os_cp:scripting
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os_cp:scripting [2024/03/25 21:20] jkonczak [Parameters and variables in shell] |
os_cp:scripting [2025/04/02 21:22] (current) jkonczak |
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| |5. sets up redirections|''echo'', ''a'', ''b'', ''c d 2+3=5''|…| | |5. sets up redirections|''echo'', ''a'', ''b'', ''c d 2+3=5''|…| | ||
| |6. executes the command||| | |6. executes the command||| | ||
| - | (cf. [[https://pubs.opengroup.org/onlinepubs/009604499/utilities/xcu_chap02.html#tag_02_01|POSIX documentation]]) | + | (cf. [[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_01|POSIX documentation]]) |
| ===== Splitting line into commands ===== | ===== Splitting line into commands ===== | ||
| Line 128: | Line 128: | ||
| ===== Shell expansions ===== | ===== Shell expansions ===== | ||
| - | The shell //[[https://pubs.opengroup.org/onlinepubs/009604499/utilities/xcu_chap02.html#tag_02_06|expands]]// | + | The shell //[[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_06|expands]]// |
| the following tokens: | the following tokens: | ||
| + | <html><div style="margin-top:-1.2em"></div></html> | ||
| - 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, | - 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 | - ''$//NAME//'' and ''${…}'' are substituted by a variable/parameter value | ||
| Line 140: | Line 141: | ||
| \\ | \\ | ||
| Inside single quotes no expansions are performed. | Inside single quotes no expansions are performed. | ||
| + | \\ | ||
| + | <small>The [[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_02_04|most recent release]] | ||
| + | of the POSIX standard also requires expanding **''%%$'…'%%''** by transforming | ||
| + | backslash-escaped sequences into characters they represent((This sanctions the | ||
| + | feature that has been there for a long time in most shells.)).</small> | ||
| **Importantly, the results of expansions 2-4 are split again into tokens whenever | **Importantly, the results of expansions 2-4 are split again into tokens whenever | ||
| Line 163: | Line 169: | ||
| \\ | \\ | ||
| <small>The C language functions ''[[https://en.cppreference.com/w/c/program/getenv|getenv]]'' | <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]]'' | + | and ''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/setenv.html|setenv]]'' |
| provide access to the environment variables.</small> | provide access to the environment variables.</small> | ||
| Line 169: | Line 175: | ||
| For the shell, the name | For the shell, the name | ||
| - | [[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05|parameter]] | + | [[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_05|parameter]] |
| denotes either script/function arguments or some special variables, while the name | 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]] | + | [[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_05_03|variable]] |
| denotes an ordinary shell variable (that is closely related to an environment variable). | denotes an ordinary shell variable (that is closely related to an environment variable). | ||
| \\ | \\ | ||
| Line 198: | Line 204: | ||
| <small> | <small> | ||
| Moreover the following expansions are performed: | Moreover the following expansions are performed: | ||
| + | <html><div style="margin-top:-1.2em"></div></html> | ||
| * ''${#X}'' → the length of the value (in characters) | * ''${#X}'' → the length of the value (in characters) | ||
| * ''${X:-expr}'' → value of ''X'' if ''X'' is set and nonempty, ''expr'' otherwise | * ''${X:-expr}'' → value of ''X'' if ''X'' is set and nonempty, ''expr'' otherwise | ||
| Line 332: | Line 339: | ||
| \\ | \\ | ||
| The shell understands assignments and operators similar to those in C (cf. | The shell understands assignments and operators similar to those in C (cf. | ||
| - | [[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html#tag_17_01_02_01|table of operators]]). | + | [[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap01.html#tag_18_01_02_01|table of operators]]). |
| \\ | \\ | ||
| Inside ''%%$((…))%%'' one can leave out the ''$'' preceding variable names, e.g., ''%%X=$((X+2))%%''. | Inside ''%%$((…))%%'' one can leave out the ''$'' preceding variable names, e.g., ''%%X=$((X+2))%%''. | ||
| Line 372: | Line 379: | ||
| ===== Aliases ===== | ===== Aliases ===== | ||
| - | [[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_03_01|Aliases]] | + | [[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_19_03_01|Aliases]] |
| are intended to provide user-defined alternative names for a command. | are intended to provide user-defined alternative names for a command. | ||
| \\ | \\ | ||
| Line 404: | Line 411: | ||
| ====== The read command ====== | ====== The read command ====== | ||
| - | The ''**[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/read.html|read]]**'' | + | The ''**[[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/read.html|read]]**'' |
| command reads a line from the standard input. | command reads a line from the standard input. | ||
| \\ | \\ | ||
| Line 507: | Line 514: | ||
| ==== if ==== | ==== if ==== | ||
| - | The ''[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_07|if]]'' statement has the following syntax: | + | The ''[[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_09_04_07|if]]'' statement has the following syntax: |
| <html><pre style="margin:0"> | <html><pre style="margin:0"> | ||
| <b>if</b> <i>command1</i> | <b>if</b> <i>command1</i> | ||
| Line 544: | Line 551: | ||
| ==== case ==== | ==== 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: | + | The ''[[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_09_04_05|case]]'' statement (in many other languages called the switch statement) has the following syntax: |
| <html><pre style="margin:0"> | <html><pre style="margin:0"> | ||
| <b>case</b> <i>value</i> <b>in</b> | <b>case</b> <i>value</i> <b>in</b> | ||
| Line 578: | Line 585: | ||
| ==== for ==== | ==== 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: | + | The ''[[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_09_04_03|for]]'' loop iterates over a set of values. It has the following syntax: |
| <html><pre style="margin:0"> | <html><pre style="margin:0"> | ||
| <b>for</b> <i>VAR</i> <b>in</b> <i>value1</i> <i>value2</i> <i>…</i> | <b>for</b> <i>VAR</i> <b>in</b> <i>value1</i> <i>value2</i> <i>…</i> | ||
| Line 615: | Line 622: | ||
| ==== while and until ==== | ==== 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"> | + | <html><p style="margin:0">The syntax of the <a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_09_04_09" class="urlextern" title="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_09_04_09" rel="nofollow">while</a> loop is as follows:</p><pre style="margin:0"> |
| <b>while</b> <i>command1</i> | <b>while</b> <i>command1</i> | ||
| <b>do</b> | <b>do</b> | ||
| Line 621: | Line 628: | ||
| <b>done</b> | <b>done</b> | ||
| </pre></html> | </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: | + | and the syntax of the ''[[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_09_04_11|until]]'' loop is as follows: |
| <html><pre style="margin:0"> | <html><pre style="margin:0"> | ||
| <b>until</b> <i>command1</i> | <b>until</b> <i>command1</i> | ||
| Line 646: | Line 653: | ||
| ====== Functions in shell ====== | ====== Functions in shell ====== | ||
| - | To define a [[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_05|function]] | + | To define a [[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_09_05|function]] |
| in the shell, one has to use the following syntax: ''//name//() //body//'', | 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 | where the //body// shall be a compound command such as a list of statements | ||
| Line 694: | Line 701: | ||
| ====== Signals in shell ====== | ====== Signals in shell ====== | ||
| - | The command ''[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_28|trap]] "//statements//" //CONDITION//'' | + | The command ''[[https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_29|trap]] "//statements//" //CONDITION//'' |
| is used to set up either handing a signal (when the //CONDITION// is a name of a | 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 | signal) or set the statements to be executed upon terminating the shell (when the | ||
os_cp/scripting.1711398055.txt.gz · Last modified: 2024/03/25 21:20 by jkonczak
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