| Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
|---|---|---|
| Prev | Chapter 10. Manipulating Variables | Next |
Manipulating and/or expanding variables
Same as $parameter, i.e., value of the variable parameter. In certain contexts, only the less ambiguous ${parameter} form works.
May be used for concatenating variables with strings.
1 your_id=${USER}-on-${HOSTNAME}
2 echo "$your_id"
3 #
4 echo "Old \$PATH = $PATH"
5 PATH=${PATH}:/opt/bin # Add /opt/bin to $PATH for duration of script.
6 echo "New \$PATH = $PATH" |
If parameter not set, use default.
1 var1=1
2 var2=2
3 # var3 is unset.
4
5 echo ${var1-$var2} # 1
6 echo ${var3-$var2} # 2
7 # ^ Note the $ prefix.
8
9
10
11 echo ${username-`whoami`}
12 # Echoes the result of `whoami`, if variable $username is still unset. |
![]() | ${parameter-default} and ${parameter:-default} are almost equivalent. The extra : makes a difference only when parameter has been declared, but is null. |
1 #!/bin/bash
2 # param-sub.sh
3
4 # Whether a variable has been declared
5 #+ affects triggering of the default option
6 #+ even if the variable is null.
7
8 username0=
9 echo "username0 has been declared, but is set to null."
10 echo "username0 = ${username0-`whoami`}"
11 # Will not echo.
12
13 echo
14
15 echo username1 has not been declared.
16 echo "username1 = ${username1-`whoami`}"
17 # Will echo.
18
19 username2=
20 echo "username2 has been declared, but is set to null."
21 echo "username2 = ${username2:-`whoami`}"
22 # ^
23 # Will echo because of :- rather than just - in condition test.
24 # Compare to first instance, above.
25
26
27 #
28
29 # Once again:
30
31 variable=
32 # variable has been declared, but is set to null.
33
34 echo "${variable-0}" # (no output)
35 echo "${variable:-1}" # 1
36 # ^
37
38 unset variable
39
40 echo "${variable-2}" # 2
41 echo "${variable:-3}" # 3
42
43 exit 0 |
The default parameter construct finds use in providing "missing" command-line arguments in scripts.
1 DEFAULT_FILENAME=generic.data
2 filename=${1:-$DEFAULT_FILENAME}
3 # If not otherwise specified, the following command block operates
4 #+ on the file "generic.data".
5 # Begin-Command-Block
6 # ...
7 # ...
8 # ...
9 # End-Command-Block
10
11
12
13 # From "hanoi2.bash" example:
14 DISKS=${1:-E_NOPARAM} # Must specify how many disks.
15 # Set $DISKS to $1 command-line-parameter,
16 #+ or to $E_NOPARAM if that is unset. |
See also Example 3-4, Example 31-2, and Example A-6.
Compare this method with using an and list to supply a default command-line argument.
If parameter not set, set it to default.
Both forms nearly equivalent. The : makes a difference only when $parameter has been declared and is null, [1] as above.
1 echo ${var=abc} # abc
2 echo ${var=xyz} # abc
3 # $var had already been set to abc, so it did not change. |
If parameter set, use alt_value, else use null string.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, see below.
1 echo "###### \${parameter+alt_value} ########"
2 echo
3
4 a=${param1+xyz}
5 echo "a = $a" # a =
6
7 param2=
8 a=${param2+xyz}
9 echo "a = $a" # a = xyz
10
11 param3=123
12 a=${param3+xyz}
13 echo "a = $a" # a = xyz
14
15 echo
16 echo "###### \${parameter:+alt_value} ########"
17 echo
18
19 a=${param4:+xyz}
20 echo "a = $a" # a =
21
22 param5=
23 a=${param5:+xyz}
24 echo "a = $a" # a =
25 # Different result from a=${param5+xyz}
26
27 param6=123
28 a=${param6:+xyz}
29 echo "a = $a" # a = xyz |
If parameter set, use it, else print err_msg and abort the script with an exit status of 1.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, as above.
Example 10-7. Using parameter substitution and error messages
1 #!/bin/bash
2
3 # Check some of the system's environmental variables.
4 # This is good preventative maintenance.
5 # If, for example, $USER, the name of the person at the console, is not set,
6 #+ the machine will not recognize you.
