonzero length |
|''[ -z "$X" ]''| ''"$X"'' has __z__ero length |
|''[ "$X" = "$Y" ]''| The strings are the same |
|''[ "$X" != "$Y" ]''| The strings differ |

Warning: the expression <del>''[ $X = $Y ]''</del> is going to work if ''$X''
and ''$Y'' expand to single words, but if either of the variables is empty or
contains a space, then a syntax error will be raised.

=== Numeric tests ===

^  Expression  ^  True when  ^
|''[ "$X" -eq "$Y" ]''|The number  ''"$X"'' is __eq__ual to ''"$Y"''|
|''[ "$X" -ne "$Y" ]''|The number  ''"$X"'' is  __n__ot __e__qual to ''"$Y"''|
|''[ "$X" -lt "$Y" ]''|The number  ''"$X"'' is __l__esser __t__han ''"$Y"''|
|''[ "$X" -le "$Y" ]''|The number  ''"$X"'' is __l__esser than or __e__qual to ''"$Y"''|
|''[ "$X" -gt "$Y" ]''|The number  ''"$X"'' is __g__reter __t__han  ''"$Y"''|
|''[ "$X" -ge "$Y" ]''|The number  ''"$X"'' is __g__reter than or __e__qual to ''"$Y"''|

=== File tests (a choice of) ===

^  Expression  ^  True when the file ''"$X"'' exists and…  ^
|''[ -e "$X" ]''| (just exists) |
|''[ -s "$X" ]''| is nonempty |
|''[ -f "$X" ]'' \\ ''[ -d "$X" ]'' \\ … | is an ordinary file \\ is a directory \\ … |
|''[ -r "$X" ]'' \\ ''[ -w "$X" ]'' \\ ''[ -x "$X" ]''|the current user can read the file \\ the current user can write to the file \\ the current user can execute the file|

=== Negating and grouping tests ===

''**!** //arg//'' negates //arg//.
\\
''//arg1// **-a** //arg2//'' stands for //arg1// __a__nd //arg2//
\\
''//arg1// **-o** //arg2//'' stands for //arg1// __o__r //arg2//
\\
Parentheses ''( … )'' group the expressions, but one has to escape them (else
the shell sees them as shell special characters).

~~Exercise.#~~
Using the ''&&'' and ''||'' syntax to group commands:
\\       • test whether a directory ''//dirname//'' exists, and if it does exist, enter it,
\\       • test whether the variable ''X'' is larger than the variable ''Y'', and if the test fails, print ''Y≥X''.

====== Control flow statements ======

===== Conditional constructs =====

==== if ====

The ''[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_07|if]]'' statement has the following syntax:
<html><pre style="margin:0">
<b>if</b> <i>command1</i>
<b>then</b>
    statements executed when <i>command1</i> returned the exit value indicating success
<b>elif</b> <i>command2</i>
<b>then</b>
    statements executed when <i>command2</i> returned the exit value indicating success
<b>else</b>
    statements executed when neither <i>command1</i> nor <i>command2</i> returned the exit value indicating success
<b>fi</b>
</pre></html>
The ''elif'' and ''else'' branches are optional, and ''elif'' can appear multiple
times.

It is worth underlining that the ''if'' and ''elif'' require a command and check
the exit value **returned** by the command, and disregard the data written to the
standard output/error streams by the command.

A one-liner example:
<code bash>
if grep -q ^user: /etc/passwd; then echo "'user' present"; elif grep -q ^student: /etc/passwd; then echo "'user' absent, 'student' present"; else echo "neither present"; fi
</code>

~~Exercise.#~~
Write an ''if'' statement that tries to remove a file //filename// and upon
success outputs ''Removed the file'' while upon failure it outputs
''Failed to remove the file''.

~~Exercise.#~~
Create a script that verifies if it was run with exactly two arguments and
if the first argument is lesser then or equal to the second argument.
If any of the conditions does not hold, print a corresponding text and exit the
script.

