Diff for "BashFAQ/013"

Differences between revisions 4 and 10 (spanning 6 versions)

How can I concatenate two variables? How do I append a string to a variable?

There is no (explicit) concatenation operator for strings (either literal or variable dereferences) in the shell; you just write them adjacent to each other:

   1 var=$var1$var2

If the right-hand side contains whitespace characters, it needs to be quoted:

   1 var="$var1 - $var2"

If you're appending a string that doesn't "look like" part of a variable name, you just smoosh it all together:

   1 var=$var1/.-

Otherwise, braces or quotes may be used to disambiguate the right-hand side:

   1 var=${var1}xyzzy
   2 # Without braces, var1xyzzy would be interpreted as a variable name
   3 
   4 var="$var1"xyzzy
   5 # Alternative syntax

CommandSubstitution can be used as well. The following line creates a log file name logname containing the current date, resulting in names like e.g. log.2004-07-26:

   1 logname="log.$(date +%Y-%m-%d)"

There's no difference when the variable name is reused, either. A variable's value (the string it holds) may be reassigned at will:

   1 string="$string more data here"

Concatenating arrays is also possible:

   1 var=( "${arr1[@]}" "${arr2[@]}" )

Bash 3.1 has a new += operator that you may see from time to time:

   1 string+=" more data here"     # EXTREMELY non-portable!

It's generally best to use the portable syntax.

CategoryShell

-  ⇤ ← Revision 4 as of 2007-11-07 07:28:17 → 
  Size: 1486
  Editor: c-68-56-54-232
  Comment:
+   ← Revision 10 as of 2018-07-06 10:01:46 → ⇥
  Size: 1604
  Editor: 83
  Comment: Added howto concatenate arrays
-Deletions are marked like this.
+Additions are marked like this.
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-[[Anchor(faq13)]]
+<<Anchor(faq13)>>
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+There is no (explicit) concatenation operator for strings (either literal or variable dereferences) in the shell; you just write them adjacent to each other:
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+Line 5:
-There is no (explicit) concatenation operator for strings (either literal or variable dereferences) in the shell, *BUT* one can concat the values of variables by sequentially naming eaching variable to be joined together. The variables whose values are to be concatenated are written one after the other:

{{{
    var=$var1$var2
+{{{#!highlight bash
var=$var1$var2
-Line 13:
+Line 11:
-{{{
    var="$var1 - $var2"
+{{{#!highlight bash
var="$var1 - $var2"
-Line 19:
+Line 17:
-{{{
    var=$var1/.-
+{{{#!highlight bash
var=$var1/.-
-Line 25:
+Line 23:
-{{{
    var=${var1}xyzzy
    # Without braces, var1xyzzy would be interpreted as a variable name
+{{{#!highlight bash
var=${var1}xyzzy
# Without braces, var1xyzzy would be interpreted as a variable name
-Line 29:
+Line 27:
-    var="$var1"xyzzy
    # Alternative syntax
+var="$var1"xyzzy
# Alternative syntax
-Line 35:
+Line 33:
-{{{
    logname="log.$(date +%Y-%m-%d)"
+{{{#!highlight bash
logname="log.$(date +%Y-%m-%d)"
-Line 39:
+Line 37:
-There's no difference when the variable name is reused, either:
+There's no difference when the variable name is reused, either.  A variable's value (the string it holds) may be reassigned at will:
-Line 41:
+Line 39:
-{{{
    string="$string more data here"
+{{{#!highlight bash
string="$string more data here"
}}}

Concatenating arrays is also possible:

{{{#!highlight bash
var=( "${arr1[@]}" "${arr2[@]}" )
-Line 47:
+Line 51:
-{{{
    string+=" more data here"     # EXTREMELY non-portable!
+{{{#!highlight bash
string+=" more data here"     # EXTREMELY non-portable!
-Line 52:
+Line 56:
+----
CategoryShell