7
8 : ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?}
9 echo
10 echo "Name of the machine is $HOSTNAME."
11 echo "You are $USER."
12 echo "Your home directory is $HOME."
13 echo "Your mail INBOX is located in $MAIL."
14 echo
15 echo "If you are reading this message,"
16 echo "critical environmental variables have been set."
17 echo
18 echo
19
20 # ------------------------------------------------------
21
22 # The ${variablename?} construction can also check
23 #+ for variables set within the script.
24
25 ThisVariable=Value-of-ThisVariable
26 # Note, by the way, that string variables may be set
27 #+ to characters disallowed in their names.
28 : ${ThisVariable?}
29 echo "Value of ThisVariable is $ThisVariable".
30
31 echo; echo
32
33
34 : ${ZZXy23AB?"ZZXy23AB has not been set."}
35 # Since ZZXy23AB has not been set,
36 #+ then the script terminates with an error message.
37
38 # You can specify the error message.
39 # : ${variablename?"ERROR MESSAGE"}
40
41
42 # Same result with: dummy_variable=${ZZXy23AB?}
43 # dummy_variable=${ZZXy23AB?"ZXy23AB has not been set."}
44 #
45 # echo ${ZZXy23AB?} >/dev/null
46
47 # Compare these methods of checking whether a variable has been set
48 #+ with "set -u" . . .
49
50
51
52 echo "You will not see this message, because script already terminated."
53
54 HERE=0
55 exit $HERE # Will NOT exit here.
56
57 # In fact, this script will return an exit status (echo $?) of 1. |
Example 10-8. Parameter substitution and "usage" messages
1 #!/bin/bash
2 # usage-message.sh
3
4 : ${1?"Usage: $0 ARGUMENT"}
5 # Script exits here if command-line parameter absent,
6 #+ with following error message.
7 # usage-message.sh: 1: Usage: usage-message.sh ARGUMENT
8
9 echo "These two lines echo only if command-line parameter given."
10 echo "command-line parameter = \"$1\""
11
12 exit 0 # Will exit here only if command-line parameter present.
13
14 # Check the exit status, both with and without command-line parameter.
15 # If command-line parameter present, then "$?" is 0.
16 # If not, then "$?" is 1. |
Parameter substitution and/or expansion. The following expressions are the complement to the match in expr string operations (see Example 16-9). These particular ones are used mostly in parsing file path names.
Variable length / Substring removal
String length (number of characters in $var). For an array, ${#array} is the length of the first element in the array.
![]() | Exceptions: |
Example 10-9. Length of a variable
1 #!/bin/bash
2 # length.sh
3
4 E_NO_ARGS=65
5
6 if [ $# -eq 0 ] # Must have command-line args to demo script.
7 then
8 echo "Please invoke this script with one or more command-line arguments."
9 exit $E_NO_ARGS
10 fi
11
12 var01=abcdEFGH28ij
13 echo "var01 = ${var01}"
14 echo "Length of var01 = ${#var01}"
15 # Now, let's try embedding a space.
16 var02="abcd EFGH28ij"
17 echo "var02 = ${var02}"
18 echo "Length of var02 = ${#var02}"
19
20 echo "Number of command-line arguments passed to script = ${#@}"
21 echo "Number of command-line arguments passed to script = ${#*}"
22
23 exit 0 |
${var#Pattern} Remove from $var the shortest part of $Pattern that matches the front end of $var.
${var##Pattern} Remove from $var the longest part of $Pattern that matches the front end of $var.
A usage illustration from Example A-7:
1 # Function from "days-between.sh" example.
2 # Strips leading zero(s) from argument passed.
3
4 strip_leading_zero () # Strip possible leading zero(s)
5 { #+ from argument passed.
6 return=${1#0} # The "1" refers to "$1" -- passed arg.
7 } # The "0" is what to remove from "$1" -- strips zeros. |
Manfred Schwarb's more elaborate variation of the above:
1 strip_leading_zero2 () # Strip possible leading zero(s), since otherwise
2 { # Bash will interpret such numbers as octal values.
3 shopt -s extglob # Turn on extended globbing.
4 local val=${1##+(0)} # Use local variable, longest matching series of 0's.
5 shopt -u extglob # Turn off extended globbing.
