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7.1. Test Constructs

  • An if/then construct tests whether the exit status of a list of commands is 0 (since 0 means "success" by UNIX convention), and if so, executes one or more commands.

  • There exists a dedicated command called [ (left bracket special character). It is a synonym for test, and a builtin for efficiency reasons. This command considers its arguments as comparison expressions or file tests and returns an exit status corresponding to the result of the comparison (0 for true, 1 for false).

  • With version 2.02, Bash introduced the [[ ... ]] extended test command, which performs comparisons in a manner more familiar to programmers from other languages. Note that [[ is a keyword, not a command.

    Bash sees [[ $a -lt $b ]] as a single element, which returns an exit status.

  • The (( ... )) and let ... constructs also return an exit status, according to whether the arithmetic expressions they evaluate expand to a non-zero value. These arithmetic-expansion constructs may therefore be used to perform arithmetic comparisons.

    (( 0 && 1 ))                 # Logical AND
    echo $?     # 1     ***
    # And so ...
    let "num = (( 0 && 1 ))"
    echo $num   # 0
    # But ...
    let "num = (( 0 && 1 ))"
    echo $?     # 1     ***
    
    
    (( 200 || 11 ))              # Logical OR
    echo $?     # 0     ***
    # ...
    let "num = (( 200 || 11 ))"
    echo $num   # 1
    let "num = (( 200 || 11 ))"
    echo $?     # 0     ***
    
    
    (( 200 | 11 ))               # Bitwise OR
    echo $?                      # 0     ***
    # ...
    let "num = (( 200 | 11 ))"
    echo $num                    # 203
    let "num = (( 200 | 11 ))"
    echo $?                      # 0     ***
    
    # The "let" construct returns the same exit status
    #+ as the double-parentheses arithmetic expansion.
  • An if can test any command, not just conditions enclosed within brackets.

    if cmp a b &> /dev/null  # Suppress output.
    then echo "Files a and b are identical."
    else echo "Files a and b differ."
    fi
    
    # The very useful "if-grep" construct:
    # ----------------------------------- 
    if grep -q Bash file
      then echo "File contains at least one occurrence of Bash."
    fi
    
    word=Linux
    letter_sequence=inu
    if echo "$word" | grep -q "$letter_sequence"
    # The "-q" option to grep suppresses output.
    then
      echo "$letter_sequence found in $word"
    else
      echo "$letter_sequence not found in $word"
    fi
    
    
    if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED
      then echo "Command succeeded."
      else echo "Command failed."
    fi
  • These last two examples courtesy of StИphane Chazelas.

Example 7-1. What is truth?

#!/bin/bash

#  Tip:
#  If you're unsure of how a certain condition would evaluate,
#+ test it in an if-test.

echo

echo "Testing \"0\""
if [ 0 ]      # zero
then
  echo "0 is true."
else          # Or else ...
  echo "0 is false."
fi            # 0 is true.

echo

echo "Testing \"1\""
if [ 1 ]      # one
then
  echo "1 is true."
else
  echo "1 is false."
fi            # 1 is true.

echo

echo "Testing \"-1\""
if [ -1 ]     # minus one
then
  echo "-1 is true."
else
  echo "-1 is false."
fi            # -1 is true.

echo

echo "Testing \"NULL\""
if [ ]        # NULL (empty condition)
then
  echo "NULL is true."
else
  echo "NULL is false."
fi            # NULL is false.

echo

echo "Testing \"xyz\""
if [ xyz ]    # string
then
  echo "Random string is true."
else
  echo "Random string is false."
fi            # Random string is true.

echo

echo "Testing \"\$xyz\""
if [ $xyz ]   # Tests if $xyz is null, but...
              # it's only an uninitialized variable.
then
  echo "Uninitialized variable is true."
else
  echo "Uninitialized variable is false."
fi            # Uninitialized variable is false.

echo

echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ]            # More pedantically correct.
then
  echo "Uninitialized variable is true."
else
  echo "Uninitialized variable is false."
fi            # Uninitialized variable is false.

echo


xyz=          # Initialized, but set to null value.

echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ]
then
  echo "Null variable is true."
else
  echo "Null variable is false."
fi            # Null variable is false.


echo


# When is "false" true?

echo "Testing \"false\""
if [ "false" ]              #  It seems that "false" is just a string.
then
  echo "\"false\" is true." #+ and it tests true.
else
  echo "\"false\" is false."
fi            # "false" is true.

echo

echo "Testing \"\$false\""  # Again, uninitialized variable.
if [ "$false" ]
then
  echo "\"\$false\" is true."
else
  echo "\"\$false\" is false."
fi            # "$false" is false.
              # Now, we get the expected result.

