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Bash:Test

lastmod 2024-12-05created 2015-08-246 edits

Tests (Test Constructs).

  • if/then tests whether the exit status of a command list is 0 (since by Unix convention 0 means "success"), and if so, runs the following commands.
  • There is a dedicated test command, [ (the left bracket special character). It is a synonym for the test command and is a built-in for efficiency. This command treats its arguments as comparison expressions or file tests and returns an exit status according to the result of the operation (0 for true, 1 for false).
  • Also, since version 2.02, Bash provides [[ ... ]], which offers comparison operations somewhat more familiar to programmers coming from other languages. Note that [[ is a keyword, not a command.
  • Bash understands [[ $a -lt $b ]] as a single element that returns an exit status.

Operator

Returns true if the following condition holds

  • -e: file exists
  • -f: regular file (not a directory or device file)
  • -s: file size is not 0
  • -d: file is a directory
  • -b: file is a block device (floppy, CD-ROM, etc.)
  • -c: file is a character device (keyboard, modem, sound card, etc.)
  • -p: file is a pipe
  • -h: file is a symbolic link
  • -L: file is a symbolic link
  • -S: file is a socket
  • -t: file descriptor is associated with a terminal device. Can be used to check whether a script's standard input ([ -t 0 ]) or standard output ([ -t 1 ]) is a terminal.
  • -r: the user running the test has read permission
  • -w: the user running the test has write permission
  • -x: the user running the test has execute permission
  • -g: the set-group-id (sgid) flag is set on the file or directory
  • -u: the set-user-id (suid) flag is set on the file
  • -k: sticky bit (sticky bit) is set
  • -O: you are the owner
  • -G: group ID is the same as yours
  • -N: modified since it was last read
  • f1 -nt f2: file f1 is newer than file f2
  • f1 -ot f2: file f1 is older than file f2
  • f1 -ef f2: files f1 and f2 are hard links to the same file
  • !: negation.
  • -n: the string length is not 0.
  • -z: the string length is 0.

[ vs [[

  • [[ ]] is the same as [ ] in the shell. This command is an extended test command borrowed from ksh88.
  • Between [[ and ]], filename expansion and word splitting do not occur, but parameter expansion and command substitution do.
  • Using [[ ... ]] instead of [ ... ] can prevent many logic errors. For example, the &&, ||, <, > operators cause errors in [ ] but work fine in [[ ]].

Another advantage of using double brackets is that it is a bit easier to read when there are two or more conditions combined with AND/OR. 

num=1
name="john"

# 조건식: num=1 AND name="john"
if [ $num -eq 1 -a $name = "john" ]
then echo "$num: $name"
fi

# 대괄호 두개 사용
if [[ $num -eq 1 ]] && [[ $name = "john" ]]
then echo "$num: $name"
fi

((

  • (( ... )) and let ... return exit status 0 if the arithmetic expression they evaluate has a nonzero value. Therefore, this arithmetic expansion can be used for arithmetic comparisons. 
# test if file exists (test actual file, not symbolic link)
if [ ! -e "$F" ] ; then
    # code if the symlink is broken
fi

See also

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