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--- becki:linux:bash [2011-11-08 12:27]
becki
+++ becki:linux:bash [2016-03-03 11:25] (aktuell)
becki [File Inclusion, Command Execution]
@@ Zeile 1: / Zeile 1: @@
-====== Bash Programming ======
+====== Bash Programming ==
+===== Documentation ==
+  - [[kman>man1/bash.1|Bash man page]]
+  - [[http://tldp.org/LDP/abs/html/|Advanced Bash-Scripting Guide]]
 ===== File Inclusion, Command Execution ==
@@ Zeile 13: / Zeile 16: @@
 **command**
-when the shell encounters a command, it forks off a child process to actually execute the command (external commands only; from abs-guide) FIXME: Is this true? Isn't this ''%%command &%%''?
+when the shell encounters a command, it forks off a child process to actually execute the command (external commands only; from abs-guide). Parent script blocks until child process ends
+**command &**
+forks off a child process to execute the command and put child process into background so that the parent script continues immediately
 **eval command ...**
@@ Zeile 20: / Zeile 27: @@
 For me useful if command contains spaces witch are protected by single quotes, eg ''%%wget -S --header'Content-Type: application/json'%%''. Without eval, ''%%--header'Content-Type:%%'' and ''%%application/json'%%'' would be interpreted as 2 single arguments
+===== Subshells ==
+FIXME
+===== Coprocesses ==
+The following example starts a shell function as a coproc and uses the coproc to manipulate a string:
+<code bash>
+#!/bin/bash
+test_coproc() {
+    read rein
+    echo ">$rein<"
+}
+coproc test_coproc
+echo "Hallo" >&${COPROC[1]}
+read -u ${COPROC[0]} rein
+echo "$rein"
+#cat <&${COPROC[0]} # alternative
+</code>
+===== Expansion ==
+FIXME doc ''*'' and ''?''
+<code bash>
+echo {a,}quota.{group,user}.new
+# prints: aquota.group.new aquota.user.new quota.group.new quota.user.new
+# without any regard to the content of the working directory
+</code>
+===== Variables ==
+$meli is a shortcut for ${meli}
+==== Variable Variables ==
+<code bash>
+# get a value from a variable (meli) over a 'pointer' (who): (from abs-guide)
+meli=Melanie
+who=meli    # who is used as pointer to meli
+adr=${!who} # now $adr contains Melanie
+# set a value into a variable (meli) over a 'pointer' (who): (tried myself)
+meli=""
+who=meli
+eval $(echo $who)=Melanie # now $meli contains Melanie
+</code>
+==== Special Variables ==
+<code bash>
+$# # number of command line arguments
+$* # Arguments as one word
+$@ # Arguments as separate words
+$$ # Pid of script
+$? # return value of command (& function?)
+$! # PID of last job run in background
+$0 # full name of script
+$1 # first argument of script
+</code>
+Following a ''shift'' , the ''$@'' holds the remaining command-line parameters, lacking the previous ''$1'', which was lost(([[tldp>LDP/abs/html/othertypesv.html#EX19|Using shift]])).
+==== Default Values ==
+If variable is not set, expand to default value:
+<code bash>
+${variable:-defaultvalue}
+# Example:
+#Keep Value of VERSION or if not set yet, set it to 3.9:
+VERSION=${VERSION:-3.9}
+</code>
+[[tldp>LDP/abs/html/parameter-substitution.html|Source]]
 ===== Arrays ==
@@ Zeile 32: / Zeile 121: @@
 echo ${app[appix]} # XINE
 </code>
+===== Maps ==
+See http://stackoverflow.com/questions/1494178/how-to-define-hash-tables-in-bash
 ===== Loops ==
@@ Zeile 74: / Zeile 167: @@
 done
 </code>
-===== Variables ==
-$meli is a shortcut for ${meli}
-==== Variable Variables ==
-<code bash>
-# get a value from a variable (meli) over a 'pointer' (who): (from abs-guide)
-meli=Melanie
-who=meli    # who is used as pointer to meli
-adr=${!who} # now $adr contains Melanie
-# set a value into a variable (meli) over a 'pointer' (who): (tried myself)
-meli=""
-who=meli
-eval $(echo $who)=Melanie # now $meli contains Melanie
-</code>
-==== Special Variables ==
-<code bash>
-$# # number of command line arguments
-$* # Arguments as one word
-$@ # Arguments as separate words
-$$ # Pid of script
-$? # return value of command (& function?)
-$! # PID of last job run in background
-$0 # full name of script
-$1 # first argument of script
-</code>
-Following a ''shift'' , the ''$@'' holds the remaining command-line parameters, lacking the previous ''$1'', which was lost(([[tldp>LDP/abs/html/othertypesv.html#EX19|Using shift]])).
 ===== Quoting ==
@@ Zeile 127: / Zeile 187: @@
 ===== Functions ==
+==== Parameters ==
+FIXME
 <code bash>
-query() {
+myfunc() {
-    echo $*; echo $@; echo $1
+    echo $* #-> eins zwei drei
+    echo $@ #-> eins zwei drei
+    echo $1 #-> eins
 }
-query # call the function
+myfunc eins zwei drei # call the function
+</code>
+==== Exit Status and return Value ==
+The exit status may be explicitly specified by a return statement, otherwise it is the exit status of the last command in the function. Exit status can be checked with ''$?''.
+Here is a method to return a string along with the exit status:
+<code bash>
+myfunc() {
+    echo "Hello $1"
+}
+res=$(myfunc 'crazy world') # Oddly no quotes necessary in bash and ash
+echo ">>$res<<"
+</code>
+By using a separator (tab) and the cut command it is possible to return more than one value:
+<code bash>
+myfunc() {
+    echo -e "EINS\tZWEI"
+}
+buf=$(myfunc)
+eins=$(echo "$buf" | cut -f1)
+zwei=$(echo "$buf" | cut -f2)
+echo "$eins - $zwei" #-> EINS - ZWEI
 </code>
@@ Zeile 168: / Zeile 261: @@
 -s file True if file exists and has a size greater than zero
 -x file has execute permission (for the user running the test)
+-z string is null, that is, has zero length
+-n string is not null, that is, has not zero length
 # Example:
 if [ ! -r "$sourcefile" ]; then

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