==== case ====

The ''[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_05|case]]'' statement (in many other languages called the switch statement) has the following syntax:
<html><pre style="margin:0">
<b>case</b> <i>value</i> <b>in</b>
  <i>pattern1</i><b>)</b> <i>statements1</i> <b>;;</b>
  <i>pattern2</i><b>)</b> <i>statements2</i> <b>;;</b>
  <i>*</i><b>)</b> <i>statements3</i> <b>;;</b>
<b>esac</b>
</pre></html>
The //value// is consecutively matched against //pattern1//, //pattern2//, … and
upon first match the corresponding //statements// are executed. Subsequent
patterns are not taken under account.
\\
Notice the double semicolon '';;'' terminating statements corresponding to a pattern.
\\
There is no special syntax for the default match – simply a pattern of ''*'' is used.

A one-liner example:
<code bash>
case $(LANG= date "+%A") in Monday) echo "Ugh...";; S*day) tput bel; echo "Weekend!";; *) echo "a day.";; esac
</code>

~~Exercise.#~~
Write a script that accepts a filename as an argument, and if the file has an extension:
\\       • ''pdf'', then run the ''pdftotext //filename// -'' command,
\\       • ''zip'', then run the ''unzip -l //filename//'' command,
\\       • any other extension, then run the ''cat //filename//'' command.

===== Loops =====

In the loops one can use the **''break''** and **''continue''** keywords to,
respectively, leave the loop and start the next loop iteration.

==== for ====

The ''[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_03|for]]'' loop iterates over a set of values. It has the following syntax:
<html><pre style="margin:0">
<b>for</b> <i>VAR</i> <b>in</b> <i>value1</i> <i>value2</i> <i>…</i>  
<b>do</b>
   <i>statements</i>
<b>done</b>
</pre></html>
In subsequent iterations, subsequent //value//s from the list are assigned to
the //VAR// variable. Once the loop finishes, the //VAR// contains the last
value assigned in the loop.
\\
<small>When the ''in //value1// //value2// …'' part is omitted, the ''for''
iterates over the positional parameters.</small>

Usually the list of values is //not// hand-written, but is a result of an
expansion, for instance: \\
''for NUM in $(seq 7)'' → ''for NUM in 1 2 3 4 5 6 7'' \\
''for IMG in *.jpg'' → ''for IMG in alice.jpg bob.jpg eve.jpg'' \\
''for USERNAME in $(getent passwd | cut -f1 -d:)'' → ''for USERNAME in root student ...'' \\
''for FILE in $(grep -il 'ldap' /etc/*.conf 2>/dev/null)'' → ''for FILE in /etc/autofs.conf /etc/ldap.conf ...''((Warning: this example is not whitespace-proof.))

A one-liner example:
<code bash>
for X in `seq 3`; do echo -n "Iteration $X: "; date +%N; done
</code>

~~Exercise.#~~
Write a script that accepts a list of files as the arguments, and for each file
prints its name, its first line and its last line.

~~Exercise.#~~
Write a loop that outputs all powers of two up to a given exponent.
\\
<small>(To calculate the power, you can use the expression ''$%%((2**exponent))%%''.) </small>

==== while and until ====

<html><p style="margin:0">The syntax of the <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_09" class="urlextern" title="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_09" rel="nofollow">while</a> loop is as follows:</p><pre style="margin:0">
<b>while</b> <i>command1</i>  
<b>do</b>
   <i>statements2</i>
<b>done</b>
</pre></html>
and the syntax of the ''[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_11|until]]'' loop is as follows:
<html><pre style="margin:0">
<b>until</b> <i>command1</i>  
<b>do</b>
   <i>statements2</i>
<b>done</b>
</pre></html>
The ''while'' executes //''statements2''// as long as the return value of //''command1''// indicates success. \\ 
The ''until'' executes //''statements2''// as long as the return value of //''command1''// indicates failure.

A one-liner example:
<code bash>
X=7; until [ $X -eq 1 ]; do [ $((X%2)) -eq 1 ] && X=$((3*X+1)) || X=$((X/2)); echo $X; done
</code>

~~Exercise.#~~
Write a loop that outputs all powers of two up to a given number.

