Linux command – file & test

file

因為 Linux 不依據檔尾的名稱來判斷是什麼檔案,所以我們可以使用 #file 這個程式來判斷檔案的格式.而這個程式是依據 /usr/share/magic 所定義的檔頭來做分辨.

[root@benjr ~]# file /etc/passwd
/etc/passwd: ASCII text
[root@benjr ~]# file /bin/ls
/bin/ls: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), for GNU/Linux 2.2.5, dynamically linked (uses shared libs), stripped

下次遇到不確定的檔案可先用 #file 來確認.

test

test 一樣可以檢查檔案,方式很多種,先以下面的範例來做說明.

• -e FILE
  FILE exists

當執行指令時都會回傳一個執行後的代碼給變數 $?,成功的執行完指令後會回傳一個 0 值(有錯誤時會回傳 錯誤代碼).
利用 test 的指令來看看 變數 $? 的變化.

root@ubuntu:~# test -e /etc/passwd
root@ubuntu:~# echo $?
0

root@ubuntu:~# test -e /etc/passwd11
root@ubuntu:~# echo $?
1

可以很明顯看到當 test 成功後回傳得值為 0 ,失敗則回傳 1 .

通常變數 $? 常會與 && || 一起使用,我們現來看一下 && || 的用法

• cmd1 && cmd2 : 若 cmd1 執行正確且無錯誤 ($?=0),則執行 cmd2 ,反之若 cmd1 執行完畢且為錯誤 ($?≠0),則 cmd2 不執行.
• cmd1 || cmd2 : 若 cmd1 執行正確且無錯誤 ($?=0),則 cmd2 不執行 ,反之若 cmd1 執行完畢且為錯誤 ($?≠0),則開始執行 cmd2.

下面的範例的語法是常用的架構 (這 && 與 || 的順序就是固定),用以判斷 command1 的執行是成功還是失敗,成功則執行 command2 失敗則執行 command 2

command1 && command2 || command3

使用上面常用的 && || 的判斷式的架構範例.

root@ubuntu:~# test -e /etc/passwd && echo "File exist" || echo "File not exist"
File exist

root@ubuntu:~# test -e /etc/passwd11 && echo "File exist" || echo "File not exist"
File not exist

其他可以用參數.

• -b FILE
  FILE exists and is block special
• -c FILE
  FILE exists and is character special
• -d FILE
  FILE exists and is a directory
• -f FILE
  FILE exists and is a regular file
• -g FILE
  FILE exists and is set-group-ID
• -G FILE
  FILE exists and is owned by the effective group ID
• -h FILE
  FILE exists and is a symbolic link (same as -L)
• -k FILE
  FILE exists and has its sticky bit set
• -L FILE
  FILE exists and is a symbolic link (same as -h)
• -O FILE
  FILE exists and is owned by the effective user ID
• -p FILE
  FILE exists and is a named pipe
• -r FILE
  FILE exists and read permission is granted
• -s FILE
  FILE exists and has a size greater than zero
• -S FILE
  FILE exists and is a socket
• -t FD
  file descriptor FD is opened on a terminal
• -u FILE
  FILE exists and its set-user-ID bit is set
• -w FILE
  FILE exists and write permission is granted
• -x FILE
  FILE exists and execute (or search) permission is granted

除了比較一個檔案,test 還可以比較兩個檔案.

• FILE1 -ef FILE2
  FILE1 and FILE2 have the same device and inode numbers
• FILE1 -nt FILE2
  FILE1 is newer (modification date) than FILE2
• FILE1 -ot FILE2
  FILE1 is older than FILE2

下面的例子用來比較兩個檔案的新舊,並觀察變數 $? 的變化.

root@ubuntu:~# test /etc/passwd -nt /etc/group
root@ubuntu:~# echo $?
0
root@ubuntu:~# test /etc/passwd -ot /etc/group
root@ubuntu:~# echo $?
1

其他可以用參數.

• STRING1 == STRING2
  the strings are equal
• STRING1 != STRING2
  the strings are not equal
• INTEGER1 -eq INTEGER2
  INTEGER1 is equal to INTEGER2
• INTEGER1 -ge INTEGER2
  INTEGER1 is greater than or equal to INTEGER2
• INTEGER1 -gt INTEGER2
  INTEGER1 is greater than INTEGER2
• INTEGER1 -le INTEGER2
  INTEGER1 is less than or equal to INTEGER2
• INTEGER1 -lt INTEGER2
  INTEGER1 is less than INTEGER2
• INTEGER1 -ne INTEGER2
  INTEGER1 is not equal to INTEGER2
• -n STRING
  the length of STRING is nonzero
• -z STRING
  the length of STRING is zero