6 _strip_leading_zero2=${val:-0}
7 # If input was 0, return 0 instead of "".
8 } |
Another usage illustration:
1 echo `basename $PWD` # Basename of current working directory.
2 echo "${PWD##*/}" # Basename of current working directory.
3 echo
4 echo `basename $0` # Name of script.
5 echo $0 # Name of script.
6 echo "${0##*/}" # Name of script.
7 echo
8 filename=test.data
9 echo "${filename##*.}" # data
10 # Extension of filename. |
${var%Pattern} Remove from $var the shortest part of $Pattern that matches the back end of $var.
${var%%Pattern} Remove from $var the longest part of $Pattern that matches the back end of $var.
Version 2 of Bash added additional options.
Example 10-10. Pattern matching in parameter substitution
1 #!/bin/bash
2 # patt-matching.sh
3
4 # Pattern matching using the # ## % %% parameter substitution operators.
5
6 var1=abcd12345abc6789
7 pattern1=a*c # * (wild card) matches everything between a - c.
8
9 echo
10 echo "var1 = $var1" # abcd12345abc6789
11 echo "var1 = ${var1}" # abcd12345abc6789
12 # (alternate form)
13 echo "Number of characters in ${var1} = ${#var1}"
14 echo
15
16 echo "pattern1 = $pattern1" # a*c (everything between 'a' and 'c')
17 echo "--------------"
18 echo '${var1#$pattern1} =' "${var1#$pattern1}" # d12345abc6789
19 # Shortest possible match, strips out first 3 characters abcd12345abc6789
20 # ^^^^^ |-|
21 echo '${var1##$pattern1} =' "${var1##$pattern1}" # 6789
22 # Longest possible match, strips out first 12 characters abcd12345abc6789
23 # ^^^^^ |----------|
24
25 echo; echo; echo
26
27 pattern2=b*9 # everything between 'b' and '9'
28 echo "var1 = $var1" # Still abcd12345abc6789
29 echo
30 echo "pattern2 = $pattern2"
31 echo "--------------"
32 echo '${var1%pattern2} =' "${var1%$pattern2}" # abcd12345a
33 # Shortest possible match, strips out last 6 characters abcd12345abc6789
34 # ^^^^ |----|
35 echo '${var1%%pattern2} =' "${var1%%$pattern2}" # a
36 # Longest possible match, strips out last 12 characters abcd12345abc6789
37 # ^^^^ |-------------|
38
39 # Remember, # and ## work from the left end (beginning) of string,
40 # % and %% work from the right end.
41
42 echo
43
44 exit 0 |
Example 10-11. Renaming file extensions:
1 #!/bin/bash
2 # rfe.sh: Renaming file extensions.
3 #
4 # rfe old_extension new_extension
5 #
6 # Example:
7 # To rename all *.gif files in working directory to *.jpg,
8 # rfe gif jpg
9
10
11 E_BADARGS=65
12
13 case $# in
14 0|1) # The vertical bar means "or" in this context.
15 echo "Usage: `basename $0` old_file_suffix new_file_suffix"
16 exit $E_BADARGS # If 0 or 1 arg, then bail out.
17 ;;
18 esac
19
20
21 for filename in *.$1
22 # Traverse list of files ending with 1st argument.
23 do
24 mv $filename ${filename%$1}$2
25 # Strip off part of filename matching 1st argument,
26 #+ then append 2nd argument.
27 done
28
29 exit 0 |
Variable expansion / Substring replacement
These constructs have been adopted from ksh.
Variable var expanded, starting from offset pos.
Expansion to a max of len characters of variable var, from offset pos. See Example A-13 for an example of the creative use of this operator.
First match of Pattern, within var replaced with Replacement.
If Replacement is omitted, then the first match of Pattern is replaced by nothing, that is, deleted.
As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing, that is, deleted.
Example 10-12. Using pattern matching to parse arbitrary strings
1 #!/bin/bash
2
3 var1=abcd-1234-defg
4 echo "var1 = $var1"
5
6 t=${var1#*-*}
7 echo "var1 (with everything, up to and including first - stripped out) = $t"
8 # t=${var1#*-} works just the same,
9 #+ since # matches the shortest string,
10 #+ and * matches everything preceding, including an empty string.