#  What would happen if we tested the uninitialized variable "$true"?

echo

exit 0

Exercise. Explain the behavior of Example 7-1, above.

if [ condition-true ]
then
   command 1
   command 2
   ...
else  # Or else ...
      # Adds default code block executing if original condition tests false.
   command 3
   command 4
   ...
fi

Note

When if and then are on same line in a condition test, a semicolon must terminate the if statement. Both if and then are keywords. Keywords (or commands) begin statements, and before a new statement on the same line begins, the old one must terminate.

if [ -x "$filename" ]; then

Else if and elif

elif

elif is a contraction for else if. The effect is to nest an inner if/then construct within an outer one.

if [ condition1 ]
then
   command1
   command2
   command3
elif [ condition2 ]
# Same as else if
then
   command4
   command5
else
   default-command
fi

The if test condition-true construct is the exact equivalent of if [ condition-true ]. As it happens, the left bracket, [ , is a token [1] which invokes the test command. The closing right bracket, ] , in an if/test should not therefore be strictly necessary, however newer versions of Bash require it.

Note

The test command is a Bash builtin which tests file types and compares strings. Therefore, in a Bash script, test does not call the external /usr/bin/test binary, which is part of the sh-utils package. Likewise, [ does not call /usr/bin/[, which is linked to /usr/bin/test.

bash$ type test
test is a shell builtin
bash$ type '['
[ is a shell builtin
bash$ type '[['
[[ is a shell keyword
bash$ type ']]'
]] is a shell keyword
bash$ type ']'
bash: type: ]: not found
	      

If, for some reason, you wish to use /usr/bin/test in a Bash script, then specify it by full pathname.

Example 7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[

#!/bin/bash

echo

if test -z "$1"
then
  echo "No command-line arguments."
else
  echo "First command-line argument is $1."
fi

echo

if /usr/bin/test -z "$1"      # Equivalent to "test" builtin.
#  ^^^^^^^^^^^^^              # Specifying full pathname.
then
  echo "No command-line arguments."
else
  echo "First command-line argument is $1."
fi

echo

if [ -z "$1" ]                # Functionally identical to above code blocks.
#   if [ -z "$1"                should work, but...
#+  Bash responds to a missing close-bracket with an error message.
then
  echo "No command-line arguments."
else
  echo "First command-line argument is $1."
fi

echo


if /usr/bin/[ -z "$1" ]       # Again, functionally identical to above.
# if /usr/bin/[ -z "$1"       # Works, but gives an error message.
#                             # Note:
#                               This has been fixed in Bash, version 3.x.
then
  echo "No command-line arguments."
else
  echo "First command-line argument is $1."
fi

echo

exit 0

Note
Following an if, neither the test command nor the test brackets ( [ ] or [[ ]] ) are strictly necessary.
dir=/home/bozo

if cd "$dir" 2>/dev/null; then   # "2>/dev/null" hides error message.
  echo "Now in $dir."
else
  echo "Can't change to $dir."
fi
The "if COMMAND" construct returns the exit status of COMMAND.
Similarly, a condition within test brackets may stand alone without an if, when used in combination with a list construct.
var1=20
var2=22
[ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2"

home=/home/bozo
[ -d "$home" ] || echo "$home directory does not exist."

The (( )) construct expands and evaluates an arithmetic expression. If the expression evaluates as zero, it returns an exit status of 1, or "false". A non-zero expression returns an exit status of 0, or "true". This is in marked contrast to using the test and [ ] constructs previously discussed.

Example 7-3. Arithmetic Tests using (( ))

#!/bin/bash
# arith-tests.sh
# Arithmetic tests.

# The (( ... )) construct evaluates and tests numerical expressions.
# Exit status opposite from [ ... ] construct!

(( 0 ))
echo "Exit status of \"(( 0 ))\" is $?."         # 1

(( 1 ))
echo "Exit status of \"(( 1 ))\" is $?."         # 0

(( 5 > 4 ))                                      # true
echo "Exit status of \"(( 5 > 4 ))\" is $?."     # 0

(( 5 > 9 ))                                      # false
echo "Exit status of \"(( 5 > 9 ))\" is $?."     # 1

(( 5 == 5 ))                                     # true
echo "Exit status of \"(( 5 == 5 ))\" is $?."    # 0
# (( 5 = 5 ))  gives an error message.

(( 5 - 5 ))                                      # 0
echo "Exit status of \"(( 5 - 5 ))\" is $?."     # 1

(( 5 / 4 ))                                      # Division o.k.
echo "Exit status of \"(( 5 / 4 ))\" is $?."     # 0

(( 1 / 2 ))                                      # Division result < 1.
echo "Exit status of \"(( 1 / 2 ))\" is $?."     # Rounded off to 0.
                                                 # 1

(( 1 / 0 )) 2>/dev/null                          # Illegal division by 0.
#           ^^^^^^^^^^^
echo "Exit status of \"(( 1 / 0 ))\" is $?."     # 1

# What effect does the "2>/dev/null" have?
# What would happen if it were removed?
# Try removing it, then rerunning the script.

# ======================================= #

# (( ... )) also useful in an if-then test.

var1=5
var2=4

if (( var1 > var2 ))
then #^      ^      Note: Not $var1, $var2. Why?
  echo "$var1 is greater than $var2"
fi     # 5 is greater than 4

exit 0

Notes

[1]

A token is a symbol or short string with a special meaning attached to it (a meta-meaning). In Bash, certain tokens, such as [ and . (dot-command), may expand to keywords and commands.


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