~~Exercise.#~~
Write a script that will create a file ''myProg_1.log'' if it does not exist,
or if such file exists, a file ''myProg_2.log'', or if such file also exists, a
file ''myProg_3.log'', and so on. Put in the newly created file the current date.

====== Functions in shell ======

To define a [[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_05|function]]
in the shell, one has to use the following syntax: ''//name//() //body//'',
where the //body// shall be a compound command such as a list of statements 
contained within a curly braces (e.g., ''mkcd() { mkdir -p $1 && cd $1; }'').
\\
To call a function, one has just to issue ''//name// [//argument//]...'' (e.g.,
''mkcd /tmp/dir''). \\
Inside a function the variables ''$#'' and ''$1'', ''$2'', … refer to the
//argument//s.
\\
The command ''return [//n//]'' issued inside a function terminates the function
returning ''n'' (when specified) or the return value of the last executed command.

<html><div style="line-height:1em"><p>Consider the following example:</p><pre style="margin: 0 0 -1em 0"><span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> colorEcho() {
&gt; case &quot;$1&quot; in
&gt;   red) CCODE=31;;
&gt;   green) CCODE=32;;
&gt;   blue) CCODE=34;;
&gt;   *) CCODE=5;;
&gt; esac
&gt; echo -ne &quot;\033[${CCODE}m&quot;;
&gt; echo -n &quot;$2&quot;;
&gt; echo -e &quot;\033[0m&quot;;
&gt; }
<span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> colorEcho red riding hood
<span style="color:#cd0000;">riding
</span><span style="font-weight:bold;color:#00ff00;">user&#64;host</span><span style="font-weight:bold;color:#5c5cff;"> ~ $</span> colorEcho green &quot;anne of green gables&quot;
<span style="color:#00cd00;">anne of green gables</span>
</pre>
</div>
</html>

~~Exercise.#~~
Write a function that displays ''Hello world!''

~~Exercise.#~~
Write a function that takes two arguments: a number and a text. The function
shall output the text the number of times.

<small>
~~Exercise.#~~
Write a function that resembles the ''tree'' program. It should traverse the
filesystem depth first and output for each file its name with appropriate
indentation.
</small>

====== Signals in shell ======

The command ''[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_28|trap]] "//statements//" //CONDITION//''
is used to set up either handing a signal (when the //CONDITION// is a name of a
signal) or set the statements to be executed upon terminating the shell (when the
//CONDITION// is ''EXIT'').
\\
**Shell runs the commands associated with a signal once the current foreground
command terminates or suspends.**
\\
Notice that the commands to be executed upon a signal / upon exit must be
provided as text.
\\
The ''trap - //CONDITION//'' syntax restores the default handler.

~~Exercise.#~~
Handle the INT signal in the current shell by running the ''fortune'' command.
Trigger a SIGINT and see what happens.

~~Exercise.#~~
Create a script with the following contents:
<html><div style="margin-top:-1.4em;margin-bottom:-1.4em"></html>
<small><code bash>
TEMPFILE=$(mktemp)
echo "Created a temporary file $TEMPFILE"
date > "$TEMPFILE"
sleep 12s
date >> "$TEMPFILE"
echo "Cleaning up the temporary file $TEMPFILE"
rm "$TEMPFILE"
</code></small>
<html></div></html>
Run the script and terminate it with ''Ctrl+c'', or ''Ctrl+\'', or ''pkill -f //scriptname//''.\\
Then modify the script so that it cleans up the file regardless of how it was terminated.

====== .bashrc ======

<small>
Most of the shells read certain configuration files upon start.
There is no standard name for a user shell configuration file.
</small>

For all interactive shells, __the Bash shell__ executes the commands from ''~/.bashrc'' file
upon start.
\\
Therefore, a Bash user that wants to permanently define aliases, functions,
set environment variables etc., shall add appropriate commands to the
''~/.bashrc'' file.
\\
The commands in ''.bashrc'' must output no text to standard streams when the
shell starts in non-interactive mode.

~~META:
language = en
~~
Jan Kończak

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