11 # (Thanks, Stephane Chazelas, for pointing this out.)
12
13 t=${var1##*-*}
14 echo "If var1 contains a \"-\", returns empty string... var1 = $t"
15
16
17 t=${var1%*-*}
18 echo "var1 (with everything from the last - on stripped out) = $t"
19
20 echo
21
22 # -------------------------------------------
23 path_name=/home/bozo/ideas/thoughts.for.today
24 # -------------------------------------------
25 echo "path_name = $path_name"
26 t=${path_name##/*/}
27 echo "path_name, stripped of prefixes = $t"
28 # Same effect as t=`basename $path_name` in this particular case.
29 # t=${path_name%/}; t=${t##*/} is a more general solution,
30 #+ but still fails sometimes.
31 # If $path_name ends with a newline, then `basename $path_name` will not work,
32 #+ but the above expression will.
33 # (Thanks, S.C.)
34
35 t=${path_name%/*.*}
36 # Same effect as t=`dirname $path_name`
37 echo "path_name, stripped of suffixes = $t"
38 # These will fail in some cases, such as "../", "/foo////", # "foo/", "/".
39 # Removing suffixes, especially when the basename has no suffix,
40 #+ but the dirname does, also complicates matters.
41 # (Thanks, S.C.)
42
43 echo
44
45 t=${path_name:11}
46 echo "$path_name, with first 11 chars stripped off = $t"
47 t=${path_name:11:5}
48 echo "$path_name, with first 11 chars stripped off, length 5 = $t"
49
50 echo
51
52 t=${path_name/bozo/clown}
53 echo "$path_name with \"bozo\" replaced by \"clown\" = $t"
54 t=${path_name/today/}
55 echo "$path_name with \"today\" deleted = $t"
56 t=${path_name//o/O}
57 echo "$path_name with all o's capitalized = $t"
58 t=${path_name//o/}
59 echo "$path_name with all o's deleted = $t"
60
61 exit 0 |
If prefix of var matches Pattern, then substitute Replacement for Pattern.
If suffix of var matches Pattern, then substitute Replacement for Pattern.
Example 10-13. Matching patterns at prefix or suffix of string
1 #!/bin/bash
2 # var-match.sh:
3 # Demo of pattern replacement at prefix / suffix of string.
4
5 v0=abc1234zip1234abc # Original variable.
6 echo "v0 = $v0" # abc1234zip1234abc
7 echo
8
9 # Match at prefix (beginning) of string.
10 v1=${v0/#abc/ABCDEF} # abc1234zip1234abc
11 # |-|
12 echo "v1 = $v1" # ABCDEF1234zip1234abc
13 # |----|
14
15 # Match at suffix (end) of string.
16 v2=${v0/%abc/ABCDEF} # abc1234zip123abc
17 # |-|
18 echo "v2 = $v2" # abc1234zip1234ABCDEF
19 # |----|
20
21 echo
22
23 # ----------------------------------------------------
24 # Must match at beginning / end of string,
25 #+ otherwise no replacement results.
26 # ----------------------------------------------------
27 v3=${v0/#123/000} # Matches, but not at beginning.
28 echo "v3 = $v3" # abc1234zip1234abc
29 # NO REPLACEMENT.
30 v4=${v0/%123/000} # Matches, but not at end.
31 echo "v4 = $v4" # abc1234zip1234abc
32 # NO REPLACEMENT.
33
34 exit 0 |
Matches names of all previously declared variables beginning with varprefix.
1 # This is a variation on indirect reference, but with a * or @.
2 # Bash, version 2.04, adds this feature.
3
4 xyz23=whatever
5 xyz24=
6
7 a=${!xyz*} # Expands to *names* of declared variables
8 # ^ ^ ^ + beginning with "xyz".
9 echo "a = $a" # a = xyz23 xyz24
10 a=${!xyz@} # Same as above.
11 echo "a = $a" # a = xyz23 xyz24
12
13 echo "---"
14
15 abc23=something_else
16 b=${!abc*}
17 echo "b = $b" # b = abc23
18 c=${!b} # Now, the more familiar type of indirect reference.
19 echo $c # something_else |
| [1] | If $parameter is null in a non-interactive script, it will terminate with a 127 exit status (the Bash error code for "command not found"). |